Spirent Positioning Blog

Will your GNSS system manage the upcoming leap seconds insertion?

Mon, 30 Jan 2012 19:00:00 GMT

This free of charge Spirent Application Note tells you how.

The International Earth Rotation and Reference Systems Service (IERS), the organisation which monitors and manages the difference between the atomic Universal Coordinated Time (UTC) and Earth-rotation-based time, UT1, has decided that a further leap-second will be inserted on 30th June 2012 at 23 hours, 59 minutes and 59 seconds. This insertion - the first since 2008 - is intended to compensate for the accumulated difference between UTC and UT1, and the fact that the Earth’s rotation is currently running slower than UTC. GNSS systems use time references based on atomic clocks linked to UTC, so it is beneficial to keep these time systems aligned with Earth-rotation-based time.

Testing GNSS systems to see how they manage leap second insertions is crucial, particularly as some of the most critical systems around the world rely on precise time, rather than navigation accuracy. The world’s global financial organisations and major energy and communications systems are just three examples.

Users of Spirent GNSS test equipment have been able to test their systems for leap second events for many years. The test is very straightforward, and can be performed easily and repeatedly. An application note Simulating UTC leap second insertion events provides the background on leap seconds, how they are documented in the relevant ICD and describes how you can perform the test with ease on a Spirent simulator.

So, as the saying goes, “Time and tide waits for no man”. Have you planned your GNSS testing to account for 30th June 2012? Talk to Spirent, with proven expertise and world-wide, comprehensive technical support we are ready to help you, so get in touch.

Automate your GNSS Receiver Testing

Tue, 13 Dec 2011 17:39:00 GMT

“We are in Testing Times” A phrase being used more and more these days in relation to many things. However ‘Testing Times’ is also a concern to anyone responsible for planning, resourcing and carrying out GNSS testing. We all want our products to be designed, developed and out to the market in ever-shorter timescales. The time taken to perform important testing is often significant. How great it would be to be able to reduce this time, make things more efficient?

Well, there is good news. Spirent recognises the problems faced by test programmes in reducing cost and time. Indeed our engineers have just the same problems as you do. So, we will soon be introducing a new way of automating the fundamental GNSS tests required for characterisation of GNSS devices.
TestDrive-GNSS is that new tool. It is a piece of software which controls your Spirent simulator and GNSS device under test. It provides a suite of test cases that enables fundamental characterization tests to be performed in an automated and controlled way. Examples of fundamental characterization tests include time-to-first-fix (TTFF), acquisition sensitivity, tracking sensitivity, re-acquisition time, and static and dynamic navigation accuracy. The tool also provides a formal test report at the end of the process.

Characterization tests can be cumbersome to execute manually, especially when iterative repetition is required as test signals and conditions are changed. For example, a TTFF test typically involves 100 or more repeat measurements which, if undertaken manually, require significant operator involvement. Similarly, consider a navigation accuracy test under a full range of Dilution of Precision (DOP) conditions where the satellite constellation geometry needs to be changed many times. Automation of such a test is essential in a world where the timescales for completing test programs are being constantly reduced.

TestDrive-GNSS unlocks the full power of a Spirent GNSS simulation system, enabling the test equipment to do the work with minimal intervention. It not only saves time, but helps to increase test quality and accountability with its comprehensive test reporting. With TestDrive-GNSS, these Testing Times are now much shorter.

Watch this space in early 2012 for TestDrive-GNSS. Your route to more efficient, less costly GNSS testing.

A Review of the Possible Sources of Common GPS Errors

Mon, 24 Oct 2011 11:30:00 GMT

Today, GPS applications and services are everywhere, and accessible to almost everyone.

In the UK you can now purchase a portable “SatNav” system, admittedly from a manufacturer you might not have heard of, for less than it costs to fill your car with fuel! This is great news indeed, right? Well, maybe … but then maybe not. When was the last time you opened your morning paper and did NOT find an article about a lorry blocking a country road or a car getting stranded somewhere remote? Many of us in this industry are concerned about how this bad PR about “SatNav” might end us all up in a bad place.

Being in the GNSS industry, we know, of course, that the GNSS itself is not to blame for these misdemeanours. We know that the GPS receiver is reporting a position accurately on flawed route instructions. The GPS accurately takes the lorry to the country road. We also know that this issue is being actively worked on and, hopefully solved. Improvements underway include smart routeing to avoid minor roads, systems that take the size and weight of vehicles into account, and improved mapping information. These issues are squarely in the commercial domains, where the customer demands perfect performance for extremely low investment. In more professional applications, it’s usually reasonable to assume that the user will want to understand the limitations of the technology. The golden rule is not to rely slavishly and without thought in the GNSS position. The position you see may not always be entirely accurate or without risk.

A comprehensive review of the possible sources of GPS error are well beyond the scope of this article. However, we can touch here on some of the most common, and not always most well known, error sources. Looking at an overall “error budget” for a single frequency GPS receiver in an open sky environment, the major component of error is atmospheric. The L-band GPS signals will be delayed dependent on the electron count of the atmosphere. GPS receivers attempt to back-out these atmospheric errors via standard models in the receiver itself. The most common is known as the Klobuchar model. These models are quite good at giving us sub five metre accuracy we expect today. In fact today the accuracy is often even better than this, say 2-3 metres, partly due to us currently being at in the middle of an 11-year solar cycle. The next solar maximum will peak around 2011, and there is disagreement among experts how big the electron count at that time will be. What is certain is that the high solar activity is more difficult to model and will cause higher inaccuracy in single frequency receivers. For dual-frequency receiver users the problem is much less, as the atmosphere affects the two frequencies differently and it is therefore possible for the receiver to measure, and back-out, the ambiguity of the atmosphere from the receiver’s position determination.

Multipath contributes a much more dynamic and potentially disruptive element to receiver errors. Multipath occurs when a signal arrives indirectly at a receiver antenna. Because of the indirect path, the signal length is longer. While receivers try to identify and reject multipath signals from their position calculation, this is not always successful. Multipath effects can introduce high magnitude errors, of the order of tens of metres not being uncommon. In addition, for moving vehicles, multipath can vary quickly, introducing see-sawing effects in speed and position. Bear in mind also that multipath will never be the same twice, even at the same location, due to the ever changing positions of the GPS satellites in the sky.

Very occasionally, GPS has been known to have signal transmission errors. Correction messages, based on reference data from GPS monitoring stations, are often used to inform receivers of a bad satellite or signal condition.

There can be a lag between an error occurring and a health flag being set and transmitted to the receiver. The good news for all of us is that, used with a degree of knowledge, GPS is an efficient and reliable professional tool. With the addition of new GPS signals (e.g. L2C and L5) multi-frequency professional receivers will be much more readily available. New GLONASS satellites have been added quite recently. And Galileo and potentially the Chinese Compass signal will also be available during this decade.

We still have some way to go, particularly in relation to navigation anywhere, but exciting times indeed are ahead for those of us tracking GNSS technology developments.

For more in depth information regarding testing GNSS System Errors, download the free Spirent eBook.

Simulating roadside buildings with SimGEN Vertical Planes Feature

Fri, 21 Oct 2011 12:00:00 GMT

One of the major considerations in designing a GNSS receiver for automotive applications is to ensure that the performance is not unduly affected by roadside buildings. Such structures can both obscure satellite signals from the receiver and reflect them, leading to potentially complex multipath effects.

Clearly, it is impractical for a receiver designer to take his or her latest project “on the road” to test out its performance in the presence of every possible size, shape and combination of roadside buildings. However, help is at hand in the form of the Vertical Planes feature of Spirent's SimGEN™ Windows software.

The Vertical Planes feature enables users to define a series of vertical, rectangular obstructions (or planes) on either or both sides of the vehicle's direction of travel. The software gives complete freedom to define the distance, height and width of each obstruction. Quite simply, the simulator signals are obscured if the planes occur in the line-of-sight path between the simulated satellites and the vehicle antenna. And the level of obscuration will depend on both the height of the planes and the relative distance from the vehicle.

The Vertical Planes feature can also be used to simulate the effect of a road running through a cutting by replicating the obscuration of low-level satellites while the vehicle is in the simulated cutting.

High-precision GNSS market set for growth

Wed, 19 Oct 2011 11:00:00 GMT

With the latest report from market research guru ABI Research predicting that the high-precision GNSS sector is set to double in size between 2011 and 2016, there will be strong temptation for new companies to enter this potentially lucrative part of the market.

Already consumer chipset manufacturers are looking to exploit the potential of the forthcoming open L2C and L5 signals, and with China accounting for around half of the market in 2010, new manufacturers from that locality are sure to claim a share of the spoils.

Driving the market growth are a small number of specific application areas, including continuously operating reference stations (CORS), advances in real-time kinematic (RTK) techniques and various augmentation technologies.

However, it is important to note that the predictions in the ABI Research report are specifically for the high-precision GNSS market, one in which accuracy and reliability are paramount. Therefore, any companies considering expanding into this sector must above all be confident in the quality of their designs and on-going manufacturing output.

The importance of testing GNSS receivers throughout their product lifecycle can never be over-stated. And, in the case of GNSS receivers that are intended to exploit as-yet unbroadcast L2C and L5 signals, this can only mean embracing the world of GNSS simulation at the very start of the project.

What is PRN code?

Mon, 17 Oct 2011 12:00:00 GMT

Pseudorandom noise (PRN) codes are an important element of code division multiple access (CDMA) based satellite navigation systems. Each satellite within a GNSS constellation has a unique PRN code that it transmits as part of the C/A navigation message. This code allows any receiver to identify exactly which satellite(s) it is receiving.

The PRN codes act as spreading codes in the spread-spectrum communications system, and must be carefully chosen to minimise interference between each satellite signal. Failure to do so would leave the system open to so-called CDMA noise, potentially degrading performance to unworkable levels.

It is not only satellites that are allocated PRN codes, they are also necessary for augmentation systems and pseudolites. Therefore, the PRN codes for each GNSS have to be carefully managed.

In the GPS system, this management is performed by the GPS Directorate, which has already defined a large set of GPS PRN sequences that provide good auto- and cross-correlation properties. Operators of augmentation systems and other pseudolites must then apply to the GPS Directorate to be allocated one of the codes from this sequence.

Compass on track for global coverage

Fri, 14 Oct 2011 11:30:00 GMT

While the Russian GLONASS constellation is slowly approaching full operational status, developers of future multi-GNSS systems also need to focus on the likely timeline for availability of the Chinese Beidou-2 or Compass system.

The successful launch of the latest satellite for the Beidou-2 constellation means that nine such satellites have been placed into orbit since 2007. Further launches are planned for this year and next. And the Chinese authorities reckon to be on course to provide satellite navigation, time and short message services throughout the Asia-Pacific region some time during 2012.

Further launches will follow, with a stated aim of providing global coverage by 2020. The Compass constellation will eventually comprise 35 geostationary satellites.

The Compass GNSS will offer two different services. The freely available civilian service aims to provide an accuracy of 10m for user position, 0.2m/s for user velocity and 50ns for time. The second military service will provide greater accuracy levels.

So with Asia-Pacific coverage imminent and a promise of global coverage to follow, developers of GNSS systems and applications with worldwide markets need to factor Compass functionality into their future projects. Fortunately, testing of systems can begin well in advance of the launch of the remainder of the Compass constellation using the Spirent range of multi-GNSS simulators.

If you have an interest in testing the Compass constellation, get in touch.

Why test GNSS receivers for EMC?

Wed, 12 Oct 2011 11:30:00 GMT

Although there is no specific product standard for EMC in GNSS receivers, testing for immunity to electromagnetic interference has to be an essential part of the product development process.

Any receiver designed for low-level signals will inevitably be susceptible to electromagnetic interference. And GNSS receivers inevitably are required to operate reliably in areas where they will be subject to any number of sources of interference. These range from natural phenomena such as solar radio bursts to man-made sources such as broadcast transmitters and electrical noise from all manner of electromechanical machinery and even power lines and the like.

Add to these the specific problems of certain application areas, such as vehicle ignition systems interfering with automotive sat nav or telemetry systems, and the operating environment for a GNSS receiver could scarcely be more hostile.

In addition, with many GNSS receivers integrated into larger pieces of equipment, the scope for electromagnetic interference is even greater. Earth loops, oscillators operating in close proximity to each other and intentional emitters sitting on a common PCB with the receiver add to the potential for problems.

So while EMC testing of GNSS receivers may not be a mandatory requirement, it is an essential part of the quality control process to ensure that the receivers will provide reliable accurate outputs.

What is CNAV?

Mon, 10 Oct 2011 11:30:00 GMT

CNAV is the name for the civilian navigation message that will be carried by the modernized GPS system. And while the CNAV message will carry similar data to the existing NAV message, its structure will be completely different, with a packetised format that will increase message bandwidth to allow for greater information density and pave the way for future system expansion. To this end, the system is designed to support 63 satellites, compared with 32 for the L1 NAV message.

Each packet of the CNAV message is 300bit in length. Although only a small proportion of the available packet types have so far been defined, the basic structure has been set. Two out of every four packets will contain ephemeris data and at least one in four packets will include clock data. One of these clock packets will incorporate a GPS time offset to simplify timebase integration between GPS and other GNSSs, such as Galileo and GLONASS. A further packet is available for differential correction, which can be used to correct the L1 NAV clock data.

As part of the new message structure, the CNAV message uses forward error correction to effectively double its bandwidth. This means that while the 300bit packet would normally take 12 seconds to transmit, each packet is transferred in 6 seconds.

Each packet incorporates an error flag that can be tripped if the satellite data cannot be trusted. Therefore, users will be aware within 6 seconds if any satellite in the constellation is no longer usable, which is important for safety-of-life applications.

Developers of GNSS receivers designed to support modernised GPS and other systems such as GLONASS and Galileo can test their systems today using Spirent's range of multi-GNSS constellation simulators.

GNSS Chipset Expansion Must be Underpinned by Quality

Wed, 29 Jun 2011 13:00:00 GMT

With a recent report predicting that consumer and precision GNSS receiver IC shipments will reach two billion units by 2016, it is important to remember that quantity must be underscored by quality. The report, by ABI Research, highlights the importance of new vertical markets, such as tablets, cameras and fitness equipment, as well as the growth of LBS (location-based services) and LBA (location-based advertising) applications, all of which will contribute to a quadrupling of the market within five years.

The report also predicts that multi-GNSS GPS + GLONASS functionality will become standard by 2013, with additional support for Compass following shortly thereafter. Hybrid systems will also come to the fore, with both Wi-Fi positioning and inertial sensors becoming standard features in the cellphone market.

Such rapid advances, particularly in the consumer sphere, can only come about with the entry to the market of a whole raft of manufacturers with little or no experience in GNSS receiver design. And while the quality, reliability and accuracy of the semiconductors themselves are not in question, it is the expertise of these new manufacturers in applying them and integrating them that could lead to unwanted consequences.

While the consumer's appetite for location enabled devices continues to grow, it would only take a few high-profile failures for the market to be set back years. So it is essential, both for the health of the market and for the reputations of the manufacturers involved, that the quality, reliability and accuracy of the products reaching the market are not compromised.

With test solutions available now for GPS, GLONASS and Compass GNSS receivers, together with a great deal of research into systems capable of testing hybrid technologies, there is no excuse for releasing untried and untested designs onto the market. It doesn't matter whether the product is a navigation system, a tablet PC or a running shoe, the consumer expects and deserves that it will function exactly as expected under all reasonable conditions.

Recently Spirent sponsored an informational webinar titled, “Hybrid Positioning & Technologies for Mobile Users,” During the presentation, four recognized industry experts:

Brock Butler, Spirent
Farshid Alizadeh, Skyhook Wireless
Tony Haddrell, Pinpoint Positioning
Naser El-Sheimy, Trusted Positioning

Discussed a variety of technologies that can be integrated into mobile devices for use in places where GPS signals are not always available. The webinar explored the uninterrupted continuous positioning technology that is no longer a “nice to have” option but, for many applications, a “must have” requirement.

The live webinar attracted almost 900 registrations. View our on-demand Webinar anytime and find out why your peers found this subject so interesting.

If you would like to learn even more about hybrid positioning and technologies for mobile users contact: gnsseditor@spirent.com

How will GPS/GLONASS Chipset Integration Help the Consumer?

