Pesky Unhealthy AHRS. Can't get rid of it!

I have two planes which present a nagging “Unhealthy AHRS” in the HUD. Most of my other fixed wings will resolve this once I’m airborne for a few seconds. I also notice that when checking EKF status during flight, “pos_horizon_abs Off” is red. I am running with compass disabled. Flight behavior is normal. RTH, Guided, TKOFF, LAND, Cruise and FBWA all work as intended and have no strange flight characteristics. FC is Matek F405 Wing. I have tried the following to resolve the issue:

Recalibrated accelerometer with the flight controller in the planes (using a spirit level).
GPS.HDop, GPS.NSats all good.
Shielded GPS cable and GPS unit from rf using copper tape.
Performed reset to default of all parameters and reloaded.
Flashed other firmware then re-flashed plane and loaded parameters again.

I’ve seen that Unhealthy AHRS is a bit ambiguous and there were mentions to even remove this back in 2019. Can I trouble some of the experts to review my log and perhaps pickup something I’m missing?

https://drive.google.com/file/d/1jaz1cKBjBIrZIYeh4EIoeLZto-vj26MI/view?usp=sharing

Hi @Ed_209

Add a compass. Calibrate and enable the compass.
Result:

  • all
    AHRS: EKF3 active
    AHRS: DCM active
    switching is gone.
  • no more “Unhealthy AHRS”
  • Better flight performance on windy days too.

I’d prefer not to enable compass. Lost 2 planes due to compass errors over the years. It’s unnecessary for my use case. I’ve never had an issue flying without a compass. I’m not concerned about dead reckoning benefits of a compass if there was gps failure. Never had gps fail but have lost 2 planes due to compass. :man_shrugging:

Hi @Ed_209

That’s bad to hear.
I can understand the preference of the simplicity of a plane without a compass.

For me the living in the Netherlands it’s often windy and the flight performance improvement especially in cross wind conditions was worth all the effort to have a healthy compass.

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Yes totally agree and that’s why I tried with compass twice. Both times ended in disaster :joy:.

There are several recent topics on unhealthy AHRS. I’m not sure if something changed with the requirement to trigger this warning but I have started noticing it a lot more on my fixed wing platforms with various flight controllers. I do not use compass on fixed wing so maybe the EKF requirement for good position estimate is set high? As you can see in my logs, I have min 10 sats 17 max, hdop min=.61 max=1.16 so GPS is healthy and providing good position information.

Here is a similar topic and it seems @rmackay9 mentioned that it could be due to no GPS and that it’s on the todo list to remove when the AHRS isn’t really not healthy. I’m wondering if this is a similar case?

Here is another with rover where it was said that it was going to be removed.

@rmackay9 can you please provide some insight? The EKF reports “pos_horizon_abs Off” as red when I click on it in the HUD but everything seems fine. Guided flights, RTL and LAND all are perfect.

For anyone who stumbles on this with the same issue, the fix was that I had some non-default EK3 parameters that may have been carried over from an old 4.1.0dev build. Changing these parameters for compass-less operation in Plane solved my Unhealthy AHRS issue.

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