Satellites

Types of satellites

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Meteorological Satellites - help meteorologists to predict the weather or see what is happening at the moment. The satellites generally have cameras that can take pictures of the earth's climate, both from a point geostacionário fixed as polar orbits.

Satellite Communications - allows data and telephone conversations can be retransmitted via a satellite. Satellites to cover the Telstar communications and Intelsat.

Satellite broadcasting of television - send signals from one point to another (similar to the satellites of communication).

Satellites Scientific - perform a variety of scientific missions. The Hubble space telescope is one of the most famous scientific satellites, but there are several others to observe a bit of everything from patches to solar gamma rays.

Satellite Navigation - to help ships and aircraft to navigate. The most famous are the GPS satellites NAVSTAR.

Rescue Satellite - to respond to radio signals at the request of relief.

Terrestrial Observation Satellites - of analyzing the planet, looking for changes, from deforestation to the temperature and coverage of the ice pack. The most famous are the series LANDSAT.

Military Satellites - many their real applications remain a secret. The possibilities for the collection of information using electronic, high-tech equipment and a sophisticated recognition of photographic are unlimited.

Different satellite orbits

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The artificial satellites occupy different orbits that have different characteristics. Normally, these routes are defined in relation to Earth.

Most of telecommunication satellites are geostationary satellites, or occupy an orbit geostacionária.

Depending on the altitude at which the satellites are, and depending on the characteristics that describe the orbit, it is possible to define various types of orbits: - GEO (Geosynchronous Earth Orbit): circular orbit equatorial geossíncrona.´

Lperíodo of revolution: 23h56 m4, 091 s

Laltitude average: 35786 km

LEO (Low Earth Orbit): órblaltitude typical: 500-1500 km Lperíodo of revolution 1h 30m-2h

MEO (Medium Earth Orbit): circular orbit of average altitude Laltitude typical: 10400 km Lperíodo of revolution 6 hours

HEO (Highly Elliptical Orbit): elliptic orbit Lórbita elliptic inclined Lperigeu low altitude

Orbit Helium

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Orbit Helium - Synchronous - the helium-synchronous orbit is a particular case of a nearly polar orbit. The satellite travels from the North Pole to the south pole and vice versa, but their plan orbit is always fixed to an observer who is facing Sun Once the satellite passes always approximately the same point on the surface of the Earth every day at the same time. Thus the satellite can transmit all data obtained for an antenna fixed terrestrial during their orbits. Usually the satellites helium-synchronous satellites are of medium and low orbit, with altitudes ranging from 550 to 850 km. Orbitam with an inclination to the equator of 97 to 98 º. As the Earth moves around the sun, to keep your plan in orbit, the satellites of helium-synchronous orbit must make a rotation about 1 ° to the east every day.

Orbit transfer of Hohmann

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Orbit transfer of Hohmann - in space, orbit transfer of Hohmann is an intermediate orbit that the satellite must go to pass a circular orbit to another. It was calculated for the first time by German engineer Walter Hohmann in 1925.

Orbit Geossíncrona

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Orbit Geossíncrona - An orbit is considered geossíncrona when their rotation comes exactly the rotation of the Earth. Unlike orbit geostacionária, the orbit may have an inclination and eccentricity different from zero. When the fuel an artificial satellite communications is over, it is often fail to control such orbital inclination, so that its orbit becomes geossíncrona.

Polar Orbit

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Polar Orbit - a satellite into polar orbit passes over (or almost over) both poles of the planet (or other celestial body) in each of its revolutions. Thus, the orbit inclination is equal or near to 90 degrees at the equator. As the satellite is fixed orbital plane and perpendicular to the rotation of the planet, it will pass on a region with a different longitude each of its orbits. The polar orbits are generally used for satellites, geographical maps, observation or recognition, including spy satellites, as well as some weather satellites. To stay on a polar area, for a long time, despite the distance, is used an elliptic orbit with great eccentricity and with peak on this area, known as the orbit of Molniya.