Thu, 16 Jun 2011 11:30:00 GMT

The integration of GPS and GLONASS systems within the same receiver offers many advantages to both the manufacturer and the end user. And now that this integration is happening at the chipset level, several further advantages can be added to the list.

On the basic technology level, the combination of GPS with GLONASS will provide greatly improved performance for the end user. The ability for the receiver to access signals from both constellations will provide greater availability, improved accuracy, faster time to first fix, wider coverage and improved performance in “difficult” environments, such as urban canyons. However, initial implementations have dictated that all these benefits come at a cost to the manufacturer (both in terms of design complexity and bill of materials), which is inevitably passed on to the end user.

Now, though, with the receiver integration taking place at the chipset level, the technology immediately becomes available to a wider ecosystem of manufacturers, and the benefits of this will clearly flow on to the consumer.

The latest devices recently released use state-of-the-art 45nm semiconductor processing, making the overall GNSS receiver both smaller and less power-hungry than previous implementations. The ability to build a complete GPS/ GLONASS receiver with a reduced bill of materials and covering just 25 square millimetres of PCB real estate will mean that smaller and sleeker receivers can be built into an ever-wider range of end applications, with the reduced power consumption further saving space by reducing the space needed for battery power sources.

In the end, the consumer gets a GNSS receiver that is smaller, cheaper, more reliable, more accurate and more efficient.

Do you Need a Screened Enclosure to Test a GNSS Receiver?

Tue, 14 Jun 2011 11:30:00 GMT

The traditional logic that all RF testing should be carried out in a screened enclosure has much to recommend it. But do you always need to go to this level of protection when testing a GNSS receiver with a simulator? The short answer is no, because the preferred method of testing is to remove the antenna from the equation and connect the simulator directly to the receiver.

However, there will be occasions where the antenna cannot be removed. And there will also be certain acceptance tests that demand the testing of the complete system – including the antenna.

The screened enclosure (or TEMS cell), which only needs to be large enough to accommodate the test piece and a dipole antenna to emit the simulator signals, will act as a Faraday cage to keep unwanted signals from the outside away from the receiver's antenna. And while testing will inevitably be performed indoors, within a building that itself will act as a Faraday cage so far as live-sky GNSS signals are concerned, the building will also be home to its own array of intentional wireless signals and accidental radio frequency interference that could compromise the validity of the receiver tests. What's more, RF-reflecting surfaces within the test area can also create complex multipath effects that could also compromise the results.

A wide variety of screened enclosures are available with a wide range of sizes and a huge variation in costs. The enclosure must be large enough to accommodate the unit under test and the half-wavelength dipole antenna, and should provide RF shielding of at least 40dB at 1.5GHz and have all its internal surfaces covered in suitable RF absorbing material to minimise internal reflections and multipath effects.

For more information on the importance of antenna modeling in GNSS testing download the FREE Spirent Application Note “Keeping your eye on the sky”.

GNSS Record & Playback Complements Testing With Simulators

Fri, 10 Jun 2011 11:30:00 GMT

All Global Navigation Satellite Systems (GNSS) work by measuring the transmission-time delay from a satellite to the receiver. With a clear view of the sky and an unobstructed path to multiple satellites a modern GNSS receiver is able to calculate its position rapidly and accurately. However atmospheric propagation effects can alter the speed of the signals to an indeterminate degree, signals can be obscured by buildings or reflected off surfaces such as the sea, all of which will compromise the performance of the receiver, reducing accuracy, extending acquisition time or reducing sensitivity.

Thorough evaluation of receiver performance requires that the impact of these various sources of impairment is assessed. An emerging technique for performing this testing is by Recording the RF signal for subsequent Playback in the lab.

Spirent’s GSS6400 Record & Playback System now enables you to bring the complexity of the GNSS world back into your lab.

The GSS6400 complements Spirent's established range of navigation and positioning simulation systems. While simulators generate the specific controlled signals which are needed to develop and test the performance of navigation and positioning systems, the GSS6400 captures the full "richness" of the real world environment which can then be replayed in the lab. The captured data includes real world fades, multipath and in-band interference as well as direct signals from GNSS satellites.

The GSS6400 Record & Playback System is:

Simple to operate
Provides repeatable test environments
Represents real world conditions
Makes collaboration easy with scenarios portable between units

Find out more about Record & Playback for GNSS testing and

View the recent RPS webinar
Download the Free Record & Playback eBook
View the GSS6400 video

Spirent STR4760 Currently Testing Navigator Space Qualified Receiver Used on Hubble Space Mission

Mon, 06 Jun 2011 12:12:00 GMT

We very rarely talk about specific products in this Blog, we prefer to discuss applications and trends within the GNSS community but for this particular entry we thought we’d make an exception.

The Navigator is a space-qualified GPS receiver, being developed by a team led by Dr. Carl Adams at the NASA Goddard Space Flight Center (GSFC), designed to highly elliptical and geosynchronous orbits. It has flown as part of a Remote Navigation Sensor experiment on the Hubble Space Telescope Servicing Mission-4 (HST SM-4), and is being used for benchmark performance testing for the geosynchronous GOES-R mission.

Navigator is undergoing testing for the MMS mission via a Spirent STR4760 GPS simulator available in the Flying Formation Test Bed at GSFC.

The STR4760 was launched by Spirent in 1997 as the first GPS simulator to offer more than 24 satellite channels (32) in a single chassis. Since then the STR4760 has been superseded, firstly by the GSS7700 and currently by the GSS8000 Multi-GNSS Constellation Simulator which offers test capability for GPS, GLONASS, Galileo and QZSS systems and applications.

More information on the background of Navigator can be found here.

What is Binary Offset Carrier Modulation?

Tue, 31 May 2011 11:30:00 GMT

Binary offset carrier modulation (or BOCM) is a split-spectrum modulation scheme used by the Galileo navigation satellite system. BOCM is a square subcarrier modulation. A signal is multiplied by a rectangular subcarrier of a frequency that is equal to or higher than the CDMA rate. Following this subcarrier multiplication, the spectrum of the signal is divided into two parts.

The purpose for using BOC modulation in Galileo is to reduce interference with BPSK-modulated signals such as C/A GPS codes. These signals have sinc function shaped spectra, with most of their spectral energy concentrated around the carrier frequency. In comparison, the BOC-modulated signals have low energy around the carrier frequency and two main spectral lobes further away from the carrier.

As a result, the Galileo signals can easily coexist with GPS signals without danger of interference, despite sharing the same 20MHz band centred at 1575.42MHz. This greatly simplifies the circuitry of any combined GPS/Galileo receiver design.

With the first Galileo-capable GNSS receiver chipsets just starting to appear, manufacturers can immediately get to work perfecting their multi-GNSS receiver designs in advance of the launch of the Galileo system in 2014 using Spirent simulators. Spirent is the official supplier of RF constellation simulators for the Galileo programme.

What is Wi-Fi Positioning?

Fri, 27 May 2011 13:00:00 GMT

Although satellite-based navigation systems are the preferred means for establishing location in “open” terrain, they do suffer shortcomings in areas where the satellites are obscured from the receiver, particularly by man-made structures such as buildings. In short, the performance of GNSS receivers cannot be guaranteed indoors or in densely populated “urban canyon” environments. Wi-Fi positioning is a technique that has been developed to overcome these limitations and augment GNSS-based positioning systems in just such areas.

The system, dubbed WPS, was developed in 2005 by Boston-based Skyhook Wireless, exploiting the growing popularity of IEEE 802.11 wireless routers (or Wi-Fi “hotspots”). It uses a database of known fixed Wi-Fi hotspots, and triangulates the position of the receiver based on the relative position and signal strength from each hotspot that can be received. And in suitably populated environments, position fixes accurate to 10-20 metres are possible within a couple of seconds.

WPS is seldom used alone. But smartphones and other mobile devices that combine cellular, Wi-Fi and GNSS technologies can use all three to provide hybrid positioning systems that improve on the performance of each individual technology.

However, such hybrid systems are potentially prey to interference issues between each of the technologies, and the consequences of this can be catastrophic for location-based services that rely on precise positioning information. Exhaustive testing of any hybrid location technology is essential to ensure reliable and accurate operation in the field.

If you are a product designer, system integrator or other engineering professional who works with GNSS and other positioning, communications, and sensor technologies, register for the “Hybrid Positioning and Technology” webinar hosted by Inside GNSS which takes place on 9th June.

A panel of experts from Spirent, INS, Skyhook Wireless and the University of Calgary will discuss the issues surrounding the technologies and techniques that can be incorporated into equipment to meet mobile users’ need for accurate, robust, and continuous positioning through a variety of operating environments: indoors, in urban canyons and steep outdoor terrain, under tree canopies, etc. — places in which GNSS signals are not always (or never) available.

Register here

What is GAGAN?

Mon, 23 May 2011 11:30:00 GMT

The GPS Aided Geo Augmented Navigation or GPS and Geo Augmented Navigation system (GAGAN) is a space-based augmentation system (SBAS) funded by the Indian government. Interestingly, Gagan is not just an acronym: it is also a Sanskrit word for the sky.

The GAGAN space transponder is due to be launched on 19th May 2011 on an Ariane-5 launcher from the European Space Agency (ESA) spaceport in Kourou, French Guiana. This will be the second attempt to get GAGAN into orbit. The first satellite was lost in the Bay of Bengal on 15th April 2010 along with its Indian GSLV-D3 launch vehicle.

In addition to the space-based transponder, the initial system will comprise eight reference stations located in Delhi, Guwahati, Kolkata, Ahmedabad, Thiruvananthapuram, Bangalore, Jammu and Port Blair, with the master control centre located at Bangalore.

The Indian government will use GAGAN as the basis of an autonomous regional navigation system to be known as the Indian Regional Navigational Satellite System (IRNSS). The system will improve navigational accuracy to 3m over Indian airspace, and will increase safety by allowing pilots to use a three-dimensional approach operation with course guidance to the runway. This is particularly important in areas such as India, which combines a high proportion of mountainous terrain with unpredictable and volatile weather patterns.

Before GAGAN is operational, designers of avionics GNSS receivers will need to test the performance of their products by simulating Indian airspace. If you’d like more information on this testing, contact your local Spirent representative.

Record and Playback Systems in Multi-GNSS Receiver Testing

Fri, 20 May 2011 11:30:00 GMT

Record and playback systems (RPSs) such as the new Spirent GSS6400 can be used at many stages of the GNSS receiver design cycle and for many different tasks. However, one application that may not seem immediately obvious is in testing the ability of multi-GNSS receivers to work with both existing and future satellite systems.

Clearly, the RPS cannot be used to playback signals from non-existent satellites. But used in conjunction with a simulator, the RPS can play a key role in investigations into interoperability.

Under this test setup, the user would capture real-world GPS signals out in the field using the GSS6400 RPS and then return to the laboratory where the signals can be replayed with total fidelity and repeatability. The addition of a multi-GNSS simulator would then allow simultaneous creation of signals from as-yet-unlaunched satellites, say from the Galileo constellation. Then, so long as the replayed and simulated signals are correctly synchronised, then the operator will be able to assess the ability of the receiver under test to work with both the current and future GNSSs.

Alternatively, the same test setup could also be of use to developers of future GNSS constellations in determining exactly which signals will work well with the existing family of GNSSs.

Finally, the GSS6400 RPS can also be used to capture and replay the output from any of Spirent's family of GNSS simulators, offering the advantage that the simulator can be freed up for future projects while the RPS performs those tests that require multiple iterations.

If you would like more information on Spirent’s Record & Playback System, download the eBook or if you’re involved in Multi-GNSS download the Application Note “Multi-GNSS for Technology Developers”

LightSquared GPS Testing: Spirent’s Perspective

Wed, 18 May 2011 14:00:00 GMT

Introduction

The FCC, LightSquared and GPS industry have all agreed that the many questions around the potential for LightSquared’s proposed L-Band terrestrial/satellite network to cause interference with GPS can only be answered with accurate and thorough testing.

As mandated by the FCC, LightSquared and the United States Global Positioning System Industry Council (USGIC) have established a GPS Technical Working Group (TWG) to investigate the issues surrounding potential interference.

Spirent has been active in providing test equipment to help with the investigation and in contributing test plan automation to ensure that results are available in time to meet the TWG deadlines. Test equipment provided includes GPS simulators and, for testing in the “cellular” category, Spirent is supplying its complete, automated GPS/A-GPS test solutions. Spirent is a supplier of the test equipment and automation; the actual test execution, results analysis and reporting is carried out by the organisations appointed by the TWG.

If the test data shows that GPS and A-GPS performance metrics are indeed degraded by terrestrial L-Band signals in any particular part of the overall cellular receiver test plan, Spirent solutions will be able to precisely determine the severity and likely impact, helping to provide the industry and the FCC with the data needed to obtain a clear picture of how GPS and the L-Band terrestrial signals from LightSquared’s network will coexist in the real world.

Spirent’s position is neutral on the question of whether interference, or the risk of interference, will indeed be an issue. Spirent’s role has been to provide test systems and test automation expertise to enable quantification and analysis of the likely impact in a wide range of scenarios.

For more information on the testing required, please refer to the following web site: http://www.pnt.gov/interference/lightsquared/. For advice on systems capable of testing interference effects on GNSS please contact globalsales@spirent.com.

What is CNSS?

Wed, 18 May 2011 12:30:00 GMT

CNSS is the Compass Navigation Satellite System, which will eventually comprise up to 30 medium-earth-orbit satellites and five geosynchronous satellites to provide true global coverage. This Chinese system is distinct from that country's existing Beidou I satellite system, which has been operating since 2003 but provides only domestic coverage using three geosynchronous satellites.

Like other systems, the CNSS will provide two levels of service. The free service for civilian users will offer positioning accuracy of within 10m, velocity accuracy of within 0.2m/s and timing accuracy of within 50ns. There will also be a licensed service with higher accuracy for authorised and military users only. The CNSS will initially cover China and its neighbouring countries only, but will eventually extend into a global navigation satellite system.

The CNSS ranging signals are based on the CDMA principle, like GPS and Galileo. And the frequencies for Compass are allocated in four bands - E1, E2, E5B and E6 - that overlap with Galileo. This overlapping could be convenient for multi-GNSS receiver design, but it also creates the potential for interference, especially within E1 and E2 bands allocated for Galileo’s publicly regulated service.

Although no signals are yet available for the CNSS, Spirent's constellation simulators are “Compass-ready”, and will be capable of simulating both Compass and Galileo signals to allow designers to address these issues with receiver design.

Record and playback systems in multi-GNSS receiver testing

Mon, 16 May 2011 15:12:00 GMT

Clearly, the RPS cannot be used to playback signals from non-existent satellites. But used in conjunction with a simulator, the RPS can play a key role in investigations into interoperability.

Alternatively, the same test setup could also be of use to developers of future GNSS constellations in determining exactly which signals will work well with the existing family of GNSSs.

If you would like more information on Spirent's Record & Playback System, download the eBook or if you're involved in Multi-GNSS download the Application Note "Multi-GNSS for Technology Developers"

GPS Modernization: What is M-Code?

Mon, 02 May 2011 12:00:00 GMT

An important part of the current GPS modernization program, M-code is the name given to a new signal that is designed to improve both the security and anti-jamming properties of military navigation using GPS. Importantly, the M-code is designed to be autonomous, and so users will be able to calculate their positions using only the M-code signal (unlike the existing military P(Y) code, which also requires use of the C/A code).

Radically, in addition to the normal wide-angle broadcast, the M-code will also be transmitted from a high-gain directional antenna, in a so-called “spot beam” (albeit focused on an area several hundred kilometers in diameter). This will increase the local signal strength by 20dB. And, as a side effect of the two antennas, inside the spot beam the satellite will appear to be two separate GPS satellites occupying the same position.

The M-code signal will carry a new MNAV navigational message, which is packetised instead of framed, allowing for greater flexibility of data content. There will be four effective data channels, and different data can be sent on each frequency and on each antenna. It can also include FEC and error detection.

The full twin-antenna realisation of the M-code signals will not be available until the launch of the GPS Block III satellites, which is tentatively scheduled for 2013. In the meantime, developers of military navigation equipment can prepare their new designs using Spirent's constellation simulators, which can support all the new signals scheduled for use under the GPS modernization program.

What is a GPS almanac?

