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Compass /GPS Failure and Phantom 4 RTK
902 7 2019-7-8
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DJI4Survey
Second Officer

Australia
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All,

Had a pretty big failure on one of our new Phantom 4 RTK.

Prior to take-off the unit prompted for a compass calibration, which was a bit unusual as was flying the unit in the same area the day before.

Completed compass cal all fine, sent the unit on a survey mission and completed 90% of it as usual.

On the second last line of the mission it completely varied from the flight path.  Upon seeing this I attempted to take control by toggle P to A and back to P.  No dice.  The UAV would not take inputs from me at that stage.

I tried RTH, no dice.  I managed to counteract the direction it was travelling, however, not sure if it was coincidence or not.  It then shot the other direction for about 800m.

I got out of the mission planned screen and went into the "FLY", and repeated attempts to take control.  I managed to get some control back but the compass was completely messed up.

Managed to limp the unit home and land with 8% battery remaining.

Have taken in for diagnostics, however, the GPS must have failed also as the flightlog did not show the actual behaviour of the drone.  It had it hovering and darting in a small area (50m x 50m)

Very strange, but thought I would share.  If  you find yourself in trouble, may be worth getting out of the "Plan" screen and into the "FLY" screen to try to take control.

Cheers,
2019-7-8
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Labroides
Captain
Flight distance : 9991457 ft
Australia
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Prior to take-off the unit prompted for a compass calibration, which was a bit unusual as was flying the unit in the same area the day before.
The compass was warning you of a problem close to the launch area - not a problem with the compass.
Recalibrating the compass couldn't possibly fix the problem the compass was warning you about.
The correct action would have been to move away from the problem.

Your compass shouldn't ever need recalibrating unless you modify or rebuild the drone.
If you think it's telling you to recalibrate it, check again and move away from the problem.

2019-7-9
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DJI4Survey
Second Officer

Australia
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That is definitely not the case.  If you have the drone in the back of a metal ute tray, on an aircraft or near high levels of interference between flights it will nearly always ask for a compass calibration prior to the next flight.

The compass calibration is specifically to calibrate the compass based on your current environment and 'reset' any abnormalities that have occurred between flights.

If the compass shouldn't ever need re calibrating then we have had a lot of broken DJI Phantoms over the years.
2019-7-9
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Geebax
Captain
Australia
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DJI4Survey Posted at 7-9 13:50
That is definitely not the case.  If you have the drone in the back of a metal ute tray, on an aircraft or near high levels of interference between flights it will nearly always ask for a compass calibration prior to the next flight.

The compass calibration is specifically to calibrate the compass based on your current environment and 'reset' any abnormalities that have occurred between flights.

What Labroides says is quite correct. In the scenario you are quoting, you should not be firing up and taking off from the back of a 'metal tray ute', as that is almost guaranteed to produce the effect you described.

It is the act of starting up the aircraft under the close influence of ferro-magnetic materials that causes the problem. Then once the aircraft gets into the air, it finds the compass is giving confusing readings, so it internally switches the aircraft to ATTI mode, ignoring the compass and GPS data. This exact situation is reported here time and time again.

Contrary to urban myth, the compass calibration is not intended to compensate for 'current environment and reset any abnormalities that have occured between flights'. It is intended to help the aircraft determine what part of its magnetic footprint is due to the materials used in its construction. This was very necessary when the early Phantoms could be fitted with a variety of cameras and gimbal units, but is not needed any longer since DJI made an integrated aircraft.
2019-7-9
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Labroides
Captain
Flight distance : 9991457 ft
Australia
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DJI4Survey Posted at 7-9 13:50
That is definitely not the case.  If you have the drone in the back of a metal ute tray, on an aircraft or near high levels of interference between flights it will nearly always ask for a compass calibration prior to the next flight.

The compass calibration is specifically to calibrate the compass based on your current environment and 'reset' any abnormalities that have occurred between flights.

That is definitely not the case.  If you have the drone in the back of a metal ute tray, on an aircraft or near high levels of interference between flights it will nearly always ask for a compass calibration prior to the next flight.
Re-read my explanation above and Geebax further details in his post.
Not understanding what compass calibration actually does (and what it can't do) has been complicated by DJI's poor wording in the error message and completely explains the issue you encountered.
The error message actually reads:  Move aircraft or calibrate compass.
In probably 99% of cases, the correct action is to move away from the problem the compass is warning you about.
If the compass shouldn't ever need re calibrating then we have had a lot of broken DJI Phantoms over the years.
Your compasses probably haven't been broken or needed recalibrating.
They have been warning you of problems in the environment you put the drone.
2019-7-9
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DJI4Survey
Second Officer

Australia
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"What Labroides says is quite correct. In the scenario you are quoting, you should not be firing up and taking off from the back of a 'metal tray ute', as that is almost guaranteed to produce the effect you described."

The drone was not taken off from the back of a metal tray, and never is.  It is stored in the boxes on the back of a metal ute for transport on some projects.

" it finds the compass is giving confusing readings, so it internally switches the aircraft to ATTI mode, ignoring the compass and GPS data"


That is not what was described in this instance.  Switching to ATTI mode in the instance of compass error would be completely understandable.  It did not do that.  It would not take inputs after switching between all modes.  

2019-7-9
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DJI4Survey
Second Officer

Australia
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Not all failures are the same, and it is dangerous to suggest that they are.  

I was advising that the only way that I was able to gain any semblance of control over the aircraft was to get out of the planned mission mode and re-enter FLY mode.   This is possibly a software issue with the GSR app when the compass switches off.  I am not sure, but it definitely is a non-standard issue.

If people find themselves in a similar situation, maybe that will help them.
2019-7-9
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Geebax
Captain
Australia
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DJI4Survey Posted at 7-9 20:58
Not all failures are the same, and it is dangerous to suggest that they are.  

I was advising that the only way that I was able to gain any semblance of control over the aircraft was to get out of the planned mission mode and re-enter FLY mode.   This is possibly a software issue with the GSR app when the compass switches off.  I am not sure, but it definitely is a non-standard issue.

The symptoms you describe still sound like a sequence of compass errors, but there is an easy way to find out, go to this location: https://www.phantomhelp.com/LogViewer/Upload/ Follow the instructions to upload your flight log, then come back here and post a link to the displayed log.

And just one small point, carrying the aircraft around in the ute will not have any effect on the compass. It is switching on the aircraft and taking off from a surface that contains ferro-magnetic materials that causes the problem.

It is possible that the software you are using does not know how to handle a compass error if it encounters it, which may explain why you got control back after exitting the mission mode and re-entering the fly mode. That would be a serious deficiency in the program if it were the case.
2019-7-9
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