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I saw a question posted by @Rocketmagnet (Why do I need a Kalman filter?) about the need for a Kalman filter. In that question, he says he's designing an UAV and lists the following sensors he will incorporate into the design:

  • 3-axis accelerometer

  • 3-axis gyroscope

  • 3-axis magnetometer

  • horizon sensor

  • GPS

  • downward facing ultrasound.

This list made me question my understanding about a few of these sensors:

My questions as a newbie are : 1) do you need a magnetometer if you have a GPS sensor? 2) Do you need a horizon sensor if you have an accelerometer and a gyroscope

Are some of these sensors listed redundant for the purposes of tracking a UAV's position/orientation/navigation?

Thanks!

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    $\begingroup$ Please edit the question and include a link to the question to which you refer $\endgroup$
    – Greenonline
    Commented Apr 18, 2019 at 13:50
  • $\begingroup$ What is a 'horizon sensor'? $\endgroup$
    – FooTheBar
    Commented Apr 18, 2019 at 15:04

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1) Yes, a megnetometer is an instrument that measures magnetism — either the magnetization of a magnetic material like a ferromagnet, or the direction, strength, or relative change of a magnetic field at a particular location. I imagine the GPS sensor gives some approximate latitude and longitude - this isn't something a magnetometer could give you.

2) As for the horizon sensor, it seems unlikely that you would need one. However, if calibrated correctly, it does give you an absolute measurement of the horizon. Accelerometers and gyroscopes could be relative to their starting position so it could help in calibrating those sensors - however, newer accelerometers and gyroscopes (MEMS) are able to observe (near-)absolute measurements

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  • $\begingroup$ Thanks for your response. I was looking at the magnetometer from the standpoint of drone navigation and location. I assumed it was used for location relative to the North Pole, in which case , that is the sort of info you’d get from a gps sensor (and more). Similarly, I figured one wouldn’t need a horizon sensor if you combined the data provided by an accelerometer and the gyroscope. But as @user3150208 mentions below, maybe the reason some of these seemingly redundant sensors are used together is that some of them provide noisy data.... $\endgroup$
    – jrive
    Commented Apr 19, 2019 at 15:52
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Yes, some of the sensors are redundant they complement one another. For example, if you want to do a "position hold" and stay fixed in space the GPS, in theory, should be enough. But the using only GPS you would have to move some number of meters before you notice at the GPS accuracy is measured in meters. But the accelerometer will detect very slight movement. But the accelerometer is so noisy that you can not use it for longer distances. Likewise, the gyros are very sensitive to the rotation but drift badly while the compass is not so sensitive and VERY noise but it has no long term drift

The Kalman filter is very good can be combining complementary sensors and using the strengths of each one. You get much better state estimation if you have a suite of different sensors such that their errors are all from different causes.

AlsoI amVERYsurprized a barometer is not on the list. A downward-looking radar or lidar will have trouble when it flies over a picnic table

But what to use depends on the goals. A high-performance racing drone might us only a gyro and nothing else while a camera drone does not need GPS because the pilot can see the drone. An autonomous drone does need GPS and more.

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