Mon, 02 May 2011 12:00:00 GMT

In the wider world, an almanac is an annual publication dedicated to information such as weather forecasts, tide tables, lunar cycles etc. A typical almanac will contain tabular information covering a particular field or fields, and will be arranged according to the calendar.

However, in the world of satellite navigation systems, the almanac is a regularly updated digital schedule of satellite orbital parameters for use by GNSS receivers.

The almanac for any given GNSS consists of coarse orbit and status information covering every satellite in the constellation, the relevant ionospheric model and time-related information. For example, the GPS almanac provides the necessary correction factor to relate GPS time to co-ordinated universal time (UTC).

The major role of the almanac is to help a GNSS receiver to acquire satellite signals from a cold or warm start by providing data on which satellites will be visible at any given time, together with their approximate positions. An ephemeris message is still required from each satellite for the receiver to compute the exact position, but it is the almanac for the constellation that gives the receiver its starting point.

The ionospheric model contained within the almanac is essential for single-frequency receivers to correct for ionospheric errors - the largest error source for GPS receivers. However, modern dual-frequency receivers have no need for this data as the dual-frequency design can correct for such errors without any assumed model.

What is Dilution of Precision in GNSS Receivers?

Fri, 29 Apr 2011 12:00:00 GMT

Although a GNSS receiver requires only four satellite signals to provide a complete positional fix in three dimensions, the accuracy of this fix depends to some degree on the exact positions of the four satellites relative to the receiver. If the four signals acquired come from satellites spread throughout the sky relative to the receiver, then the fix should be highly accurate. But if all four are observed in close proximity to each other within a single quadrant, then the fix will be less accurate. This phenomenon is known as dilution of precision (or DOP).

Indeed, if two or more of the satellites are aligned so as to appear to occupy the same space, then it may be impossible to obtain any fix. Alternatively, the fix obtained may be out to the tune of 150 or even 200 metres.

There are five different types of dilution of precision: GDOP, PDOP, HDOP, VDOP and TDOP refer to geometric, positional, horizontal, vertical and time dilution of precision, respectively. And the different classes of DOP have differing effects on the accuracy of a receiver. For example, a poor VDOP rating implies that the visible satellites are low in the sky. Hence urban canyon performance will be poor.

Spirent's SimGEN™ software allows designers to test and optimise the DOP performance of their receivers by allowing the user to excluding certain satellites from the simulated visible constellation.

Using Inertial Sensors to Improve GPS Performance

Wed, 27 Apr 2011 11:30:00 GMT

GPS and inertial sensors complement each other extremely well.

GPS works best with a good view of the sky. When the sky view gets obstructed, for example in an aircraft roll or when a smartphone gets taken indoors, GPS will not continue to track and the navigation capability is lost. Inertial sensors continue working in all conditions – move them and you get an output from the sensor. The biggest weakness of inertial sensors is that errors are cumulative.

By combining GPS and inertial, the GPS can provide the reference positions and inertial can provide continuity at all times. The basic principles are similar whether in a military jet or a smartphone.

Military grade systems generally use dual-frequency GPS for better accuracy and resilience. The inertial systems in military systems are high grade, often with inertial drift of only a few degrees per hour. This enables the inertial sensors to be used for navigation with GPS providing re-calibration periodically.

MEMS sensors and GPS are increasingly common now in smartphones and consumer grade navigation systems such as PNDs. However, the sensors used are very low cost and accuracy can be poor even over short distances or time periods.

What is RAIM?

Mon, 25 Apr 2011 11:30:00 GMT

RAIM stands for receiver autonomous integrity monitoring, a technology used in GNSS receivers to assess the integrity of the GNSS signals that are being received at any given time. It is particularly applicable to receivers intended for safety-critical applications, and in particular in aviation applications.

The RAIM concept makes use of redundant satellite signals – i.e., any that are available above and beyond those needed to produce a position fix. If the pseudorange data in any of the signals is at odds with the position computed from the other signals it may indicate a fault in that satellite, such as a clock error. Alternatively, the error may be due to unexpected atmospheric conditions.

In general, to obtain a 3D positional fix at least four satellite signals are required. To detect a fault, at least five signals are required, and to isolate and exclude a fault, at least six satellite signals are required. However more signals are often needed, depending on the geometry of the satellite constellation, and so RAIM is not always available.

In aviation applications, pilots can check whether RAIM will be operational on any given route and/or approach by checking one or other of the RAIM prediction websites operated by the Federal Aviation Authority and Eurocontrol. These sites allow pilots to predict RAIM status during pre-flight checks.

Applicable GNSS receivers can be tested for RAIM performance using Spirent's SimGEN™ software, which incorporates a pseusorange ramp feature that changes the simulated position of a satellite in a controlled (but abnormal) manner. This deviation is not declared in the navigation message, but any receiver with a RAIM algorithm should be able to detect the anomaly.

What is WGS-84?

Fri, 22 Apr 2011 12:30:00 GMT

The World Geodetic System provides a standard co-ordinate frame for the Earth, a standard spheroidal reference surface (or ellipsoid) for raw altitude data, and a gravitational equipotential surface (or geoid) that defines the nominal sea level. WGS-84 is the most recent version of the system, which was originated in 1984 and revised in 2004. Earlier schemes included WGS-72, WGS-66 and WGS-60. WGS-84 is the reference coordinate system used by GPS.

The origin of the WGS-84 co-ordinate system is centred close to the Earth's centre of mass. Indeed, the error is reckoned to be less than 20mm. The meridian of zero longitude is the IERS Reference Meridian, which lies 5.31 arc seconds east of the Greenwich Prime Meridian.

The WGS-84 datum surface is an oblate spheroid, with a major radius of 6,378,137m at the equator, and minor radius of 6,356,752m at the poles.

WGS-84 currently uses the 1996 Earth Gravitational Model (EGM96) geoid, which was revised in 2004. This defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100km horizontal resolution).

What is a Terrain Obscuration Model?

Wed, 20 Apr 2011 12:00:00 GMT

Particularly relevant for ground-vehicle-based GNSS receivers, terrain obscuration is the phenomenon of temporary and intermittent masking of GNSS signals when manoeuvring at low altitude in mountainous terrain. This can be a critical limiting factor on the performance of GNSS receivers in certain applications, and so the ability to simulate such effects is an important tool in improving the reliability and performance of GNSS receiver designs.

Spirent SimGEN™ software contains a comprehensive terrain obscuration model for terrestrial vehicles that allows GNSS receiver manufacturers to assess the performance of their designs under such conditions.

Within the SimGEN software, the user has full control over the proximity of the terrain, its maximum and minimum height above the WGS-84 ellipsoid, and its maximum and minimum width. When running the model, the terrain height and width are varied pseudorandomly, and the pattern of interruption is accurately repeated on consecutive simulation runs with the timing of changes proportional to the vehicle speed. In addition, the terrain type can be modified during the simulation, with the horizontal distance travelled determining the point at which the terrain changes.

Time Discrepancies in Multi-GNSS Receivers

Mon, 18 Apr 2011 12:00:00 GMT

One of the major issues in testing GNSS receivers designed for use with multiple satellite systems is that the different systems do not necessarily share the exact same time-bases. And while the differences may be tiny, time is such a critical quantity in satellite navigation that even microsecond differences can create large accuracy errors.

The problem is a legacy of the GPS system, which has used its own time-base (GPS time) since it began in the early 1980s, rather than the globally accepted co-ordinated universal time (UTC) used elsewhere. GPS time is not corrected to match the rotation of the Earth, and so it does not contain leap seconds or other corrections that are periodically added to UTC. However, periodic corrections are performed to the clocks onboard each GPS satellite to correct relativistic effects and keep them synchronized with ground clocks. And the almanac that is broadcast with each GPS navigation message includes the difference between GPS time and UTC.

In comparison, both GLONASS and Galileo are locked to UTC (or a derivative thereof), and so do not require such corrections.

Fortunately, Spirent multi-GNSS simulators give the user full control of GPS time, allowing multi-GNSS tests to use a single time-base. They also account for timeshifts between the various UTC derivatives, using a second-order Markov process and adding the shift to a state vector to be estimated in the multi-GNSS receiver's PVT.

GPS Modernization and the L2C Signal

Wed, 13 Apr 2011 11:10:00 GMT

L2C is the name given to one of the new signals to be broadcast from the satellites in the modernised GPS constellation. This new signal is intended for civilian use (hence the “C”), and will be broadcast on the L2 frequency at 1227.6MHz by all satellites from block IIR-M onwards.

The L2C signal is one of the key means by which the modernized GPS will offer improved accuracy and availability for civilian applications based on dual-frequency receivers. Not only is the signal intended to be easy to track, but it will also offer redundancy for the L1 signal in areas prone to interference.

In addition, having the L1 and L2C signals broadcast simultaneously on different frequencies will provide a relatively simple means for dual-frequency receivers to measure and mitigate the ionospheric delay error for each satellite.

The structure of the L2C will be different from the L1 signal (which will continue to be broadcast unchanged to assure backward compatibility with existing receivers), and will comprise two separate code chains, dubbed CM and CL, which are multiplexed to form a 1,023,000bit/s signal. The L2C signal characteristics will provide 2.7dB greater data recovery and 0.7dB greater carrier tracking than the existing L1 C/A.

The full nature of the L2C signal is defined in the IS-GPS-200 standards document, and the signals are not likely to be broadcast until at least 2013. However, designers working on dual-frequency receiver designs for use with the modernised GPS can test their ideas and prototypes today using Spirent's family of Multi-GNSS constellation simulators.

Japanese Earthquake Highlights Importance of GNSS Timing

Mon, 11 Apr 2011 12:00:00 GMT

One of the more surprising consequences from the March 2011 earthquake in Japan was that the forces unleashed shifted the earth's mass sufficiently to accelerate its rotation, shortening each day by no less than 1.8 microseconds, according to calculations by NASA. And while such effects are clearly imperceptible to humans, their cumulative effect on GNSS timekeeping would cause significant inaccuracies that would be unacceptable to any GNSS application.

Each GNSS will deal with the time variation in its own way, generally by transmitting a correction factor to each satellite in the constellation, which will then be embedded in the navigation message received by each GNSS receiver. This therefore emphasises the importance of ensuring that any GNSS receiver design has the ability to cope with time corrections sent from its constellation.

Fortunately, Spirent's SimGEN™ software allows users to modify any part of the navigation message simulated by the company's GNSS simulators. This includes the ability to simulate timebase changes that might be required to compensate for unusual phenomena such as the effects of the Japanese earthquake. This type of test is more usually applied to ascertain the receiver's ability to recognise the insertion of leap seconds.

GPS Modernization With Improved Civilian Signals

Wed, 30 Mar 2011 11:30:00 GMT

In addition to the new signals to be broadcast under the GPS modernization project, there are to be two significant changes to the existing civilian signals, both designed to improve the performance of GPS receivers. The first is an additional data-free pilot signal and the second is the addition of forward error correction (FEC) encoding to the navigation message.

The new data-free signal will be broadcast alongside the normal data signal, acting as an easy-to-acquire pilot signal. Once acquired by a GPS receiver, the pilot signal will be used to acquire the data signal. This new scheme will improve the acquisition of the GPS signal and boosts power levels at the correlator.

The second improvement, the addition of forward error correction encoding to the navigation message, is designed to make the overall GPS signal more robust. This is because the navigation message is broadcast at such a low data-rate (around 50bit/s) that even a small interruption to the signal can have a significant effect on the reception of the message.

The forward error correction works by adding systematically generated redundant data (or error-correcting code) to the message. The carefully designed redundancy allows the receiver to detect and correct a limited number of errors occurring anywhere in the message without the need to receive additional data.

Designers of GNSS equipment can prepare for GPS modernization by simulating all the new signals today using Spirent's range of GNSS simulators. Not only do these instruments already cover all the new and enhanced signals that will be provided by the modernized GPS constellation, they also cover all signals for the GLONASS and Galileo GNSSs as well as the regional and local augmentation systems.

When Will Galileo be Ready?

Mon, 28 Mar 2011 12:00:00 GMT

The official opening on 20th December 2010 of the Fucino Galileo Control Centre, 130km east of Rome, has brought the Galileo global navigation satellite system one step closer to fruition. However, a continuing shortage of funding for the project suggests that while Galileo will be available by 2014, the service will initially be limited.

Earlier in 2010, the European Commission confirmed that funding was available to launch four in-orbit validation (IOV) satellites by 2014, with the first two of these scheduled for launch in August 2011. Funds are also available for 14 full operational capability (FOC) satellites, with deployment planned to start in late 2012. However, this total of 18 satellites to be in orbit by 2014 is only 60% of the originally planned 30-satellite constellation.

While the original plan of 30 satellites organised in three orbital planes of ten satellites each would have guaranteed that any Galileo receiver anywhere on earth would have access to at least four satellites at any given time, the curtailed 18-satellite constellation will not be able to provide such guaranteed coverage. However, working in conjunction with GPS and other GNSSs, Galileo will still be able to play a major role in improving global navigation services using multi-GNSS receivers.

Even without the necessary satellites in place, GNSS receiver designers can now prepare their Galileo-capable receivers using Spirent family of multi-GNSS simulators, which already have the capability of simulating Galileo navigation signals, both alone and in combination with GPS and GLONASS signals.

Motion Simulation with GNSS Scenario Software

Thu, 24 Mar 2011 12:00:00 GMT

For most GNSS receivers, static navigation accuracy is only part of the story. The ubiquitous automotive satnav system is the most common case where dynamic accuracy is essential. And users will only be satisfied with the performance of their receivers if turn-by-turn navigation instructions are both accurate and timely.

So while you may be familiar with the concept of testing the performance of a GNSS receiver in the lab with a GNSS simulator, how can this be extended to take into account the dynamic navigational accuracy required of an automotive (or even marine) system in which the relative positions of the receiver and the satellites are continually changing due to the movement of the vehicle carrying the receiver? The answer comes in the form of GNSS scenario software, such as the SimREPLAY software package for the SpirentGSS6700 Multi-GNSS Constellation Simulator.

The GSS6700 generates similar RF signals to those that would be seen by a GNSS receiver when installed on a vehicle with time, place and motion pre-defined in a test “scenario”.

This enables the performance of the receiver to be assessed in the laboratory as if it were receiving RF signals from real satellites whilst stationary or performing complex user defined maneuvers.

“Truth” data from the simulation is available to facilitate results analysis which makes the GSS6700 with SimREPLAY ideal for quantifying and comparing the performance of GNSS receivers in such areas as:

Design verification
Production test in manufacturing
Comparative evaluation
Statistical data-generation through extended and repeated tests
Incoming product test

Unlike testing with live-sky signals, the users of a GSS6700 with SimREPLAY can replicate the test environment weeks, months or years apart to compare a number of receivers under identical test conditions

So by adding motion simulation to the simulation of the GNSS system, receiver manufacturers can be confident that their products will provide the dynamic navigation accuracy that today's users’ demand.

Sampling Quantization for GPS or GNSS RPS Systems

Mon, 07 Mar 2011 16:00:00 GMT

When talking about Record and Playback Systems (RPS), more bits mean more resolution which has got to be better right? Indeed if familiar with the classic graphic used to describe the operation of an ADC you may be asking yourself how GNSS signals over a 20 or 30dB range can possibly be captured by only one or two bits at all. After all the well-known rule of thumb is 6dB per bit and so 30dB dynamic range will need at least 5 bits, right?

Well wrong actually and the reason is that the classical graphic represents signals clear of noise but with GNSS the signal is right down deep buried in the noise. For all practical purposes the signal is not there until the receiver’s correlators dig it out. It’s too small to see and falls well below the quantisation steps in the DAC.

What is important is that the noise is captured (and replayed) as faithfully as possible. The tiny phase shifts around the transition and distribution of levels will be available to allow the correlators to do their thing. But nothing comes for free and the more that the “noise” is changed during the process the harder it is for the GNSS signal to be subsequently recovered. Think of it as reducing the processing gain from a starting point of 43dB. This will be seen as a lower C/N0. So the apparent effect of the reduced resolution in the ADC – storage – DAC process is a drop in GNSS signal level and the amount of the drop relates directly to the resolution. 1-bit systems “add” around 3dB noise, 2-bit systems add about 1.3dB noise whilst a 4-bit system adds around 0.6dB noise. So going from 1 to 2 bit provides a performance improvement of around 1.7dB whilst going from 2 to 4 bits only lifts performance another 0.7dB. In other words diminishing returns in performance in exchange for vastly more storage requirements.

