Lost GPS in flight today can anybody help me why?

[frogbmth]

Hi all
I went for a flight today, nice clear open skies, lots of cloud cover but everything seemed ok until I got a little away and it seems to lose GPS entirely. This a the first time this happened but recently I had two compass calibration errors a few minutes into flight despite calibrating right before take off. Possibly unrelated and I put those down to wearing Apple Watch.

So this GPS issue seemed to solve itself, I landed, restarted and had an uneventful flight. Can anybody take a loo, and suggest what the cause may be of the issue based on the flight log? Should I be worried?

Thanks in advance!
Andy

DJI Flight Log Viewer - PhantomHelp.com

 

[sar104]

The usual question - do you recall in which direction it was pointing at takeoff?

 

[frogbmth]

Pretty sure to the west

 

[sar104]

There's nothing obviously badly wrong here.



The initialized yaw at takeoff was -95°, which is consistent with your recollection. Furthermore, it is fairly consistent with the motion after takeoff. You rotated the aircraft CCW to -138° (SW) after takeoff, and then applied back elevator. The aircraft moved backwards to the NE, as expected, although not quite straight. It does show significant instability when it stops, however, which looks like a compass/IMU heading mismatch. That's visible in the distance trace above and the measured and computed tracks below.

GPS health drops to zero at that time, but it is clearly not due to a lack of locked satellites, which remain in the 12 - 15 range. That causes a switch to ATTI for 13 s.



Also suggestive is the mismatch between the reported position and the position computed from the inertial data.

It was likely a small but measurable error in the initialized yaw that caused the problem. If you retrieve the DAT file from the mobile device then the magnetometer data should give a definitive answer.

 

[frogbmth]

Wow this is brilliant work thank you so much for your efforts, very much appreciated! My first go at retrieving the files hopefully this is the correct DAT file
Dropbox - 2018-04-13_11-10-06_FLY064.DAT

2018-04-13_11-10-06_FLY064.DAT

For reference here is the takeoff location that I calibrated the compass, just above where we are standing

Thanks again I really appreciate your support with this!

 

[sar104]

That was the correct file and it shows pretty much what I expected, but for a different reason.



The yaw was initialized correctly on power up to -110°, but then you appear to have rotated and inverted the aircraft which resulted in disagreement between the magnetometer and the IMU:



Then the aircraft was stationary for 52 seconds before takeoff, during which time the IMU yaw drifted back towards the magnetic yaw, but they still disagreed by 6° at takeoff.

After takeoff things really go wrong around 34 s when the first significant pitch and roll excursions occur during braking, and the magnetic yaw value goes bad. Then there is the brief period of ATTI mode, after which the FC takes over again but with continuing compass errors. It looks to me more like a badly calibrated compass than anything else.

 

[frogbmth]

Ah ok thanks again really helpful. There is an electric fence close by and the big tower one side, some kind of underground bunker the other. May be they affected the calibration. I will take it someplace in the middle of nowhere and recalibrate.

Your efforts are truly appreciated, cheers!