Accelerometers are key because unlike standard helicopters, which use complex mechanics to allow stable flight, Quadrotors use fast onboard motor control to take care of stability. This mechanical simplicity is also their main attraction: Quadrotors can navigate in three dimensions using only four moving parts. And the high reliability of brushless motors makes them a simpler, more reliable alternative to many traditional flying platforms.
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For now most applications such as inspection of power lines, oil rigs or wind turbines, law enforcement surveillance or military reconnaissance are very real-time and do not require much autonomy beyond simple GPS waypoint navigation. However, that may soon change.