At bit depths much beyond 2-bits the system performance is dominated by the device under test RF noise figure and so further increasing the bit depth has little impact on performance.

For more information on Spirent’s new Record & Playback system download our eBook.

EGNOS: Fit to FLy

Mon, 07 Mar 2011 15:00:00 GMT

The recent certification of the European Geostationary Navigation Overlay Service (EGNOS) for safety of life applications now means that its performance is of a high enough standard that it can be used for aviation. Pilots can reliably use it as a navigation tool, enhancing the performance of stand-alone GPS.

This means navigations systems for aircraft can now be produced knowing that the systems they will use are certified, robust and reliable. Designers and manufacturers of these systems will therefore need certifiable, robust and reliable test tools and methodologies in order to help ensure their products are fit for purpose. GNSS RF simulators are the ideal tool for such work. As precision test equipment, simulators can be readily certified for use in testing safety-critical systems. Spirent’s GNSS RF simulators are able to generate EGNOS signals (and signals for the other augmentation systems such as WAAS, MSAS, LAAS and QZSS) and have been verified by the US, European and Russian authorities as the standard in GPS, Galileo and GLONASS test, so you can trust Spirent with confidence for your test needs. No other method or tool for GNSS test can be appropriately certified for critical, safety of life applications.

Why not call Spirent today to find out how your systems can take to the skies with the sure and certain confidence that in all operating conditions they will help protect human life in the air and on the ground.

Testing GPS in the Rail Network: What’s the deal with using GPS in trains?

Thu, 24 Feb 2011 18:10:00 GMT

There’s a simple, compelling, theory around using GPS in trains: by using GPS for position information, coupled with train communication systems, the need for track-side signalling goes away. The massive infrastructure cost associated with signalling goes away as everything needed is on-board each train.

The reality surrounding this vision, however, is that rail infrastructure has to be heavily regulated and standards-driven due to the safety critical nature of the industry.

The vision is compelling enough, however, that it is receiving significant attention. Spirent is experienced in testing safety critical systems and has been involved at various levels in helping those involved. Examples of questions we have helped with include:

How to accurately model the railway environment with bridges, cuttings, tunnels and even high voltage electric cables.
How to develop appropriate test cases to assess the performance of candidate systems in a range of conditions, including extreme and error states
Input to the standardisation work on train positioning
How to deal with known error possibilities such as wheel slips or interference with the navigation systems.

If you’re interested in rail positioning and GPS test solutions for rail applications please comment to this blog or get in touch with Spirent.

Threat of import duties to increase appeal of GLONASS

Tue, 22 Feb 2011 17:00:00 GMT

The Russian Federation has come up with further “encouragement” for manufacturers to support the GLONASS satellite navigation system with the threat of import duties to be levied on any mobile handsets without GLONASS functionality. Deputy Prime Minister Sergei Ivanov has revealed that the duty will be “about 25%”, will be introduced by 2012 and will be levied on handsets imported to the Russian Federation with GPS-only positioning technology.

Three more GLONASS -M satellites are scheduled for launch by the end of 2010, allowing Russia to operate a complete GLONASS network with global coverage and have three or four satellites in reserve. A further eight satellites will be launched between 2011 and 2013 to enable further redundancy and to replace some of the earlier satellites that are reaching their end of life. All this will be added to the investment of over US$2 billion over the past 10 years, which explains why Russia is so keen to see GLONASS adopted both within its own borders and in the world at large.

Manufacturers of mobile handsets and other location-based technologies can work on their multi-GNSS designs today featuring GPS and GLONASS capabilities using Spirent's range of constellation simulators. This also extends to coverage of the Galileo and Compass systems, long in advance of their physical launches.

How do you test an IVNS?

Fri, 18 Feb 2011 16:59:00 GMT

The integrated in-vehicle navigation system (IVNS) is becoming an increasingly popular feature in many automotive applications, ranging from relatively straightforward driver information systems right through to sophisticated tracking and fleet management systems. Such systems, which combine a GNSS receiver with one or more dead reckoning (DR) sensors, provide superior positional accuracy over GNSS alone, and are particularly useful in areas prone to signal obscuration or complex multipath effects such as urban canyons or deep-cut freeways with overpasses and tunnels.

Clearly, any navigation system requires rigorous testing, right through the product lifecycle from design to manufacture. But how can a system that combines both GNSS and DR sensors be tested reliably? And the question of integrated testing for an IVNS takes on even greater relevance when you consider that the whole purpose of adding the DR sensors to the system is based on the premise that the GNSS signals will be subject to interference and even failure.

Total control over the test environment is paramount. And that clearly rules out any form of “live” or “real-world” testing: the real world is simply not controllable. So simulating the required GNSS signals would appear to solve half of the problem. But how do you combine these signals with the outputs from the DR sensors? The answer is simpler than you might think.

Spirent's SimAUTO IVNS software is designed with just such applications in mind. The software can be used to emulate a variety of DR sensors, such as compasses, gyros and wheel pulses, and will co-ordinate these sensor signals with the company's range of GNSS constellations simulators. This creates a complete test environment for integrated navigation systems.

When will smartphones be able to navigate to a few metres of accuracy indoors?

Wed, 16 Feb 2011 16:33:00 GMT

Over the past few years the ability of consumer grade devices, typically smart phones but also PNDs, to navigate indoors has improved significantly. This is because GPS has been supplemented by other navigation technologies, such as cell tower identification and, more recently, navigation using Wi-Fi to localise position.

To get reliable sub-10metre accuracy indoors, however, will require more than GPS, cellular and Wi-Fi positioning. Use of other sensors, particularly MEMS inertial sensors, as a compliment to the existing positioning information is likely to provide the answer.

MEMS sensors are becoming very common on consumer devices. For example they are used to detect movement, orient screens and stabilise images. The most common sensors are accelerometers, gyroscopes and magnetometers. In vehicles, odometry (wheel pulse) is also commonly used. Other inputs to a navigation solution include barometric pressure measurement (to help determine height, such as which floor of a building) and even cameras and light sensors.

The answer to accurate indoor position, accurate enough to locate a car in a parking garage or a shop in a mall, would seem to lie in integrating as many of these sensor inputs as possible to provide a blended position.

How far away is this? I would suggest 3-5 years. If you have a different view, let us know.

JAXA Chooses Spirent’s GSS8000 to benchmark their QZSS receiver performance

Fri, 11 Feb 2011 13:00:00 GMT

In a recent post I explained the concept behind QZSS (What is QZSS) – Since the publication of that post and to further help in the development of the Quazi-Zenith Satellite System (QZSS) programme, the Japanese Aerospace Exploration Agency (JAXA) has selected Spirent’s GSS8000 Multi-GNSS Constellation Simulator to verify QZSS receivers. The highly elliptical orbits of QZSS allow satellites to dwell at high elevations, improving coverage in urban canyons and providing additional overhead ranging sources in Japan. JAXA needed to design receivers that supported multiple satellite technologies. After several discussions with Spirent we provided JAXA with a solution that not only included testing capabilities for GPS at L1, L2 and L5 signals but also tested performance of QZSS signals at the same frequencies.

If you are working on QZSS and would like more information on Spirent’s QZSS test solution contact globalsales@spirent.com or your nearest Spirent representative.

How GNSS Navigation can Improve Safety Critical Navigation on the Railway

Wed, 09 Feb 2011 13:00:00 GMT

The use of GNSS in navigation, timing and related systems for transportation is ever increasing. The railways are no exception. There are various programmes underway looking at the application of GNSS to the railway for safety-critical functions. Significant among these is train location and control. Traditional line-side signalling is expensive through installation, operation and maintenance. It is inflexible; signalling headways and hence capacity are determined by physical location of signals, track circuits and associated infrastructure, and it is vulnerable; signalling cables and apparatus are frequently stolen by metal thieves – its scrap value being very high.

Much of this fixed infrastructure could be rendered obsolete by using GNSS navigation systems fitted to the railway vehicles themselves. One of the main challenges is the performance of GNSS and whether or not it is robust enough for such a highly safety-critical application. The development and evaluation of such systems is therefore also critical, and demands robust, quantifiable and certifiable test methods. GNSS RF simulation and associated RF testing is widely accepted as such a method. RF simulation hardware and software scenarios can be certified for use in international standards, because they offer repeatability, precise quantification, accuracy and integrity. No other test method can provide that level of traceability.

Standards for certification and type approval are coming, and RF simulators will be used extensively in them. Spirent’s range of GNSS test solutions are already certified by the actual GNSS design authorities for Galileo, GLONASS and GPS. They are the first choice in test tools for many safety-critical programmes of work, both public and private sector - all around the world. The people responsible for these programmes have trusted Spirent for almost 25 years, and continue to do so. Who do you trust to test your safety-critical GNSS system, let us know?

What is QZSS?

Wed, 02 Feb 2011 13:00:00 GMT

The Quasi-Zenith Satellite System (QZSS) is a GPS augmentation system that aims to greatly improve GNSS accuracy over Japan and the rest of East Asia. The first satellite in the system, dubbed Michibiki, was successfully launched from the Tanegashima space centre on 11th September and reached its quasi-zenith orbit on 27th September 2010.

QZSS aims to enhance GPS services both by improving the availability of GPS signals and by performance enhancement to increase the accuracy and reliability of GPS navigation. The enhancement signals transmitted from Quasi-Zenith Satellites will be compatible with both existing and modernised GPS signals, minimising the need for changes to specifications and receiver designs.

The combined GPS/QZSS solution will deliver improved positioning performance using ranging correction data transmitted as dual sub-metre-class performance enhancement signals. It will also improve reliability using failure monitoring and system health data notifications. QZSS will also provide other support data that will improve GPS satellite acquisition.

Interestingly, the QZSS satellites do not carry their own high-accuracy atomic clocks. Rather they feature a synchronisation framework combined with lightweight steerable onboard clocks that act as transponders rebroadcasting the precise time from a synchronisation network located on the ground.

What is a Pseudolite?

Mon, 31 Jan 2011 12:30:00 GMT

A pseudolite (or pseudo-satellite) is any device that performs a specific task that would otherwise require a satellite. So, for example, a dedicated transmitter might be deployed to extend the reach of a GNSS to areas where the satellite signals are either blocked or jammed, and there has been considerable success with deployment of pseudolites to extend GPS coverage indoors. These are relatively simple devices that only transmit the coarse acquisition code.

Pseudolites are also used to simulate future GNSS satellites in a real-world environment as well as for robotic guidance in space exploration.

A single-channel GNSS simulator (or signal generator) can be used as a pseudolite, particularly in the research and development arena. However, care needs to be taken when operating powerful transmitters within GNSS bands. Clearly, highly sensitive GNSS receivers designed to capture minute signals from distant satellites will not react well to high-power transmissions on their receiving frequencies.

Why Does a GNSS Simulator Need a GUI?

Thu, 27 Jan 2011 13:00:00 GMT

There is no doubt that testing GNSS receivers is a complex business. Today's GNSS simulators are sophisticated instruments with increasing levels of features and complexity. Yet time-to-market pressure on GNSS receiver designers and manufacturers dictate that these simulators must be easy to understand and intuitive to operate.

The key to this ease of use is in the graphical user interface (or GUI). And properly designed, the GUI will help simulator users access important information quickly and improve the interaction between the user and the device under test. In short, a good GUI is no longer an option – it is a necessity, saving valuable time at all stages of the test process, and thereby reducing time to market.

At the most basic level, the user interface for a GNSS simulator should provide graphical representations of the sky chart, detailed channel information including power levels, true position information, the satellite ground track and a vehicle instrument panel to mirror the GNSS receiver in use.

Advanced simulator features that greatly benefit from graphical representation include visualisation of ionospheric errors and the ability to create simple and complex multipath test scenarios.

Spirent's GNSS simulators come complete with the company's highly acclaimed SimGEN™ test software, which incorporates an easy-to-use graphical user interface that allows modification of a wide range of variables from preset defaults, enabling the user to focus their time on the areas of test important to them.

What is Pseudorange?

Tue, 25 Jan 2011 14:00:00 GMT

The pseudorange is an approximation of the distance between a satellite and a GNSS receiver.

A GNSS receiver will attempt to measure the ranges of (at least) four satellites as well as their positions when their positional data were transmitted. With the satellites' orbital parameters supplied in the almanac within the message, each position can be calculated for any point in time.

The pseudoranges of each satellite are obtained by multiplying the time taken for each signal to reach the receiver by the speed of light. But because there will inevitably be accuracy errors in the time measured, the term pseudorange is used.

The problem is that the quartz oscillators used for timing in most GNSS receivers are only accurate to 1ppm at best. This means that if the clock hasn't been corrected for a week the distance error will be massive. Even when the clock is corrected, a second later the error can be hundreds of metres.

Fortunately, in a GNSS receiver the clock's time is used to measure the ranges to several different satellites simultaneously, and so all the measured ranges have the same error. Ranges with the same error are called pseudoranges. By finding the pseudorange of an additional fourth satellite, the time error can also be estimated.

What is Record & Playback

Wed, 05 Jan 2011 15:00:00 GMT

Thorough evaluation of receiver performance requires that the impact of these various sources of impairment is assessed. An emerging technique for performing this testing is by Recording the RF signal for subsequent Playback in the lab.

Spirent’s NEW GSS6400 Record & Playback System now enables you to bring the complexity of the GNSS world back into your lab.

The GSS6400 record & Playback System is:

Simple to operate
Provides repeatable test environments
Represents real world conditions
Makes collaboration easy with scenarios portable between units

Find out more about Record & Playback for GNSS testing:

View the "on demand" RPS webinar
Download the free Record & Playback eBook
View the GSS6400 video

The Importance of Static Navigation Accuracy in a GNSS Receiver

Tue, 30 Nov 2010 13:00:00 GMT

Read on and get your Free copy of the Spirent eBook explaining how to undertake controlled testing of your GNSS receiver design.

Of all the performance metrics for a GNSS receiver, navigation accuracy is probably the most important. More to the point, pinpoint static navigation accuracy is essential to the end user. And if he or she doesn't get the exact location fix expected when the receiver is powered up for the first time, there will be little confidence in the overall performance of the receiver.

Whether the initial “user test” takes place in the shop before purchase or at home when taking a new unit out of the box, the consumer has a reasonable expectation that the GNSS receiver will be able to work out its start position. So not only will he or she expect the correct map to be loaded, but also for the position fix to be displayed to the nearest metre – at worst. Any mistake at this stage and either the unit will not make it out of the shop or it will be returned for a refund!

Extensive testing of both the navigation circuitry and the user interface are essential in ensuring the navigation accuracy of any GNSS receiver design. And because there are such a large number of external factors that can impact on static navigation accuracy, it is essential that the tests be performed under controlled and repeatable conditions in the laboratory using a GNSS simulator. Only this will enable the designer to be sure that the design will perform properly, both under “ideal” conditions, and under the influence of these external interfering influences.

You can get more information on how to undertake controlled testing of your GNSS receiver design by downloading the FREE eBook from Spirent.

In this free eBook, Spirent explains the 9 key tests that together determine the performance of any GPS/GNSS receivers.

Find out how testing GNSS receivers under real world conditions alone does not guarantee their proper operation. If you're designing or manufacturing "location aware" products, you need to read this.

What is the Local Area Augmentation System?

Wed, 24 Nov 2010 14:00:00 GMT

The Local Area Augmentation System (LAAS) is a ground-based augmentation system (GBAS) specified by the US Federal Aviation Authority for use at airports to augment the accuracy of GNSS-based navigation.

The LAAS focuses its service on the airport area (around 30-50km radius) for precision approach, departure procedures and terminal area operations. It broadcasts its correction message over a VHF radio datalink from a ground-based transmitter.

The FAA plans to replace legacy navigation systems with satellite based navigation technology and has decided that the LAAS is the most cost-effective alternative to legacy Instrument Landing Systems (ILS).

Receiver designs that are intended to work with the LAAS can be tested using Spirent's novel LAAS VHF data broadcast signal simulator, the GSS4150, which works in concert with the company's multichannel constellation simulators to provide complete coverage of the tests required for civil aviation applications.

Are you Involved in GPS/GNSS Production Test?