Low Earth Orbit

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Low Earth Orbit - orbit is one in which the objects, such as satellites, are below the intermediate circular orbit and substantially below the orbit geostacionária but usually between 350 and 1400 km above the surface of the Earth. The orbits lower than this are not stable, and will be dragging atmospheric. The satellites in a low Earth orbit traveling at about 27400 km / h (8 km / s), which represents a revolution of about 90 minutes.

Communication Satellite

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A satellite is any artificial body made by man and placed in orbit around the Earth or any other planet. Today, contrary to what occurred in the early history of artificial satellites, the word satellite has been used almost as a synonym for "artificial satellite". This term has been used when either distinguí them of natural satellites, as the moon.

Currently are in orbit, in addition to the satellites of the Global Positioning System (GPS) satellites for communications, scientific satellites, military satellites, among others, and a large amount of space junk, or should not refer to only satellites as a means of transport of data or only a means spy the earth system.

Coverage of a satellite

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Like ordinary radio repeaters installed on top of a mountain have greater coverage in relation to the horizon and curvature of the earth's crust, the satellites also have the artificial horizon that allows them to broad areas of coverage in the so-called line of sight radio. The polar orbiting satellites in low orbitam the Earth from varying altitudes and that generally start around 300 km and may exceed 2000 km in altitude on poles.

This orbital position, the satellite has an artificial horizon where it is seen and illuminates, in radio terms, the same area that Continental can go from Portugal to America, or cover a substantial part of Europe and Africa.

Signal Processing to the Level Distributor

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The purpose of a data processing center is parametrizar the information received so that it complies with a single standard pre-defined, both in terms of levels of audio-video, and in terms of digital encoding. It is therefore natural that the signals received are processed in different ways since arriving in different formats.

In the case of receiving satellite, reaches the headend (common name of the place where the processing takes place) comes a signal in the band between 900Mhz and 1800Mhz, coded and sometimes encrypted (Nagravision, Viaccess, are examples of types encryption).

The first operation will be the decoding and, if necessary, the desencriptação data received. This operation causes the separation of the video and audio signals, thus creating 2 signs distinct band in the base (in practice are 3 signals since the audio signal is divided into 2-channel left and right).

General applications of satellites

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Depending on the application to be developed with a satellite, it is possible to prepare their respective classification. Thus it can be considered that there are the following types of satellites:

Comunicações:
- Comunicações fixas (FSS, Fixed Satellite Service);
- Difusão (BSS, Broadcasting Satellite Service);
- Comunicações móveis (MSS, Mobile Satellite Service).

Navigation:
- Positioning (GPS, Global Positioning System).

Observation of the earth and atmosphere:
- Meteorology;
- Detection remote. Military:
- Espionage.

In all these applications the satellites reveal be very important, because its ability to cover large areas to achieve that would be more difficult to access if it means land use.

The frequency bands allocated by the system satellite

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To perform satellite communications are used two types of communication, the first is on the upward link (Uplink), in which the transfer is made of a central land for the satellite, and the second is on the downward link (Downlink), in which is the transmission of the satellite to a central land.

Usually the bands of frequencies used for the journey downward are lower than those used in the upward path. This is in order to reduce possible interference and because the downward path is considered more critical that the upward due to the limitations of power the satellite. The fact of the frequency band of the ascending path be higher also allows maximize the gain of receipt, as the noise captured by the satellite is high.

Bands of Frequency

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Multiple Access Techniques

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There are three techniques to various stations that can communicate simultaneously with the same satellite:

Division Multiple Access by the Frequency (FDMA) In this type of access, it is possible that all stations use the satellite simultaneously, but each uses a different band of frequencies. Such access is usually used in analog transmission.
By Division Multiple Access in Time (TDMA) This type of access is only allowed to broadcast stations one at a time on a given range of frequencies, using the "slot" time allocated. This type of access is typically used in digital transmission.
Division Multiple Access for in the Code (CDMA) In this type of access, it is possible that several stations broadcast simultaneously in the same frequency signals scattered by the spectrum, encoding the signals ortogonalmente. To recover a given signal is necessary to have knowledge of the code that was used to disperse the signal in the spectrum.