EKF2: set baro bias when GNSS is alt ref but fusion is disabled

[bresch]

Solved Problem

Before my global position rework, GNSS altitude was always used to initialise the origin of the local plane frame even when the GNSS altitude is disabled (in EKF2_GPS_CTRL). Some users actually liked this as it gives some sort of quick baro calibration using the GNSS altitude "for free".

Solution

Perform the baro bias correction if EKF2_HGT_REF is set to GNSS, even if altitude fusion is disabled in EKF2_GPS_CTRL.

The screenshot below shows a sitl test with EKF2_HGT_REF = GNSS and EKF2_GPS_CTRL = 5 (lat/, on, 3D vel). We can see that the global altitude is correctly set using GNSS altitude.

Alternatives

We could also automatically trigger a baro calibration when GNSS data is available (gated by some AUTOCAL parameter, as we do for IMU and mag)

Test coverage

unit test to cover this specific case
general SITL tests

[github-actions]

🔎 FLASH Analysis

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Updated: 2024-12-13T13:42:29

[sfuhrer]

I would actually drop a comment in the param descriptions of EKF2_GPS_CTRL and possibly EKF2_HGT_REF that GNSS is still used to initialize in this condition we cover here.

[bresch]

@sfuhrer Thanks for the review, I added a comment in EKF2_HGT_REF

[dronetastic]

I have a question about this change. is the BARO bias mentioned above a different name for entering the barometric pressure similar to how we will calibrate the altimeter on a real aircraft prior to takeoff or landing?

[bresch]

is the BARO bias mentioned above a different name for entering the barometric pressure similar to how we will calibrate the altimeter on a real aircraft prior to takeoff or landing?

It's close, but not exactly the same. The "bias" we're talking about here is a height offset (in meters). The baro calibration done on altimeters is on the QNH, which is used in the equation to convert the measured pressure to an altitude. We also have this parameter available in PX4.

[dronetastic]

Thanks for that - definitely one part I was missing, and not obvious to search for.

If the GPS is providing QFE before takeoff then is it fair to say that this calibration is providing the altitude of the takeoff point? I don't mind trying to provide some additional documentation so the details are more obvious to the next person who looks for it