Fixed Wing: Extremely erratic altitude reporting with GPS

[DzikuVx]

Since my airplane is operational, let's do some FW testing :)

• INAV 1.2
• Flip32 with Baro and Mag
• NEO-6M -> fix with 5-7 sats

Symptoms: with GPS fix, altitude reported over MSP to OSD (not sure about internal data since not using PosHold/AltHold on FW yet) was extremely erratic: from -50m to 100m in few seconds.

I suspect it's a result of extremely poor fix and the fact that FW uses GPS for altitude and vertical velocity. Probably this is fine with good GPS module and fix.

@digitalentity I suspect setting w_z_gps_p and w_z_gps_v to 0 would force INAV to use BARO for altitude, but this is hard to do without extremely good knowledge of software. Maybe some kind of a CLI swith to simplify the thing?

[digitalentity]

There is a bug in GPS/BARO combined handling of altitude, #543 is a work in progress to improve things. Basically current code is unable to handle altitude correctly if BARO and GPS are misaligned and they likely are.

As a workaround one can zero out w_z_gps_p and w_z_gps_v exactly as you suggested, or zero out w_z_baro_p and w_z_baro_v to leave only GPS as a reference.

EDIT: #543 is not ready to be used for AH/PH/RTH yet, but you can try using it to collect some data for analysis.

[oleost]

Or as the wiki suggest ;)

Disable barometer (set baro_hardware = 1) on Naze32 targets due to critical bug that can make airplane dive to ground.

[DzikuVx]

@oleost I prefer to use baro over GPS altitude. Somehow I do not trust GPS altitude too much

[oleost]

Sure it's better with baro, please report back if zeroing w_z_gps_p and w_z_gps_v does the trick. Then I can update the wiki to reflect that instead.

[sppnk]

I have to say I am flying several planes without baro (CC3D and nazes) with great success. Better to get the baro code working, but not needed.

[digitalentity]

Hope to test over the weekend

[digitalentity]

This should be fixed by #543

[digitalentity]

Still unable to test. If somebody can flight-test this (GPS + BARO enabled) and provide a log it would be awesome.

[oleost]

I was supposed to test it the day I was out, but forgot to enable barometer :/

Will test as soon as weather permits.

[digitalentity]

@oleost no worries! Also no need to engage any navigation modes, just recording a simple log will do just fine.

[digitalentity]

@oleost any chance you would be able to test this in the nearest future?

[oleost]

I hope so.

Just waiting for weather. My wing and battery is charged and ready

[oleost]

@digitalentity Looking at the weather forecast I either get a chance tomorrow if not there will be heavy wind for one week.

[digitalentity]

@oleost, @DzikuVx I'm closing this for now - looks fine on the bench. Don't have the time to flight-test it here, but I doubt it will misbehave in flight.

If you encounter this bug again - feel free to reopen.

[oleost]

@digitalentity https://drive.google.com/open?id=0B-yHzNtOXxbEa181WGstVTRhTGs

Log, CLI dump and video.

I think I noticed some strange altitude loss while circle above home with RTH and first try.

Have not have timed to look at the log myself yet. ( And skip to 8:30 for møre acrobatic flight with rolls and loops )

[oleost]

@digitalentity

There if deffinitly something wierd here. First time doing RTH on this wing. Please have a look.

It also climbs during the seconds Im in GPS hold. Unsure if I had pitch applied on TX to increase altitude but I dont think so.

Height at start:

Height at end:

Keep in mind that my baro is not shieldet in any way as I was not planning to use it. Except that its below one the flightcontroller.

(To synchronize video you can watch when I flip switch its armed and blackbox record is starting.)

[digitalentity]

Ok, looks like it's not a BARO issue, but a GPS issue. During that weird altituce change GPS altitude drops from 40m to -22m (negative altitude):

Baro on the contrary indicate correct behavior (rise in altitude) when airplane climbs in attempt to compensate. Ok, the initial issue is indeed solved - BARO/GPS are synchronised, GPS is preferred, however the altitude estimation itself may be wrong. I wonder if we should favor barometer instead of GPS?

[oleost]

Hmm. How is it on multirotors? (But I would assume that if you have BARO enabled it should be correct and be the preffered way of measuring height, on both multirotors and fixedwing)

But it is wierd, granted I dont have flown this fly very much, but I have done POS/ALT hold without any issues previously. In gps only mode. Wonder why this happend now or if its only concidence.. (Or if I took off with low satelite count and I got addiotonal satelites while that drop is happening)

[martinbudden]

I wonder if we should favor barometer instead of GPS?

From what I've read GPS altitude is less reliable than baro altitude, so favouring barometer makes sense.

[DzikuVx]

I'm for favoring baro over GPS altitude

[digitalentity]

See #842