[TheOtherCliff]

You may find a slow oscillation (oscillation period is about 4 seconds or so) in GPS flight modes if you are using an authentic DJI GPS.  The DJI clone I have doesn't have this issue.
Edit: but there is at least one report of the issue happening with a clone and the fix helping.

It appears that the cause is that DJI smooths their GPS data more than LP expects, and the smoothing causes delays.

You can see the smoothing in the size of the pattern of the GPS trail displayed in OpMap (enable diagnostics and watch the green trail) when the quad is motionless on the ground.  Do this for the authentic DJI GPS, then compare the size of that pattern with the size of a pattern produced by any other supported GPS.

You can reproduce the effect with an OP GPS or other Ublox based GPS by setting GPSSettings->UbxDynamicModel to Pedestrian (really bad) or Automotive.  You need a large open area (at least 50mx50m) to test this.  Flying Velocity Roam mode, get up to speed flying east or west, release the stick, and notice that the oscillations do not die down.  Switch back to Attitude mode to recover from the oscillations.  Make sure you have a non-GPS mode to recover with.

Link to the issue tracking details:
https://librepilot.atlassian.net/browse/LP-450

[TheOtherCliff]

Anybody have LibrePilot with a DJI Naza GPS currently flying in VelocityRoam mode?  I would appreciate some feedback on this, as to whether it happens for you or not.

This issue is fairly easy to recreate.  Fly east (or west) and just before it gets to full speed (when the bank angle is still high) release the sticks and let it hover.  It will oscillate east-west and won't stop oscillating.  For me, it does it worst when pointing north and flying east-west.  For me, other directions oscillate less or do not oscillate.  When pointing north, it gets worse and worse and I have to change modes to recover.

[HighAltitude]

It finally warmed up enough to venture outside and test whether my quads exhibit the same behavior, which they do. 
My GPS/Mag modules appear to be genuine DJI units, rather than clones. 
The oscillation is east/west, regardless of the heading of the aircraft, and increases amplitude until the GPS assisted flight mode is switched off.  This includes RTB mode.  It appears that the vehicle overshoots, corrects, overshoots the other direction, etc., etc., which seems consistent with delays caused by data smoothing.
I've read the issue tracking details, but I don't have the experience or expertise to even understand the discussion there.
As long as I can switch off the GPS modes and regain manual control, it's more of an annoyance than a serious problem, but I hope this info helps. 

[TheOtherCliff]

Thanks for the recreate!

I have a 16.09 compatible firmware that helps or fixes it.  I need to tune it more and maybe you can beta test after I fly it. 

I don't know whether it will be before or after this weekend.

[TheOtherCliff]

Here is the firmware for this issue.  It's compatible with 16.09 so just flash the firmware and leave the settings as they were in stock 16.09 although it never hurts to back up your settings before messing with things you have not messed with before.  I've test flown the Sparky2 version and it seems to completely fix the issue for me.

This only helps for the DJI/Naza GPS type.

Anybody that has this issue and tests this firmware, please report back with your findings. 

I thought this was only needed for authentic DJI GPS's but there are reports of clones with the issue too.  The authentic version looks exactly like the attached picture.  This issue does not happen with some DJI GPS clones because they do not smooth (and thus delay) the GPS data as much.

[HighAltitude]

Weather permitting, I'll try it with my "development" quad this afternoon and let you know.

[HighAltitude]

Since my quad is flying very nicely (except for the oscillations), and I don't want to have to start from scratch setting it up again, is this the appropriate procedure for trying out the firmware fix?
1. Export UAV settings.
2. Download and save the firmware.
3. Upgrade (? and Erase?).
4. Import UAV settings.
5. Test fly

[TheOtherCliff]

If you are currently running 16.09:
Download firmware to some known place like Desktop.
Run GCS.
Plug in just FC USB (doesn't work via OpLink).
Press Firmware -> Halt (and wait for up to 30 seconds for Open / Flash / Retrieve buttons to appear)
Press Open.
Navigate the "Select Firmware File" file chooser to Desktop or where ever you have the new fw_revolution.opfw
Select the opfw file and press Open in the file chooser.
Chooser closes itself.
Press Flash on the main Firmware page (and wait for it to erase then flash, takes over 30 seconds)

That's all because this firmware is 16.09 compatible.

[TheOtherCliff]

It's a good idea to back up your settings with export, but it isn't necessary when the new firmware is the same as or compatible with your current firmware.
This firmware is completely compatible with 16.09

[HighAltitude]

Thanks for the detailed instructions on installing the firmware fix.  All other settings were unaltered.  The oscillations are much improved.

[Tongala]

Hi just found this thread.
Recently upgraded from RC2 and after upgrade and erase now have these oscillations in position hold, worse when facing south. Don't know if facing south is because I live in southern hemisphere Australia.
Have authentic DJI gps.
Did not have any gps problems with RC2,PH was rock solid as was auto land, so wondering what has changed from RC2 to final release?
Will try the firmware posted.
If that doesn't work might have to go back to RC2.
Any thoughts?
Thanks

[Tongala]

Just a follow up.

Installed firmware, re-calibrated mags, oscillations almost gone but still unusable for gps modes.
Quad continually drifts around and up and down until it becomes unstable, the longer it stays in PH the worse it gets.
 
Before I upgraded to Black Rhino the quad just sat in the air with no more than about +- 500mm variation.

Went back to RC2 (erase and upgrade), imported UAV file from that release (when gps worked), re-calibrated mags,same problem as new firmware. Go figure.

The Uav file I loaded into RC2 was from when gps PH worked perfectly.

Am about to give up on ever getting this authentic DJI gps to work again.

Any ideas or advice before I bin my DJI gps units (2 No.) and go to clones?

Thanks in advance.

[TheOtherCliff]

Before this fix, I had the issue come and go too, but now I don't have any of the "gets worse and worse".  There still is some damped oscillation though.

Good position hold requires a good mag calibration.  I also always set "Rotate Virtual" for perfect driftless hover in Basic mode Attitude mode and then switch to INS13 and tweak mag board rotation to get driftless hover in INS13 mode Attitude mode.  It is explained in the section titled: Fine Tuning Your Hover To Stop Drift
Search for that title in:
https://librepilot.atlassian.net/wiki/display/LPDOC/Aux+Mag+Setup+and+Calibration

[bgiannes]

I do have a DJI clone GPS, with the oscillation problem.  however my 450 size drone is solid in none GPS modes.

This firmware fixed my issue, the quad PH's very well now.  v happy with this fix, thank you.

[TheOtherCliff]

Thank you for your post.  That is good information to know.  The DJI/Naza GPS clone I tested with did not have the oscillation problem, but now we know that some clones do have this issue.