Thu, 04 Nov 2010 12:00:00 GMT

In many production environments, a single channel GPS signal simulator is the preferred way to ensure that each device meets defined parameters. The Spirent GSS6300 Multi-GNSS Signal Generator has been designed specifically for high volume production test applications for devices that use commercial GPS/SBAS, GLONASS and/or Galileo receivers.

Controlled testing is vital in ensuring correct assembly and verification of expected performance parameters in GPS only and Multi-GNSS manufacturing environments. Spirent’s GSS6300 provides a robust single channel testing solution with the minimum of operator intervention.

The GSS6300 can be configured with one channel of a specific constellation or with multiple constellations, for example:

GPS only
GLONASS only
Galileo only
GPS and GLONASS
GPS and Galileo
GPS, GLONASS and Galileo

Some production environments, however, need the flexibility to switch between single channel and multi-channel test modes. The GSS4200 unit offers 6 channels of GPS L1 C/A code to allow a positional fix with representative satellite dynamics and data as part of the production test. For assisted GPS applications, the assistance data for each scenario is provided as standard.

While the GSS6300 and GSS4200 are well placed to test the majority of GPS receivers, there is also a need to test high-end dual frequency receivers in a production facility. The Spirent multi-channel GSS7735 has been designed specifically for testing dual-frequency L1 C/A code + P code and L2 P code GPS equipment and sensors in such manufacturing environments. Visit our video library to find out how Spirent can help.

GNSS Simulation: The General Principles

Tue, 02 Nov 2010 14:00:00 GMT

The core requirement of any GNSS receiver test, whether for development, integration or production purposes, is for a controlled, repeatable signal. For many tests, the signal control includes flexibility over test case, or scenario, conditions that enable performance testing at nominal and extreme or error-state conditions.

Real-world, live-sky testing has significant drawbacks which, in practice, preclude controlled testing. These drawbacks of live-sky testing include:

An end user or test site cannot have any control over the GNSS signal being transmitted
The signals seen incident to the GPS receiver antenna are constantly changing as the GPS system constantly changes (precesses)
There are occasional signal errors, often unknown to the receiver at the time
Atmospheric conditions change significantly and have a significant impact on single frequency systems
Testing at multiple geographic locations proves to be expensive

Using a GNSS RF simulator enables the user to define and control all simulated parameters. Advantages of using a simulator include the following:

Full control over test scenarios
Repeatable
Errors can be introduced in a controlled fashion and the way the system under test deals with each error can be optimised
Atmospheric conditions can be modelled and even removed from the test
Other signal effects can be controlled, such as multipath and antenna patterns
Vehicle trajectory and associated dynamics can be modelled
Future signals (e.g. Galileo, GLONASS and modernised GPS signals) can be generated to allow testing against new signals before a complete constellation of satellites are transmitting the Signals in Space (SIS)

GPS simulators can be used in various configurations enabling, for example, use of remotely generated trajectories and generation of interference signals as well as simulated GNSS signals.

If you want to know more about why you should choose a simulation solution from Spirent, take a look here.

What is Ionospheric Scintillation?

Tue, 26 Oct 2010 12:00:00 GMT

Ionospheric scintillation is the term given to irregularities in the ionosphere caused by so-called “space weather”. Key sources of ionospheric scintillation include solar winds and magnetic storms.

Historically, the level of scintillation has been seen to follow the 11-year solar cycle and peak at the time of maximum sunspot activity. Therefore the next peak is due in 2012. Scintillation occurs most frequently at tropical latitudes at night. It occurs less frequently at high latitudes or mid-latitudes.

Scintillation has a significant effect of scattering GNSS signals so that a receiver would perceive a satellite as irregularly moving around its actual position. It causes fluctuations in both the amplitude and phase of the carrier. And it is only recently that these effects have been adequately modelled.

High levels of scintillation, such as those experienced during the peak of the solar cycle, have traditionally caused problems for GNSS receivers. However, GNSS receivers can now be tested using newly developed scintillation models available in Spirent's SimGEN software. This means that the latest generations of receivers will be far better equipped to handle the high levels of scintillation due to occur in 2012.

What is a GNSS Test Scenario?

Thu, 21 Oct 2010 12:00:00 GMT

Although the word “scenario” has been much overused (and misused) in common language, it has particular relevance in terms of testing GNSS receivers. The word can be correctly defined as “an outline or model of an expected or supposed sequence of events”, and this provides an insight to the importance of a test scenario in assessing and/or comparing the performance of one or more GNSS receivers.

A GNSS simulator under software control can be used to generate test scenarios of varying degrees of complexity. And, importantly, once the scenario has been defined and stored, it can be recreated precisely time and time again. The scenario will be made up of any combination of factors, including aspects such as multipath effects, motion profiles, electromagnetic interference, loss of satellite signal etc, but it will not be fixed to any specific location.

Spirent's GNSS simulators can generate data for multiple scenarios and multiple locations, offering massive flexibility in testing GNSS receivers. For example, a simulator with 10 scenarios and six locations is equivalent to 60 individual live sky tests, saving both the time and relocation expense traditionally associated with taking the receiver out into “the real world”.

What's more, each test is completely repeatable. In comparison, live sky testing can never be completely repeatable.

What are MEMS Sensors?

Tue, 19 Oct 2010 13:00:00 GMT

One of the simplest and most popular ways of improving the accuracy of navigation systems is to combine GNSS-based navigation with inertial navigation using MEMS sensors. But what are they?

MEMS stands for micro-electromechanical systems, and MEMS sensors comprise a class of devices that are micromachined from bulk silicon. As they are made from silicon, it’s possible to produce devices such as accelerometers that combine both a sensing element and the associated signal conditioning circuitry on a single silicon die. The resulting devices are both compact and easy to apply as they can be manufactured with easy-to-use linear or digital outputs.

The first commercial MEMS-based accelerometers were produced in the early 2000s as triggers for automotive airbags, and many millions of devices have been used in this single application. Subsequent generations of devices have become increasingly sophisticated, with capabilities that include sensing acceleration in three dimensions. These “silicon gyros” are ideal for use as inertial sensors in hybrid navigation systems.

Spirent GNSS simulators incorporate support for GNSS/inertial navigator systems to enable testing of hybrid systems.

What is the Klobuchar Model?

Thu, 07 Oct 2010 12:00:00 GMT

The ionosphere is the single largest error source in point positioning after the application of precise GNSS orbit and clock products, and there are a number of mathematical models that have been proposed to mitigate its effects. Of these, the model developed by John A Klobuchar is used by the GPS system and broadcast by every satellite.

The Klobuchar model is something of a compromise between computational complexity and accuracy. However, it is reckoned to be capable of correcting up to 70% of actual ionospheric delay, mainly during quiet space weather conditions. Unfortunately, it's performance is less than perfect during severe space weather, geomagnetic and ionospheric disturbances.

The International GNSS Service has been providing the total electron content of ionosphere on a global scale since 1998. This model, known as the Global Ionospheric Model, is reputed to provide better results than the Klobuchar model using the same GPS dataset and ephemeris.

Galileo will use the NeQuick model, which is currently used by the European Geostationary Navigation Overlay Service (EGNOS) for system assessment analysis.

Both of these models are incorporated into Spirent’s SimGENTM based constellation simulators. Find out more about GPS simulation here.

Multipath mitigation in Marine GNSS Receivers

Tue, 05 Oct 2010 12:00:00 GMT

On the face of it, the marine environment might appear relatively benign for GNSS receivers. After all, there is virtually no chance of signal obscuration from buildings or trees (although the odd cliff might come into play), so logically, the design of a marine GNSS receiver should be a piece of cake.

However, such devices are particularly prey to multipath interference – both from the surface of the sea and from the superstructure of the vessel itself. And left untreated multipath effects, whereby a reflected “echo” of any RF signal arrives at the receiving antenna a fraction of a second after the signal itself, are one of the major causes of inaccuracy in GNSS receivers. To further complicate matters, while a calm sea is an extremely efficient reflector of signals from low-level satellites, a rough or choppy surface produces a diffuse (and even more complex) effect.

While there are a number of highly efficient mathematical techniques employed for multipath mitigation in GNSS receivers, it is only by applying multipath effects to a receiver that these measures can be employed effectively. And the only way of applying these effects reliably and reproducibly is using a GNSS simulator.

Spirent's GNSS simulators can be programmed to apply all manner of multipath effects under software control, enabling marine GNSS receiver designers to apply multipath mitigation with confidence that they have addressed the specific problems of the marine environment.

Download the Free Spirent ebook: Testing Multipath Performance

Questions Facing Modern GNSS Developers

Tue, 21 Sep 2010 12:00:00 GMT

In the past, designers of satellite-based navigation systems have been mostly restricted to using a single system and service, the GPS C/A code.

The current revolution in the industry is giving rise to not only new GNSS systems, some GNSS systems, notably GPS and Galileo, have multiple services available to the commercial GNSS designer. The question of which system or blend of systems a GNSS designer should consider, along with the potential for multiple services is thus an entirely new consideration.

The application of the receiver under development may define the answer to this new conundrum to some extent, but in many cases the only way to answer the question fully before committing to a particular design path is to investigate the options available using a GNSS simulator.

While questions surrounding the pure accuracy of any given receiver using either one or multiple GNSS systems or services need to be answered, there are also a multitude of associated questions regarding device performance, such as power consumption, interface, multipath and interference resistance etc. that also need to be answered.

The availability of multi-frequency, multi system, multi service GNSS simulators allows the GNSS developer to answer these questions with complete confidence in the final design choice. Furthermore, the simulator is invaluable in the actual design effort and integration of the receiver into a given application.

Find out more about how Spirent simulators can help you by downloading the Spirent eBook: The role of simulation in the integration of GNSS receivers.

The use of Multiple Sensors in Improving Satellite Navigation Performance

Fri, 17 Sep 2010 12:00:00 GMT

Today most commercial receivers are GPS L1 C/A code only. However, for many applications, the single frequency GPS performance is inadequate and many technology developers are turning to other sensors to compliment GPS.

Today we have many forms and functions, depending on the application and hence needs. Integrated in -vehicle navigation systems often compliment the GPS position with dead reckoning navigation information from wheel rotation sensors. Often these are the same sensors used for anti-lock braking systems. The vehicle’s navigation system uses the GPS and wheel sensor information to compute the vehicle position. Additional sensors can also be used, for example, accelerometers and digital compass.

The beauty of this approach is that the GPS and additional sensors are complimentary to each other. When GPS performance is poor, such as in a tunnel or urban canyon, the additional sensors can take over to maintain a useful position solution. The additional sensors are open-loop systems, which become increasingly inaccurate over time, and are well suited to short GPS outages such as these.

Conversely in open sky situations, such as long highway drives, GPS generally works perfectly and there is no need to defer to the additional sensors. It’s not only cars that have additional sensors; aircraft systems typically use much more accurate inertial sensors than vehicles. Accuracy of inertial sensors is measured in cumulative drift in degrees per hour. Aircraft system sensors are typically accurate to a few degrees per hour. This can be accurate enough, in fact, to reverse the role of GPS and additional sensors so that the sensors are the main navigation mechanism and GPS is used only as a periodic correction mechanism to the open loop inertial system. Military systems are a degree more sophisticated than this, using so called tightly coupled technology where the GPS and inertial sensors effectively learn from each other to optimise the position solution.

At the opposite end of the GNSS applications spectrum, mobile phone handsets are increasingly packed full of sensors of one type or other. Mobile phone video capture has an anti-shake sensor so that your videos are smoother than your shaky hand. Your phone can flip the display from portrait to landscape when you turn it. There is some evidence of work being undertaken to use these sensors as an aid to navigation. Even in early 2008, at the Mobile World Congress in Barcelona, mobile phones were on display where the built-in digital compass could orient the user to assist with navigation. The logic is that it’s difficult to read a map on a small phone display, particularly when you have just emerged from an underground subway station, for example. The digital compass orientates the phone, the GPS provides the position and a picture of the intersection you are at is downloaded automatically over the phone’s internet connection. By the time you have programmed your destination, the navigation system in your phone is showing you a picture of the corner of the intersection you need to walk towards, perhaps annotated with an arrow or instruction or street name.

All this would be even easier, of course, if GNSS worked everywhere, even indoors or underground. This is a tricky problem that is yet to be adequately solved by an experimental, let alone commercially viable, solution.

Could GPS Technology Help Reduce Vehicle Emissions

Wed, 15 Sep 2010 12:30:00 GMT

We are generally being encouraged through government sponsored advertising to spend less time in our cars or not to rev our car engines too much. By doing so we can help to save fuel, the world’s resources and, by implication, do our bit towards reducing climate change. The European Union has a long-standing target for car manufacturers to reduce average CO2 emissions for their vehicles to 120g/km by 2012. A longer term target is 80g/km by 2020. The view of the industry has been that this is very challenging and unlikely to be met via conventional approaches alone. Indeed, the 2012 target already represents a slip from an original 2005 target date. As most manufacturers will not be able to meet the 2012 target, further slippages (possibly to 2015) and concessions (exclusions for heavier vehicles) are already on the table.

So what has all this got to do with GPS? The answer is that GPS is one technology that might be able to help reduce vehicle emissions. How might this work? Let’s assume that your vehicle is a hybrid, running on batteries part of the time to help meet the 120g/km and certainly the 80g/km target. The more the batteries are used, the lower the emissions. Hybrid car systems have limits set on the depth of discharge that the battery systems can be taken to before the engine kicks in. Often these limits include quite large margins.

Imagine for a minute, though, that your car also has a GPS system. This means that the vehicle can benefit from knowing not only where it is, but also from where it is going. Specifically, if the car systems know what’s coming up ahead, this information particularly related to inclines, could be used to optimize the performance of the hybrid system. In a simplistic example, if the car is going up hill, the batteries can be used more if the vehicle system knows that a down gradient is coming where a predictable level of recharging will be possible. It doesn’t take much of a leap to wonder, could this process be maximized by taking the decisions out of the hands of the driver? Automatic transmission coupled to knowledge of the road coming up (from the GPS system) could provide an answer. By knowing the nature of the road ahead, for example corner radius, duration and gradient, the transmission system could be optimised for economy.

Initially, at least, the driver would need an option to override such a system and optimise for sporty performance, fast response etc. There has been talk of “vehicle trains” for some time, whereby on higher classes of road the vehicle systems would take over and maintain optimum speed and distance from other vehicles. The barriers to these systems becoming a reality are rapidly being removed.

From a satellite navigation perspective, two key elements required are position accuracy to the lane level and high integrity, or trustworthiness, of the data. Lane level accuracy ideally requires dual frequency satellite navigation capability. This enables the atmospheric ambiguity to be backed out of the position calculation, providing sub metre accuracy.

Integrity is more challenging, particularly when autonomous vehicles travelling at high speed and in close proximity are concerned. This is likely to be the limiting factor in vehicle trains becoming a reality. In practice, a variety of approaches will be necessary to ensure the safety guarantees that will be expected. These will include augmentation systems like proximity radar and inertial sensors. Also likely, are roadside re-calibration systems that act as reference stations for the mobile satellite navigation systems in the moving vehicles.

Spirent has wide experience of helping companies in this sector understand their test issues and the best approach to testing. Spirent can help, contact gnss-solutions@spirent.com for more information.

Modelling Atmospheric Effects on GNSS Reception

Thu, 09 Sep 2010 12:00:00 GMT

No matter how well any GNSS receiver might be designed, there are a number of outside influences that can have major effects on the performance of the receiver in the real world. In particular, atmospheric conditions can significantly degrade performance in some designs, and unless suitable compensatory measures are taken companies can find themselves selling equipment that can only operate accurately under “ideal” conditions. Ionospheric Scintillation is one such phenomenon that is very likely to increase in the next year or so as we enter, what is predicted to be a significant period of solar activity. Previous solar events have led to severe disruption of GNSS systems and technologies that rely on it.

This is hardly a recipe for success in today's market. Users will reasonably expect perfect performance in all conditions, and excuses such as “the wrong sort of atmosphere” are simply not acceptable.

The ability to accurately recreate all possible atmospheric conditions – or rather their effects on GNSS signals – is essential to be able to test the performance of any GNSS receiver design and its ability to compensate for resultant navigation error. This is clearly not possible using live signals from real-world GNSS satellites. Atmospheric conditions vary not only with changing weather patterns, but also with global location, and the effort required to recreate even a representative sample of such conditions is prohibitive.

