Compass Error - bring it home

[Sberrada]

Hi Everyone,

I was flying my Mavic Pro Monday and made the mistake of taking off within a few feet of my car.
After getting the home point confirmation, I took off, but within 1 min, while the drone was about 300 feet away, I noticed a compass error on my hand control. When I looked up, I saw the drone drift sideways although I had not activated the joysticks (ie it was not holding position).
Fortunately, the drone responded to my hand control and I was able to land it back safely. Afterwards, I placed the drone about 20 feet from my car, and was able to do several flights without any issue.

My questions are as follows:
- when one gets a compass error, does the GPS reading, which identifies the drone location remain functional ? My understanding is that these are 2 different systems, where the compass identifies drone orientation, while the GPS identifies location.
- when one gets a compass error, does the Map view on the hand control, which shows the drone location, remain accurate (with the exception perhaps of the drone triangle symbol on the map, which may be oriented wrong)?
- if the drone had been further and difficult to see, could I have brought it back home by using the hand control map?

Thanks for your help and insights
Sam

 

[sar104]

It's not quite that simple. Position and heading are initially determined by GPS, compass and barometer, but after that are both primarily derived from the IMU rate gyros and accelerometers, with the GPS, compass and barometer only being used to correct for drift in the time-integrated rate sensor data via a technique called sensor fusion. That is how the aircraft is able to hover with sub-meter accuracy even though GPS and barometer data are not that accurate.

When you power up the aircraft the IMU yaw value is initialized by the compass heading. If the compass heading is wrong then the initial IMU yaw value is wrong, which is a problem because the primary computation of yaw is done by adding the time-integrated rate gyro z-axis data to the initial IMU yaw, with the compass data only used to correct for sensor drift.

Firstly, after takeoff, when the FC attempts to move the aircraft in response to stick inputs, it will move in the wrong direction as indicated by the GPS position and velocity data. That triggers heading errors. Secondly, when the aircraft moves away from the magnetic interference and starts to indicate the correct heading, it will now disagree with the IMU yaw too, leading to compass errors.

So yes - the GPS accuracy is not compromised by compass errors, but the IMU position as shown on the map may be less accurate until the aircraft switches to ATTI mode. The heading indicated may be completely wrong, which is why checking that the aircraft orientation on the map is actually correct before takeoff is a good way to ensure that there is no significant magnetic interference.

 

[Sberrada]

First of all, thank you very much for such a detailed response. Your knowledge is clearly very thorough on these incredible machines, and it makes me realize how advanced the technology is.

So considering the inter-relationship between these components: do you believe that when the drone has a compass error and falls into ATTI mode while in flight, can the operator count on the map (hand control screen) to bring the drone back home by observing its movements and controlling it back towards home ?

Thanks again
Sam

 

[Kilrah]

If you only look at position and relative movements but not orientation, yes. Also look at the distance reading.

 

[Sberrada]

Thank you for clarifying.
It is comforting to know that even if this issue happens in flight, there is an opportunity to react and take control of the drone to bring it back home. I suppose that this will work as long as the drone is away from potential obstructions (altitude helps), so that even with drift, there is enough margin and time for the operator to take control without a collision.

Thanks again
Sam

 

[sar104]

First of all, thank you very much for such a detailed response. Your knowledge is clearly very thorough on these incredible machines, and it makes me realize how advanced the technology is.

So considering the inter-relationship between these components: do you believe that when the drone has a compass error and falls into ATTI mode while in flight, can the operator count on the map (hand control screen) to bring the drone back home by observing its movements and controlling it back towards home ?

Thanks again
Sam

Yes - in ATTI mode, provided that it is in ATTI due to compass/heading errors and not loss of GPS signal, the map will be correct.

 

[Sberrada]

Thank you for clarifying the part about ATTI mode and no loss of GPS, because many other scenarios can afflict a drone in the air. Thanks again