I sent my P4A in for replacement under the care-refresh program after a bad crash on takeoff a few months ago. When I got my replacement I did a short initial flight after updating to latest firmware etc, and all seemed good, and I noticed that the gimbal vibration that was always present in my first one was completely gone.

Anyway, the weather has finally warmed  up enough to do some flying again, and I did a couple of flights yesterday, and was getting GPS errors bad enough it was going into atti mode, flying about 700 meters out from the home point in different directions.

I never experienced GPS errors like this before flying in the same area close to my home, which I have flown a lot, both with my original Phantom 4 as well as my Phantom 4 Advanced.

Should I just open a ticket with DJI up and/or send in my dat files?

When I open the log files in the GO app and go through the flight the GPS errors are showing up, but  I have not analyzed the files any more than that.


Thanks in advance.

I would do a compass calibration and try again.  

Mark The Droner Posted at 3-23 06:00
I would do a compass calibration and try again.

I did a compass calibration after the flights, but did not try any more flights after that. I  was also noticing the dreaded yaw drift during my earlier flights which is the reason I was doing the compass calibration, even though the compass readings on the app looked ok.

I am curious though, is the GPS tied that closely to the compass?

Also, should the GPS quite working while a long ways out, would that not render the RTH feature useless?  Kind of a scary thought..

Yes the ability to give a fixed position and move in the correct direction is tied closely to the compass - meaning you could have plenty of satellites but still have a relatively weak "NavHealth", whereas with a good compass and same satellites, you would have a strong NavHealth.  The ability of the AC finding a fixed position is compromised with a questionable compass, but it still may work okay or may not work okay.  It depends.  If you study your logs, you may see evidence of this.  Just be sure your compass is calibrated, and that it's a good calibration, and once it's calibrated correctly, keep an eye on it.  Don't calibrate it if it doesn't need it because you will just be undoing a good calibration.  Be sure you don't launch near ferrous metal which may confuse your compass on start-up as to where magnetic north is located.  Yes, a useless RTH is scary.  It might even be terrifying.  Good luck.  

If you could provide a .DAT via Dropbox, or similar, we could take a look.

BudWalker Posted at 3-23 07:13
If you could provide a .DAT via Dropbox, or similar, we could take a look.

Which .DAT files?

I downloaded some with the DJI assistant, they are all quite big (from 100 to 400MB)

Also took some right off the ipad mini, which are in the MCDatFlightRecords folder and are all under 10K

cameracollector Posted at 3-23 10:02
Which .DAT files?

I downloaded some with the DJI assistant, they are all quite big (from 100 to 400MB)

The  .DAT from the P4P are best. But, it can be tricky getting the right  .DAT. DJI Assistant will give you a file with a name like
EXPORT_FILE_2019-07-16_20-31-44.DAT that contains the FLYxxx.DAT that we  want. If you already have a EXPORT_FILExxxxx.DAT file then Dropbox  that.
   
The tablet .DAT may have enough info. But, less than 10K sounds small so  I'm not sure if that will have a flight demonstrating the problem.
   
You could also try submitting the .txt for a problem flight.

BudWalker Posted at 3-23 10:51
The  .DAT from the P4P are best. But, it can be tricky getting the right  .DAT. DJI Assistant will give you a file with a name like
EXPORT_FILE_2019-07-16_20-31-44.DAT that contains the FLYxxx.DAT that we  want. If you already have a EXPORT_FILExxxxx.DAT file then Dropbox  that.
   

I got the files with DJI assistant, and they are called FLYxxx.DAT, I'm just not sure which one of the 5 is the one I want. When I look at the flights thru the GO app I know which one, but I can't see a corresponding file name.

I will try and figure it out by looking at timestamps from the video files.

cameracollector Posted at 3-23 11:10
I got the files with DJI assistant, and they are called FLYxxx.DAT, I'm just not sure which one of the 5 is the one I want. When I look at the flights thru the GO app I know which one, but I can't see a corresponding file name.

I will try and figure it out by looking at timestamps from the video files.

Okay I just uploaded the two most recent FLYxxx.DAT files, since the timestamps did not correspond to anything else I had. Hopefully they have the flights where the GPS kicked out on them.

I installed the DATCON program so I could look at them, but even though I have the latest java, I still cant open it.

Anyways, the DAT files are here

cameracollector Posted at 3-23 14:29
Okay I just uploaded the two most recent FLYxxx.DAT files, since the timestamps did not correspond to anything else I had. Hopefully they have the flights where the GPS kicked out on them.

I installed the DATCON program so I could look at them, but even though I have the latest java, I still cant open it.

FLY014.DAT  shows the problem you were referring to. Looks to me like the compass  wasn't calibrated correctly. Have you attached an external device such  as a tracker?
   
In the following plot magYaw is derived from just the magnetometers. Yaw  is the value computed by the FC and is derived from the accelerometers,  gyros and, to a lesser extent, the magnetometers. Ideally Yaw - magYaw  should be a constant 13.89 ° since the geomagnetic declination is  13.89°. But, in this flight Yaw-magYaw varies with Yaw over a 2π period,  a clear indication that the compass isn't calibrated correctly.
   

As Mark The Droner explained the several different data are required for  location solution. In this case the FC can see that Yaw and magYaw  differ, and therefore, can't be confident in the location solution. That  confidence level is expressed as gpsLevel or gpsHealth and has a value  in [0, 5] with 5 being the highest. When gpsLevel drops below 4 the P4P  FC will switch to ATTI mode, as it did in this flight. Note that the  numSats wasn't the problem.
   
   
As you might imagine gpsLevel or gpsHealth is a very confusing label. That's why DatCon uses the label NavHealth.

Best post ever!  

Thanks for the very helpful replies & data analysis.

There are no external devices added, and I did do a compass calibration when I got it back from DJI, along with IMU, vision sensors etc.

Typically I have not had to redo compass calibrations when flying out of the same area all the time, but I will redo calibration again and try some more flights.

Stay safe everyone.

cameracollector Posted at 3-24 13:18
Thanks for the very helpful replies & data analysis.

There are no external devices added, and I did do a compass calibration when I got it back from DJI, along with IMU, vision sensors etc.

Update - I have flown a few times since the last post, calibrating the compass a few times as well.

The compass / GPS dropout has happened a few more times, a couple of times doing waypoint flights and a couple of just normal flights.  One day I flew several waypoint flights and had no issues at all, then other days I have had dropouts and going into atti mode several times during one flight.  Sometimes it's just for a few seconds, one time I had to fly all the way in in atti mode then when it got close to me it went back into GPS mode.

I was wondering if it has to do with location since it's only happened at home, but I've only flown 2 other places since then but the error didn't show.  I doubt it's location though, when I fly my Phantom 4 and Mavic Air I never run into these issues.  I do notice the Mavic Air does make me calibrate the compass at the start of almost every flight, but after that it's rock solid and I've never had it go into atti mode.

I also notice the Mavic Air video signal is a lot more solid that either my Phantom 4 or Phantom 4 advanced, which kind of surprised me, since it uses wifi and some of my flights are through treetop cover.  

One of my P4P is having the same problem. I heard that some of the early P4Ps had GPS module cold solder joints. I'm not sure if the P4A is included in that. I'm sending mine to DJI Colorado to be checked out.