However, with suitable software control, a GNSS simulator can be programmed to create such conditions. Indeed, simulation of atmospheric condition is one of the standard capabilities enabled through Spirent's SimGEN software, which is in use by GNSS receiver manufacturers worldwide, and with new scintillation modelling being added, the test capabilities offered by the simulator are now better than ever.

GLONASS set for Automotive Boost

Tue, 07 Sep 2010 11:30:00 GMT

With global coverage of the GLONASS constellation now scheduled before the end of 2010, Russian Prime Minister Vladimir Putin has announced plans for a major expansion in its use on the nation's roads. In announcing the deadline for total coverage, Mr Putin revealed that all new vehicles sold in the Russian market from 2012 will be required to include GLONASS receivers.

GLONASS tracking devices are already routinely installed in commercial and emergency services vehicles throughout the Russian federation. But now Russia's largest carmaker, AvtoVAZ, is expected to start producing its Lada Kalina and Lada Priora models equipped with a GLONASS/GPS navigation system as early as 2011, making it the first manufacturer to equip vehicles with a dual-GNSS receiver as standard.

Manufacturers looking to take advantage of the expansion of the GLONASS system and exploit the new opportunities in the Russian marketplace are now running short of time. The only way to reliably develop new products using GLONASS or dual-GNSS systems featuring GPS and GLONASS is to use a multi-GNSS simulator from Spirent.

Will all Receivers Eventually be Multi-GNSS?

Tue, 31 Aug 2010 13:00:00 GMT

That was one of the questions put to John Pottle Marketing Director, Spirent Communications, Positioning Group by Coordinates magazine. You can find his answer and the Spirent view on many other interesting questions regarding future GNSS trends and applications at the following link.

http://mycoordinates.org/eventually-all-receivers-will-be-multi-gnss/

What you Need to Know About Multi-GNSS Testing

Mon, 30 Aug 2010 11:00:00 GMT

As new GNSS systems appear, it becomes more apparent that you need a Multi-GNSS test solution. Here at Spirent we’ve been advocating Multi-GNSS for quite some time but now someone else has taken up the mantle.

If you haven’t already done so, check out this comprehensive article in Inside GNSS magazine. Three receiver designers and researchers explain how they view Multi-GNSS simulators as an essential tool throughout the entire receiver development cycle: from research, development, design and validation through chip, module, OEM and user device development sequences and on to consumer testing, certification, maintenance and repair.

Spirent already has solutions for many of these technologies available now. Talk to us and make sure you don't miss out!

Multipath Mitigation in Marine GNSS Receivers

Wed, 25 Aug 2010 12:30:00 GMT

However, such devices are particularly prey to multipath interference–both from the surface of the sea and from the superstructure of the vessel itself. And left untreated multipath effects, whereby a reflected “echo” of any RF signal arrives at the receiving antenna a fraction of a second after the signal itself, are one of the major causes of inaccuracy in GNSS receivers. To further complicate matters, while a calm sea is an extremely efficient reflector of signals from low-level satellites, a rough or choppy surface produces a diffuse (and even more complex) effect.

What is EGNOS?

Thu, 19 Aug 2010 12:00:00 GMT

Hosted by the European Space Agency, the European Commission and Eurocontrol, EGNOS is the European Geostationary Navigation Overlay Service, and is the first pan-European satellite navigation system. EGNOS comprises just three satellites, and acts as an enhancement to the US-based GPS system for safety critical applications in aviation and marine environments.

The EGNOS Open Service has been up and running since 1st October 2009. This provides freely available positioning data throughout Europe to any EGNOS-enabled GPS receiver. EGNOS Safety of Life and Commercial services are scheduled to commence operation later during 2010.

In addition to the three satellites, the EGNOS network comprises more than 40 ground stations Of these, 34 are ranging and integrity monitoring stations receiving signals from the US GPS satellites; there are four mission control centres to handle data processing and differential corrections counting; and there are six navigation land earth stations that create accuracy and reliability data for sending to the three satellite transponders for relay to end-user devices.

Similar services are provided in North America by the Wide Area Augmentation System (WAAS), and in Japan by the Multifunctional Satellite Augmentation System (MSAS).

Spirent offers GNSS simulation systems that support EGNOS, WAAS and MSAS, allowing manufacturers to produce receivers supporting all forms of GPS augmentation.

What is an Ephemeris?

Mon, 16 Aug 2010 12:00:00 GMT

An ephemeris is quite simply a table giving the coordinates of a celestial body at specific times during a given period. The word comes from the same Greek root as “ephemeral”, which strictly means short-lived, but has come to mean inconsequential. However, in terms of GNSS systems, the ephemeris is certainly not inconsequential.

Each GNSS satellite includes ephemeris data in the signal it transmits. This comprises a set of parameters that can be used to accurately calculate the location of a the satellite at a specific moment in time, and hence describes the path the satellite is following as it orbits Earth.

As the name accurately implies, ephemeris data is only valid for a limited time (a few hours or less). Therefore up-to-date ephemeris data is needed to minimize errors that result from minor variations in a satellite's orbit.

When testing GNSS receivers using a GNSS simulator, ephemeris data play a significant role in several tests. For example, when testing a receiver's time to first fix (TTFF) performance there are differing requirements for the cold-, warm- and hot-start tests. For a cold-start test, the receiver needs to receive time, almanac and ephemeris data. For a hot-start test, all the data are already in the receiver. But for the warm-start test, the receiver already has the time and almanac data but needs to receive fresh ephemeris data.

You may not be working on GPS/GLONASS but many of your competitors are

Thu, 12 Aug 2010 12:00:00 GMT

Work continues apace on the GLONASS constellation, with the commencement of a headquarters building that will also house an office of the United Nations IT and satellite navigation agency. And the head of the Russian Space Agency, Anatoli Perminov, took the opportunity of laying the foundation stone of the new building to confirm that the GLONASS constellation would achieve full global coverage before January 2011. He also announced preliminary details of an upgraded system, due for introduction before 2020.

With threats of trade sanctions against companies intending to market GPS-only equipment in the Russian market, the pressure is on all manufacturers of location-enabled devices to take the Multi-GNSS route, rather than sticking to GPS-only designs. Those that do not follow this edict not only run the risk of being excluded from a fast-growing market, but also fail to maximise the location aware capabilities of their devices by not following the Multi-GNSS route.

If you’re involved in the design, integration, verification or manufacture of GPS/GLONASS devices or systems you can test your Multi-GNSS devices today. Spirent constellation simulators are available offering GPS and/or GLONASS and/or Galileo capability. What's more our new platforms are field upgradable. You can test other GNSS systems well in advance of the commencement of services and many people are. Don’t get left behind, talk to us now.

Why are we Talking About Multi-GNSS

Mon, 09 Aug 2010 10:07:00 GMT

Today, navigation and positioning technology is no longer just about GPS L1 C/A code. GPS is being modernized, the GLONASS constellation is nearly complete, new systems including QZSS, IRNSS, Galileo and Compass are on the way.

Multi-GNSS offers significant opportunities and challenges to GNSS technology, system and application developers.

Spirent multi-GNSS simulation systems are now being purchased by customers developing commercial systems and most chipset manufacturers have plans to release dual constellation chipsets before the end of 2010.

If you haven’t already done so, check out this comprehensive article in Inside GNSS magazine on Multi-GNSS testing from the R&D stage through Integration, Validation to Production

Testing Multi-GNSS equipment: Systems, Simulators and the Production Pyramid

As new GNSS systems appear, the time for multi-GNSS simulator testing has arrived. Three receiver designers and researchers look at use of this essential tool throughout the entire receiver development cycle: from research, development, design and validation through chip, module, OEM and user device development sequences and on to consumer testing, certification, maintenance and repair. http://www.insidegnss.com/node/2143.

US Initiative Expands Appeal of Multi-GNSS Systems

Fri, 30 Jul 2010 13:00:00 GMT

A new US national space policy document unveiled recently by President Obama marks a major change of direction on the relationship between the country's GPS system and other GNSS systems around the world. And the change can only accelerate the development and interoperability of systems such as GLONASS, Compass and Galileo.

Whereas US policy as affirmed in a December 2004 national security directive was focused on maintaining the country's lead in GNSS on a unilateral basis, the new initiative reflects a more open attitude and calls for the USA to “engage with foreign GNSS providers to encourage compatibility and interoperability, promote transparency in civil service provision, and enable market access for US industry”. It also allows that “foreign positioning, navigation and timing services may be used to augment and strengthen the resiliency of GPS”.

The news will be a welcome shot in the arm for manufacturers that have already moved towards Multi-GNSS systems in advance of the provision of services. These manufacturers have been able to test their designs in advance of service provision using Spirent'sMulti-GNSS simulators, which are capable of simulating all published GNSS signals.

The new policy document can be found at http://www.whitehouse.gov/the-press-office/fact-sheet-national-space-policy.

China Edges Towards Global Navigation Coverage

Wed, 28 Jul 2010 11:00:00 GMT

The latest contender in the global navigation sweepstakes has moved a little closer with the launch of the fourth satellite in China's second-generation Beidou constellation during the first week of June 2010. Beidou (which means Big Dipper) will cover all of China and neighbouring lands by 2012, and will then be expanded to provide global coverage through a constellation of 35 Compass satellites by 2020.

Compass will differ from other GNSS systems in that five of the intended 35 satellites will be in geostationary orbit, while the other 30 will have medium earth orbits similar to the GPS, GLONASS and Galileo constellations.

Although very little has been officially announced about the signals to be transmitted by the new system, the launch of the first Compass satellite in 2007 did enable independent researchers to build a Compass receiver. However, the lack of official data means that no commercial work is likely in the near future.

The Spirent GSS8000 Series of Multi-GNSS constellation simulators have been designed to be compatible with the Compass system. And as soon as the Chinese authorities release the Compass ICD, Spirent will be looking to make a solution available. This will enable users to design Multi-GNSS receivers that will have true global appeal, offering compatibility with GPS, GLONASS and Galileo in addition to the Compass system.

Could GPS Technology Help Reduce Vehicle Emissions?

Wed, 21 Jul 2010 12:00:00 GMT

Many world governments have a long-standing target for car manufacturers to reduce average CO2 emissions for their vehicles. The European Union target is 120g/km by 2012 and longer term to 80g/km by 2020. The view of the industry has been that this is very challenging and unlikely to be met via conventional approaches alone. Indeed, the 2012 target already represents a slip from an original 2005 target date. As most manufacturers will not be able to meet the 2012 target, further slippages (possibly to 2015) and concessions (exclusions for heavier vehicles) are already on the table.

Working With the Strengths and Weaknesses of Satellite Navigation Systems

Mon, 19 Jul 2010 14:35:00 GMT

GPS specifically, and GNSS more generally, works fantastically well in its native mode of operation with an open view of the sky. High vehicle speeds, even in an aircraft manoeuvring at several times the speed of sound, are well within the capabilities of the GPS system. To use more specific language, the accuracy and continuity of positioning information is very high in open sky conditions.

Back down to earth, a person walking with their GPS on the edge of the street in a typical town or city could well have a very different experience. First the continuity of service could be affected by the receiver losing its lock on the visible GPS satellites. This could be due to the satellites being blocked behind buildings. Or the receiver may be located inside a building or shopping centre where the received satellite power is too low. Or the user could unwittingly point the antenna at the ground rather than the sky. In fact, these and a whole range of related effects can cause major difficulties for GPS receivers.

Secondly, even if the user is experiencing good continuity, the accuracy of the solution can be affected by multipath signals being seen and interpreted as ‘good’ by the GPS receiver. This will ‘trick’ the receiver, which will read the effective distance from the satellite to the user as being the bounced signal length, rather than the direct signal length.

Errors from a few metres to several hundred metres are quite common from multipath effects. As well as continuity and accuracy, the ability to trust the position being given, deserves careful consideration. There are multiple factors that affect integrity. Some are common, such as local interference from TV or microwave stations and, particularly near the equator, sun spot activity (sun spot activity is something we will devote more time to at a later date). Others are relatively rare such as satellite clock or transmission errors. When these do occur, however, they can cause major position errors, up to several kilometres in extreme cases.

The importance of these effects depends on what the positioning information is being used for. There are a great many systems that rely on GPS for commercial purposes. Examples are road tolling, congestion charging, tracking and logistics. In these cases, GPS alone may well be sufficient for positioning, but safeguards need to be designed to ensure proper use and system accuracy. Whatever the application, the system design team should be very clear, up front, what the accuracy, continuity and integrity requirements of the system are. In a vehicle navigation system the ultimate responsibility for safety lies with the driver, who should ignore incorrect instructions that might compromise safety. In this case, the priority of the design team may be low cost of manufacture above performance. At the other extreme, safety critical systems such as aircraft landing or rail signalling require high and guaranteed integrity, accuracy and continuity. Equally important are trade-off decisions between time, cost and quality at the project level, system level and user terminal level. Only with these defined and agreed can a proper test plan be developed. Often, such a test plan will require a mix of ‘live’ and ‘laboratory’ testing.

Often GPS alone is not sufficient and needs to be complemented, or augmented, by additional sensors or systems. A common test approach is to capture real data from the field and then recreate elements of that field data in a controlled laboratory environment. At its simplest, a navigation system developer can drive a route in the real world and capture ‘NMEA’ data from the receiver being used. This data can then be used to create a trajectory in the lab test system, also to vary factors such as satellite power levels and satellite visibility.

By using progressive test cases and approaches, the designer can design-in the performance they require. Equally important, the design team can understand the limitations of their system and ensure that processes and operational use cases take account of these to provide the appropriate level of service.

In summary, everyone working with GNSS technology development or using GNSS systems in a professional capacity should be aware of the inherent strengths and weaknesses of the GPS and other GNSS systems. At the receiver and system design level, many of the problems can be overcome by a logical and progressive test approach linked to the design objectives. The team that ignores inherent weaknesses and does not strive to take account of them is in for a miserable time indeed.

Download the Spirent eBook; Testing Multipath Performance of GNSS Receivers

GPS Time and Leap Seconds

Wed, 14 Jul 2010 12:00:00 GMT

Time is an important component of any satellite navigation system, and it is essential that any receiver attached to the system has a clock that is fully up to date. The current GPS system uses its own timescale, which is closely linked to (but not completely in sync with) Co-ordinated Universal Time (or UTC). And to allow GPS receivers to give users the precise time according to UTC, the precise value of the current offset between the two clocks is broadcast by the satellite system.

While UTC is maintained centrally using super-precise atomic clocks, it does have to be adjusted occasionally to keep in sync with the earth's changing rotation and to reflect mean solar time. And just as our calendar is periodically adjusted by the addition of a day each leap year, UTC is periodically adjusted by the addition (or subtraction) of a leap second. These events will usually take place on the last day of June or December. But they are relatively rare, amounting to approximately 0.6 seconds per year.

These leap second events are virtually imperceptible as far as telling the time goes, but are broadcast when they occur to enable clocks around the globe to stay synchronized with UTC. But for satellite navigation systems they are essential information. And so the leap second event is broadcast to each receiver as part of the navigation data message.

Clearly, a receiver's response to the arrival of such a message is critical. And that is why Spirent's GNSS simulators offer facilities for testing the response of receivers to leap second events. More information on Spirent’s GPS and GNSS test solutions can be found on our website www.spirent.com/positioning

Galileo to Bring Additional Services

Thu, 08 Jul 2010 11:30:00 GMT

Despite continuing delays in its introduction, when the new European Union funded Galileo constellation goes live in 2014 it will provide a number of novel services. Designers of next-generation Multi-GNSS systems need to factor in these new capabilities in order to keep their equipment ahead of the competition.

Importantly, Galileo is designed provide more precise location data from that provided by GPS or GLONASS, and will be accurate down to the one-meter range. The data will also include accurate altitude measurements, and improved coverage services at high latitudes. Crucial to this, each of the 27 live satellites in the constellation will broadcast no fewer then ten different navigation signals, enabling a degree of service differentiation not yet seen from any other satellite system.

Galileo will offer five main services when fully operational. The standard “free-to-air” Open Service, the high-integrity Public Regulated Service and the Search and Rescue Service are scheduled to be fully operational by 2014. At this point, trials will also begin on the remaining two services: a value-added centimeter-accurate Commercial Service and an open Safety Of Life Navigation Service for applications where guaranteed accuracy is essential.

