INAVerr driving me nuts!

[glider]

platform: Pixhawk, CSGShop Neo-M8N GPS (same error with 3DR LEA6H GPS)

If anyone knows or has an idea what may be causing these INAVerr, I'm all ears.

I've posted at DIYDrones, RCG and APM Copter support. The latest reply from Randy was:
I should be more clear. The PM message's long-loops error is a red-herring. The inav errors are likely a real problem. I don't know why in your logs it keeps having problems. Rob apparently looked at your logs and the GPS seems to be updating at the correct rate.

Nobody (literally) has stated what it is exactly, or what can cause these errors. It shows up in Auto Analysis as looping (APM Speed error), but it has been determined that is a false positive.

The real problem is the INAVerr quickly passes 255 and stays there. What are the symptoms? Twitching while in any GPS mode (poshold, Loiter etc.), momentary loss of control. Yet, sometimes it seems to smooth out and will hover 5ft off the ground perfectly.

I haven't yet uploaded the text to put in Excel, but the gaps in data are obvious in the top picture below. It looks like timed events; not random. Poor wiring is not likely because the time curve is linear (no interruptions). The bottom picture was a mission flew last year not exhibiting the problem.

In the meantime I've ordered one of these to test: http://arsovtech.com/?lang=en

 

[Bartman]

@R_Lefebvre can you help please?

 

[glider]

Thanks for the shout out Bartman. I actually found another person who is having INAVerr stuck at 255 with a Pixhawk and CSGShop M8N. He says he doesn't get the "twitching" that I observe, so who knows, maybe I'm imagining it. I flew it the other day. In stab mode it was fine (no simple/ss mode). In poshold it does the twitch, as if there's momentary loss of control.

 

[R_Lefebvre]

There has been some discussion of this recently, and new info presented by Craig Elder, formerly of 3DR:

Just for your reference, the advise we have gotten from ublox is to not move to the M8M which is why we have not produced a GPS based on that chipset.
The evaluation of which GPS is better is not as cut and dried as saying one GPS is inferior to another. There are some places on the planet where using both the Glonass and GPS constellations will give you a faster fix but there are also cases where other equipment on the vehicle can cause interference with the GPS receiver.
In the US with SBAS you won't really see much in the way of a performance increase and the increased risk of interference doesn't really achieve you a net gain unless that time to first fix is really important to you.

If you are going to use the M8N then you will need to make additional provision to shield the GPS from the rest of the electronics on the vehicle. The M8N uses a very wide front end in order to receive both GPS and GLONASS signals. That makes it much more susceptible to noise than either the Ublox 6 or 7 series.

The noise results in degraded performance in the velocity measurements and in turn degrades the inertial navigation performance regardless of HDOP or number of satellites. i.e. you are tracking 20 satellites but there is sufficient noise on each of them that the velocity measurements are worse than tracking fewer satellites with less noise.

Ublox has recommended to me to not use the M8N on a vehicle unless it can be shielded and located far away from all noise sources. They did not want give me a specific number, but the order of magnitude was >1m.

Your results are consistent with their predicted behavior and indicate that interference with the GPS is an issue with your vehicle.

I now believe that these INAV errors are cases where the GPS signal noise causes the Inertial Navigation system to reject the GPS position and/or velocity reporting. Different people have different results with these M8N, and it probably comes down to differences between manufacturers, and different EMI on vehicles.

Nobody has really put the resources in to deep-dive on this subject, so it's mostly educated guesses.