Clearly, without the necessary satellites in place—and indeed even when they are—GNSS receiver designers need a proper means of testing their products, and this comes in the form of the Spirent family of Multi-GNSS simulators. These acclaimed test tools already have the certified capability of simulating Galileo navigation signals, both alone and in combination with GPS and Glonass signals.

With early adopters likely to pay a premium for the ability to access the new Galileo services, manufacturers cannot afford to be late with their support for the Galileo system. And that means simulating the signals now well in advance of the 2014 launch.

Safety Critical Navigation on the Rails

Tue, 06 Jul 2010 11:00:00 GMT

To those engineers more familiar with automotive or marine navigation systems, the concept of using GNSS receivers for navigation on railways might seem a case of “overkill”. After all, there are only so many places a train can go, and these are firmly bounded by two steel rails. However, the exact knowledge of the position of any train on any rail system allows the rail operator to both improve service and increase traffic density by reducing the headways associated with fixed line-side signaling without compromising safety, making it key to maximising the efficiency of the network.

Unfortunately, railway networks are pretty hostile environments for GNSS receivers. Trains spend significant periods of time in deep cuttings and tunnels, obscuring signals or creating complex multipath effects, and the electrically noisy nature of the power systems involved adds further complications. So while GNSS tracking is desirable for rail operators, it is by no means easy to deploy reliably. And as safety is paramount in any public transportation system, the integrity and reliability of the systems is essential.

Although live-sky testing of GNSS systems on railway networks would appear to be obvious way of testing the effectiveness of the system, it is both unreliable and expensive. While any live-sky test scenario may be “real”, it will not be repeatable and will not be able to test the system under specific conditions that might affect its performance. What's more, the costs of obtaining a train path on a section of a busy rail network to test a GNSS system, and the cost of operating the train would be unacceptable.

GNSS simulation offers the solution, with the ability to test the system under all possible conditions. Spirent's SimGEN™ simulation software can be used to create test scenarios that recreate railway-specific conditions, giving rail operators complete confidence in the integrity and reliability of their GNSS systems. Further details can be found here.

E-Call 112 and how it Affects GNSS

Tue, 01 Jun 2010 13:00:00 GMT

When the worst happens, seconds count. E-call, the European Commission's telematics project, is expected to save 2,500 lives annually in the EU by saving time in getting the emergency services to the right place as promptly as possible. An e-call can be initiated manually by vehicle occupants or automatically by the vehicle itself. Once communications is established with the emergency services (Public Safety Answering Point or PSAP), a data stream known as the Minimum Set of Data or MSD can be sent from the scene directly to the emergency services.

As you’d expect the MSD contains position information and so for our purposes in the area of positioning technology we need to establish that the vehicle knows its position at all times even if inverted in a ditch!

Testing to meet standards requires a degree of control and repeatability not readily available from field trials. If running such a test program you may reasonably expect a substantial reduction in uncertainty, test time and spend by incorporating a simulator. Using features such as antenna gain and phase masks, lever arms and the ability to switch between antenna patterns (to simulate an inverted car?) allows the rapid assessment of performance which may simply not be practicable using live-sky and real vehicles.

For further information check out the following links:

eCall--Calls that 'dial' 112

eSafety Support

www.spirent.com/positioning

How to Reduce the Challenge of GPS Integration

Fri, 21 May 2010 12:00:00 GMT

Have you been tasked with the integration of GPS (or GLONASS) chips into a product or system? If so you will be facing a number of challenges.

Selecting which chipset or module to use
Ensuring that the tiny RF signals are getting through
Evaluating the performance
Designing for test and manufacturability whilst providing rapid time to market.

Your first step should be to consider what characteristics you need in your receiver… they are NOT all the same! For example a receiver designed for high accuracy may not be optimised for a rapid start-up. Look at the manufacturers’ datasheets and you will find that they are often difficult to compare using slightly different parameter definitions. Testing the units yourself may be the only way to get a truly accurate and directly comparative set of results and repeated testing under identical test conditions is a job for a GNSS constellation simulator.

Having identified the receiver you wish to use, the next challenge is obtaining the same performance you observed on the evaluation board, but in your circuit. Once again, GNSS simulation provides a controllable, repeatable test. Any issues that arise during this stage can often be resolved by sharing the test scenario with your chosen vendor. Scenarios are portable between Spirent machines so provided your vendor has a Spirent simulator (and most do) problem resolution can be significantly fast.

Finally, you need to define a baseline performance that can be used for manufacturing and regression testing. Once again the control and repeatability aspects of the GNSS simulator are powerful allies in this task.

In summary, if you follow these 3 steps you'll sleep more peacefully:

Choose your vendor after consideration of the cost and performance trade-offs offered. Perform an evaluation yourself. Confirm they use a GNSS simulator so that you can share troublesome scenarios.
Develop your circuit and layout with consideration for RF losses and interference sources. Confirm operation using your GNSS simulator.
Define a baseline performance that should be obtainable in production testing. A controlled, repeatable test using a GNSS simulator allows rapid evaluation of changes in the GPS (e.g., firmware) or surrounding circuitry (e.g., WiFi, Bluetooth, board layout, etc.).

Testing GNSS Receivers in a Production Environment

Wed, 19 May 2010 13:00:00 GMT

Manufacturers of consumer products routinely perform functional testing on all production output, and it would appear that adding some form of location testing to these production test routines would be sufficient to verify the reliability of the GNSS receiver within the end product. However, it is all too easy to adopt the attitude that the simplest of tests will suffice – particularly when the duration of each test can have a significant impact on productivity.

Although it may be the case that all the other functions of the end product can be assessed with a relatively straightforward go/no-go test, taking such an attitude with a GNSS receiver is fraught with danger. The end user will expect the product to perform adequately under a wide variety of conditions. This means that the receiver will need to be tested not just for an “ideal” situation, but also for adequate performance in the presence of multipath interference, all manner of jamming signals and less than ideal atmospheric conditions.

Key test challenges in a Production line environment

The first obstacle that will be encountered in integrating GNSS receiver testing into a production test setup is pretty obvious. As such tests are performed at the end of the production line, they are inevitably performed indoors. And regardless of whether the equipment is designed to work indoors or outdoors, the roof and walls of the building will introduce variables into the test that will negate its effectiveness. So-called “live-sky” testing is therefore impossible without relaying the GNSS signals from outdoors to the production tester.

It is a relatively simple exercise to capture live GNSS signals and re-radiate them within the production test environment. However, this comes with its own set of shortcomings.

First, radiating any signal in such an environment might have unforeseen consequences on other tests that are performed on the product; and conversely, other RF signals and noise within the production test area may well impact on the integrity of the GNSS signals.

More importantly, though, the inherently dynamic nature of GNSS signals means that while each unit may well be tested in the same physical location (i.e. in the production tester fixture), the relative positions of the GNSS satellites will be different for every unit tested. And, not surprisingly, this makes direct comparison between results unreliable at best.

GNSS tests within a Production line environment

In order to fully assess the performance of a GNSS receiver embedded in any piece of equipment, it is important to work out exactly what response is required. There will be varying degrees of performance required, depending on the end application, but the requirement will be for a combination of navigational accuracy and sensitivity under a wide range of operational conditions. There will also be a requirement for the equipment to work with not just the existing Global Positioning System, but also with the forthcoming enhanced GPS, GLONASS and Galileo systems at the very least.

Some systems may have no direct output. Or, more to the point, the output may only be in the form of an alarm or trigger that is supposed to be produced with proximity to certain co-ordinates. This does not however mean that the performance demands on the receiver are any less arduous. It would however, dictate the pass/fail criteria for the production test.

GNSS test solutions for a Production line environment

Given the inherent variability of any type of live-sky testing, it is logical to seek a more precise and repeatable stimulus against which the performance of an embedded GNSS receiver can be assessed. And this can be supplied in the form of a GNSS simulator.

A multichannel multi-GNSS simulator under software control can produce all the necessary signals required to test the relevant performance criteria of any embedded GNSS receiver in any location-enabled device. Most importantly, it can do so consistently and repeatably for every unit to be tested, ensuring that manufacturing output is 100% fit for purpose.

The tests typically performed on any navigation device are inherently complex, covering the full range of performance criteria from navigational accuracy and sensitivity to acquisition time and immunity to interference. These tests have been designed to ensure the performance of dedicated GNSS receivers, and have been proved over successive generations of personal navigation devices.

Fortunately, once the desired performance of the design has been characterised, the production tests for the end product can be refined into a considerably smaller set of acceptance criteria that can be performed in a relatively short time (as little as 5 minutes).

The Spirent GSS6300 Multi-GNSS Signal Generator has been designed specifically for high volume production test applications for devices that use commercial GPS/SBAS, GLONASS and/or Galileo receivers. Visit our video library to find out more on how Spirent can help.

Certification of GNSS Devices

Tue, 18 May 2010 13:00:00 GMT

Traditionally, civilian use of GPS was seen as free and to a large extent “at your own risk”. The typical performance one might expect was stated in the relevant Interface Control Documents (ICD’s) but no guarantee of service was given. The reason for this was the historical remit of GPS as a system to satisfy US military requirements, the civilian use of the coarse acquisition (C/A) ranging code being essentially a by-product of its primary use, which was to provide classified receivers a ‘first step’ towards fast acquisition of the precise ‘P(Y)’ code.

With the increasing number of GNSS services that will offer certain guarantees of service scheduled to be available over the coming years, there is an increasing requirement for certification of these services and the GNSS devices that use them.

To facilitate these requirements, new performance standards are being written which are dictating the tests to be performed.

Test integrity is essential

All proper testing must have good integrity – that’s a given, however, it is especially so with certification, verification and type approval testing, particularly when the device and / or application is safety critical. This is why using quantifiable, traceable and accurate test methods are important. Accredited test laboratories use precision test equipment which is calibrated to standards traceable to National references for all certification and type approval testing. The same must therefore apply to GNSS testing. This is why using calibrated precision RF constellation simulators is the only way to perform this kind of testing. The inherent uncertainty and variability of using other methods (such as real GNSS signals) rules them out completely.

Spirent’s pedigree in producing high-fidelity, precision test equipment is well proven over more than 25 years. Spirent has supplied equipment into critical test programmes across the world. These equipments have often been put through rigorous certification and validation programmes to ensure they are themselves fit for certification testing. The most recent examples of this are;

The certification by the European Space Agency and Galileo Supervisory Authority of the world’s first Galileo RF constellation simulator—Spirent’s GSS7800 for the Ground Receiver Chain (GRC) and Test User Segment (TUS) receiver developments in the IOV phase of the Galileo programme.
The certification of Spirent’s GLONASS implementation by the Russian authorities

So, to ensure your testing is up to standard, make sure you select test equipment from Spirent, the world’s leading and most proven simulator provider.

GPS Modernization and the L5 Signal

Mon, 10 May 2010 13:00:00 GMT

One of the most significant additions among the raft of changes that are being made to the GPS system is the addition of a second safety-of-life signal for civilian use. This new L5 signal is centred at 1176.45MHz in the worldwide Aeronautical Radio-navigation Services band, and will be broadcast at roughly twice the power of the existing L1 and L2C signals. It also features wider bandwidth and longer spreading codes, and will be particularly useful for enabling aircraft to make precision landings in high multipath environments as well as reducing errors due to the ionosphere.

The first of the new constellation of satellites capable of broadcasting the L5 signal, the GPS IIR-20(M), was launched from Cape Canaveral Air Force Station, Florida, on 23rd March 2010, and began broadcasting the L5 signal on 10th April 2010. But while this satellite will serve as ample proof of concept, the full L5 signal coverage will not be available until the GPS modernisation is complete, which is unlikely before 2013.

However, designers of equipment can prepare for GPS modernisation by simulating all the new signals today using Spirent's range of GNSS simulators. Not only do these instruments cover the new L5 signal, as defined in the new ICD-GPS-705 standard, they also cover all the other enhanced signals that will be provided by the new modernised GPS constellation. Beyond GPS, they also cover all signals for the GLONASS and Galileo GNSS’s as well as the regional and local augmentation systems.

Making the Connection in GNSS Testing

Fri, 07 May 2010 12:45:00 GMT

One question that regularly crops up in discussions about GNSS receiver testing concerns exactly how the signals get from the GNSS simulator to the device under test. Is it better to radiate the simulator signal to the receiver's antenna, or should you couple them directly?

The short answer to this is that a direct connection from the simulator to the receiver's antenna port will always provide the most controlled test environment with no risk of outside influence. The connection is usually performed using a simple coaxial cable that acts as a 50 ohm transmission line, or it may require the addition of a low-noise amplifier in cases where the receiver is designed to use an active antenna.

In either case, making the direct connection – always using high-quality cables and components – will enable all tests to be made under controlled conditions.

Of course, some types of location-enabled equipment have no external antenna port, and in many cases the antenna is entirely hidden from view. So here practicality dictates that a radiated signal must be used. This does, however, leave the test setup open to all manner of outside interference, and this can make results unreliable. So for full confidence in the results of the test we recommend putting the equipment under test inside an RF screened enclosure together with the radiating antenna from the simulator.

NMEA Data Explained

Wed, 28 Apr 2010 12:00:00 GMT

The navigation industry often refers to NMEA data. But what is it? And why is it so important for the GNSS receiver industry?

The NMEA is the US National Marine Electronics Association, which acts, among other things, as a standards body for the industry. And one of its most important standards is NMEA 0183, which defines electrical and data specifications for serial communications between all manner of marine electronic devices. These include everything from echo sounders, sonars and anemometers to gyrocompasses, autopilots and (importantly) GNSS receivers.

NMEA data comes in the form of “sentences” that are unique to each piece of equipment, but which can be read by all other equipment adhering to the standard. Each sentence begins with a dollar sign and ends with a carriage return, and can comprise no more than 80 characters of ASCII text. A number of standard sentences are defined, each identified by a prefix such as the “GP” used for GNSS (GPS) receivers.

The standard also allows hardware manufacturers to define their own proprietary sentences. All proprietary sentences begin with the letter P and are followed by three letters that identify the manufacturer controlling that sentence. For example a proprietary Garmin sentence would start with “PGRM”.

While these NMEA sentences are designed to be used for communication between navigational devices, they can also be used in the test laboratory. For example, the output recorded from a GNSS receiver in the field can be used to program motion profiles into a GNSS simulator to test other units.

Running Interference in GNSS Receivers

Tue, 27 Apr 2010 12:00:00 GMT

It goes without saying that any RF device as sensitive as a GNSS receiver will be inherently vulnerable to interference. Clearly, care needs to be taken at both the design and integration stages to minimise interference effects. But what interference sources need to be considered? And how do you know if your receiver can deal with them?

Most potential sources of interference are obvious and predictable: The effects of fixed-frequency transmitters for TV, radio and the like can easily be modelled and accounted for. Indeed, one advantage of working with multi-GNSS receivers is that some are multiple-frequency devices, and therefore inherently more resistant to interference on any specific frequency.

However, there are two sources of interference that you ignore at your peril: the first of these is internally generated interference, and this is particularly relevant in devices such as location-enabled mobile handsets. The typical handset has a transmitter capable of transmitting more than 1W in very close proximity to the GNSS receiver (and two may even share some signal-path components). Careful design partitioning is essential to prevent interference.

The second is more subtle, but will increase over time, and concerns the increasing number of constellations and signals coming on stream. So say, for example, you have designed your multi-GNSS receiver to work with both GPS and GLONASS, and then the Galileo system comes on-line, then the receiver will view the uncorrelated signals from Galileo satellites as interference. At a simpler level, a GPS-only receiver will view both GLONASS and Galileo as interference. In a similar way, all civilian receivers need to work in the presence of classified signals such as GPS Y and M codes and Galileo PRS.

Fortunately, even if you are still working with single-GNSS receivers then you can test your designs for interference from other systems using a Spirent Constellation simulator equipped with the interference option. And this can even be used to simulate unwanted GNSS signals.

GNSS Receiver Integration: Not Just the sum of the Parts

Fri, 16 Apr 2010 12:00:00 GMT

The ability to integrate a GNSS receiver into an end product offers new possibilities for manufacturers in a wide range of both consumer and industrial markets. However, designers of such products need to be aware that even the most highly integrated GNSS receiver module is not a “fit and forget” component. As with any radio frequency system, there are design rules that must be followed, and even then the interaction of the receiver with the other functions of the product can create some surprising results.

Although radio frequency design is not the “black art” it was once considered, the design rules are inevitably more complex than for digital design. Even a slight failure to follow the rules can lead to poor performance, and as far as the end user is concerned “close but no cigar” is not acceptable. Worse still, once a design has been characterised and accepted, seemingly small variations in manufacturing tolerance, or a purchasing manager saving a few pennies by substituting a cheaper component can throw the performance right out the window.

The only guarantee of continued product quality and reliable performance is 100% functional testing of the finished product. And that will mean integrating a GNSS simulator in the production test setup. Fortunately, Spirent's production test simulators have been developed with the demands of manufacturing in mind.

GLONASS Constellation Nears Readiness

Thu, 15 Apr 2010 12:00:00 GMT

The Russian Deputy Prime Minister Sergei Ivanov has confirmed that the country's GLONASS system will have 100% global availability before the end of 2010. The news follows the launch of three new satellites during March 2010, bringing the GLONASS constellation up to 19 operational satellites of the 24 required for full service.

GLONASS, or Global'naya Navigatsionnaya Sputnikovaya Sistema (literally Global Navigation Satellite System) had fallen into severe disrepair after the fall of the USSR, at one point with only eight satellites operational. Two further launches are planned during 2010, bringing the total number of working satellites to 24, with three in reserve.

The Russian Institute of Space Device Engineering has revealed that it is close to completing a co-ordination plan that will see eight different CDMA signals on four frequencies. The first of these will be the existing GLONASS L3 frequency, with an open signal centred at 1201.743MHz and an encrypted signal at 1208.088MHz. The additional CDMA signals will be introduced at the new L1, L2 and L5 GLONASS frequencies.

Several manufacturers have begun production programmes for GPS plus GLONASS Multi-GNSS receiver chips and modules. OEMs intending to integrate these devices can test them ahead of the final launches using Spirent Constellation Simulators which can be configured for GPS, GLONASS and Galileo capability.

Location-Based Services put Pressure on GNSS Receiver Performance

Wed, 14 Apr 2010 12:30:00 GMT

The addition of GPS receivers to today's smartphones, netbooks and other internet enabled devices is allowing mobile operators and other service providers to exploit a growing market for location-based services. These can range from social networks offering “find a friend” applications, to location based marketing and advertising. And you can be sure that developers will come up with many more new applications for location data as the market matures.

Regardless of the nature of the service, all location-based services put GNSS receiver performance at a premium. After all, there is no point in giving a user a mobile coupon for a business they passed just ten seconds ago, and telling them they are standing within 10 metres of it when they can clearly see they are 100 metres away, this is hardly likely to endear them to any service. While assisted GPS can play a role in the equation, operator-independent location-based services rely purely on the performance of the GNSS receiver in the user's equipment.

The growth of such services means that it is now even more important to ensure that your GNSS receiver design delivers the best possible performance under all operating conditions. And the only way of verifying this performance is by simulating all those possible conditions using a GNSS simulator.

The Importance of Time to First Fix

Tue, 13 Apr 2010 12:00:00 GMT

Time to first fix is a crucial performance parameter for any satellite navigation system because it is the first and most easily appreciated evidence that the end user will have of the quality of the receiver. When you consider that this applies equally to potential users trying out receivers in the shop and to users maintaining satisfaction with the systems they have bought, then a few seconds here and there can make the difference between a happy customer and one that buys your competitor's product.

But how can you be sure that your receivers TTFF is optimized for best performance? Clearly, you test it. And you can even run the same tests on your competitor’s products. But (and here's the rub) be sure that you are running exactly the same tests on each piece of equipment. That means having the receivers in the same position, the satellites in the same position, identical atmospheric conditions etc – otherwise comparisons are meaningless.

So while the performance you are trying to assess will be felt by the user in the real world, there are so many factors that impact on receiver performance that it is close to impossible to make meaningful comparisons using real-world testing.

Using Spirent's constellation simulators under controlled laboratory conditions is the best way to ensure that products are tested with a level playing field. What's more, the ability to run a near infinite number of test scenarios reproducible under software control on multiple receivers will provide ample evidence that your products will outperform the competition under all manner of conditions.

Multi-GNSS: The Future of Navigation

Wed, 03 Mar 2010 13:00:00 GMT

If you're a GNSS technology, system or application developer involved in the design and implementation of a GNSS project today, you need to take into account the full range of satellite systems and signals that will be available in the near future and understand the challenges and opportunities you face. Using satellites from more than one system brings special challenges and design choices for receiver design and evaluation. But what exactly is the timescale before these new systems are operational?

The first of these systems is GPS itself, which is currently being modernized with extra ground stations and new satellites, and will supply additional navigation signals for both civilian and military users.

The target date for completion is around the middle of this decade, but with major incentives available for contractors, who knows?

Before that, though, we should see the restoration of the full Russian GLONASS system. Originally a jewel in the Soviet crown, the system fell into disrepair with the fall of the USSR. However, with substantial help from India, Russia has committed to get the system back up and running during 2010.

The European Galileo system has been something of a political football, but in 2007 the European Union took over the project from the private consortium that had instigated it, and committed to complete the system by 2013. However, subsequent EU communications now talk of a 2014 start date.

Finally, the Chinese government has committed to expanding its own local Beidou system into a global network dubbed Compass. No date has been set for completion.

Anyone involved in the design and applications of these systems can't afford to wait until the signals go live before starting to develop a solution, if you do, you'll lose your market to some quicker and sharper player. Fortunately, Spirent's multi-GNSS test systems already support modernised GPS, GLONASS and Galileo and are available today!

Download our Application Note: Benefits, challenges and test considerations for GNSS Technology Developers.

How a GNSS Simulator can Help Test the Multipath Performance of GNSS Receivers

Thu, 25 Feb 2010 13:00:00 GMT

Like any form of radio receiver, a global navigation satellite system receiver will be subject to interference from multipath effects arising from the reflection and refraction of its intended satellite signals by both natural and man-made artifacts. However, unlike some radio systems in which a small degree of interference may be tolerable to the end user, multipath interference will have an unacceptable effect on a GNSS receiver, making the output both unstable and inaccurate.

There are several multipath mitigation techniques available to the GNSS receiver designer, but in order both to assess the initial requirement and to measure the effectiveness of the measures taken, extensive testing is required. However, such is the diversity of multipath effects and the factors creating them, it is simply impractical to reproduce any form of meaningful tests using real-world GNSS signals. Therefore, the use of a GNSS simulator in the laboratory offers the only practical solution.

Crucially, a simulator system with suitable software will be able to simulate all the various multipath effects both singly and in combination, enabling designers to assess the true performance of their designs and the effectiveness of their multipath mitigation strategies.

A good GNSS simulator will not only offer the pre-defined models already discussed, but will also give the user complete control of the signals being generated. This control will allow intricate manipulation of the signals at digital baseband, which in turn will allow any effect to be implemented.

Remember, the most important thing is to ensure that you have complete knowledge of your test signals at all times. The moment you stimulate your receiver with an unquantified signal is the moment you introduce unwanted uncertainty into your tests.

With more and more users coming to rely on the accuracy of their global navigation satellite system receivers and increasing numbers of location-based services leveraging this accuracy, it is clear that interference caused by multipath effects cannot be allowed to compromise the accuracy of any GNSS receiver design. Ever-more sophisticated and effective multipath mitigation techniques are becoming available, but it is only by controlled, analytical and statistical testing at all stages of the design process that these techniques can be applied and proven.

Real-world testing can never hope to repeatably replicate all the potential multipath effects that may occur in a limited test timescale, nor can it be expected to provide even a representative sample. And so the use of laboratory-based simulation offers the most reliable, accurate and repeatable test regime to ensure the performance of any GNSS receiver design.

However, not all simulators are born equal, and not all simulation software is made equal. Only by choosing high-quality proven multichannel hardware with software that provides maximum coverage of multipath effects and models can the GNSS receiver designer be sure that his or her design will provide optimum performance under all conditions. And that is the route to end-user satisfaction.

If you want more information on Spirent’s range of GNSS simulators contact globalsales@spirent.com or check out our website at www.spirent.com.

Why are we Talking About Multi GNSS?

Tue, 16 Feb 2010 16:20:00 GMT

A few years ago the Sat Nav system in your car was considered a luxury now almost every PDA, mobile phone and PC has built-in GPS technology. However, navigation and positioning technology is no longer just about GPS L1 C/A code. The GPS constellation is being modernized, the GLONASS constellation is nearly complete with 19 satellites transmitting as you read, new systems including the Japanese QZSS, the European Galileo and the Chinese Compass constellations are on the way.

GPS, the backbone of our current satellite navigation systems for the past 10 years will be only one of four Global Navigation Satellite Systems (GNSS) and four Satellite Based Augmentation Systems (SBAS) which will be available by the middle of this decade. There are already over 60 GNSS and SBAS satellites in operation (including 32 from GPS) and more than 130 are planned.

This multi-GNSS environment offers opportunities to improve performance to meet increasing user demands. In particular, end user availability is potentially improved by using more than one constellation. Benefits to end users can also include improved integrity, continuity and accuracy, depending on the situation and priorities of the application. However this multi-GNSS environment also offers significant opportunities and challenges to GNSS technology, system and application developers’ to meet these increasing user demands e.g.

How can I test future signals which do not yet exist in space such as L5 and L2C on GPS
How can I test GNSS constellations which are partially deployed or not yet deployed (e.g. Galileo)

Well, there is a solution. GPS/GNSS simulators generate the same kinds of signals transmitted by GPS/GNSS satellites, thus GPS/GNSS receivers process the simulated signals in exactly the same way as signals from actual satellites. GPS/GNSS simulators are the most powerful test method for GNSS receivers and applications and have three core attributes which are more difficult (or impossible) to achieve using live-sky test methods, these are repeatability, control and scalability:

A simulator can repeat exactly the scenario time-after-time enabling test results to be accurately compared during the development process. Further, repeatable test scenarios can be created to include failure conditions which may only occur sporadically or randomly in the real-world and are difficult to capture with live-sky methods
Simulators give full control over all the performance characteristics for GNSS. This means that single scenario can be created which tests a GNSS receiver or application in ways which might require several live-sky sessions to emulate. The control of GNSS parameters offered by a simulator also means clearly defined, repeatable and rigorous test standards can be established and documented
Simulators offer a scalable test method
Only simulators can test for GNSS constellations which are partially deployed (e.g. Galileo) and future signals such as L5 and L2C on GPS
Simulators are the best solution for establishing test standards across dispersed development centers
Simulators generate data for multiple scenarios and locations compared to live-sky, which are single-scenario single-location solutions
Simulator test scenarios can be quickly added or modified through software whereas changing test conditions for live-sky requires new data to be collected

GNSS simulators are the test method of choice for developers and test engineers. They offer a rigorous, repeatable and cost effective means of exploring, benchmarking and testing GNSS receivers, applications and systems. No other method can offer the same flexibility in generating new test scenarios or the ability to incorporate both current and future GNSS and frequencies.

If you’re not working on multi-GNSS projects right now, you probably will be soon. Find out more on how Spirent’s multi-GNSS product portfolio could help you today.

Why use a GNSS Simulator?

Thu, 11 Feb 2010 05:00:00 GMT

Why would I use a GPS / GNSS simulator, if I want to do any testing I just stick my antenna out of the window, attach it to my receiver and away I go. Well that’s all good and well but for those requiring a rigorous GPS test environment, live sky testing has some serious limitations including a lack of repeatability, control and scalability. Not to mention the fact that you can’t test future signals in space e.g. GPS L2C and L5, partially deployed constellations or constellations that don’t yet exist.

Testing with GPS / GNSS simulators is the widely-accepted best practice for validating the performance of GNSS receivers and systems in many different scenarios and operating conditions in a controlled laboratory environment. Simulators are used extensively in academia and industry, in virtually all GNSS receiver manufacturing and major system integration, and in many different application fields, including navigation, positioning, telecommunications, aviation, automotive, and space, for both civilian and military applications. Using simulators facilitates several stages of research and product development, including requirements analysis, design and development, integration, production, maintenance, and support.

GNSS simulators provide many benefits, including

Control. Simulators allow complete control over all aspects of test scenarios, including GNSS constellation signals and environmental conditions.
Flexibility. Users can easily define different scenarios for different testing needs.
Completeness. Equipment can be tested under different operating conditions, ranging from nominal to extreme, including conditions that are impractical or impossible to produce in live testing.
Repeatability. Test scenarios are the same every time they are executed.
Reliability. Because all test conditions are controlled, test results are reliable, and equipment performance can be evaluated against known truth data.
Cost. Tests are conducted in the laboratory, without extra expenses for field tests and test vehicles.
Efficiency. Many different tests can be completed in the same laboratory test bed, without reconfiguring or relocating equipment. New test scenarios can be created and executed quickly.
Realism. The performance of GNSS receivers and systems are tested using the actual hardware. Simulators with real-time control capabilities support advanced hardware-in-the-loop (HWIL) testing.
Future. Simulators provide effective means of testing new and future GNSS capabilities that are not yet supported by actual constellations, such as the GPS L2C and L5 signals and the Galileo system.

A summary of the advantages of testing with GNSS simulators, compared to live testing with actual GNSS constellations, is shown in the table below.

Live Testing with Actual GNSS Constellations	Laboratory Testing with GNSS Simulators
No control over constellation signals	Complete control over constellation signals
Limited control over environmental conditions	Complete control over environmental conditions
Not repeatable; conditions are always changing	Fully repeatable
Unintended interference from FM, radar, etc.	No unintended interference signals
Unwanted signal multipath and obscuration	No unwanted signal effects
No way to test with GNSS constellation errors	Easily test scenarios with GNSS constellation errors
Expensive field testing and vehicle trials	Cost-effective testing in laboratory
Limited to signals available in GNSS constellations	Testing of present and future GNSS signals
Competitors can monitor field testing	Testing conducted in secure laboratory

If ensuring final product quality, whilst also meeting tight project timescales is important to you then you require the capability to simulate realistic, repeatable and controlled GNSS signals that our single-channel and multi-channel test platforms offer.

If you want more information on how we can help, contact globalsales@spirent.com or download our Application Note: Benefits, challenges and test considerations for GNSS Technology Developers.

What is a GNSS Simulator?

Tue, 09 Feb 2010 05:00:00 GMT

We sometimes get carried away into thinking everyone must know what a GNSS simulator is but in reality the proliferation of GPS / GNSS applications into many aspects of technology in such a short time span means that some people have had very little experience with GPS / GNSS technologies. So for those non-experts out there, I’d like to help. A GNSS simulator is a signal generator that provides an effective and efficient means of testing GNSS receivers and the systems that rely on them. A GNSS simulator provides control over the signals generated by GNSS constellations and over the global test environment all within a box, so that testing can be conducted in controlled laboratory conditions. GNSS simulators generate the same kinds of signals transmitted by GNSS satellites, thus GNSS receivers process the simulated signals in exactly the same way as signals from actual satellites.

A GNSS simulator provides a superior alternative for testing compared to using actual GNSS signals in a live environment. Unlike live testing, testing with simulators provides full control of the simulated satellite signals and the simulated environmental conditions. With a GNSS simulator, users can easily generate and run many different scenarios for diverse kinds of tests, with complete control over

Date, time, and location. Simulators generate GNSS constellation signals for any location and time. Scenarios for any location around the world or in space, with different times in the past, present, or future, can all be tested without leaving the laboratory.
Vehicle motion. Simulators model the motion of the vehicles containing GNSS receivers, such as aircraft, ships, or automobiles. Scenarios involving vehicle dynamics for different routes and trajectories anywhere in the world can all be tested without moving the equipment being tested.
Environmental conditions. Simulators model effects that impact GNSS receiver performance, such as atmospheric conditions, obscurations, multipath reflections, antenna characteristics, and interference signals. Various combinations and levels of these effects can all be tested in the same controlled laboratory environment.
Signal errors and inaccuracies. Simulators provide control over the content and characteristics of GNSS constellation signals. Tests can be run to determine how equipment would perform if various GNSS constellation signal errors occurred.

Hopefully that brief overview helped but if you want more information on how Spirent’s range of GNSS simulators can help you, visit our website or contact globalsales@spirent.com.