Land detector edge case in descend mode

[julianoes]

Describe the bug
The land detector seems to detect ground contact in Descend mode (RC land failsafe without GPS).

1. vertical_movement to be false: https://github.com/PX4/Firmware/blob/95dc522b99688a08402ac6462ad28f5dfd338eda/src/modules/land_detector/MulticopterLandDetector.cpp#L154
2. hit_ground to be true: https://github.com/PX4/Firmware/blob/95dc522b99688a08402ac6462ad28f5dfd338eda/src/modules/land_detector/MulticopterLandDetector.cpp#L165

This happens because the z controller only reacts slowly to the new setpoint so the z velocity down is much less than the z velocity setpoint here:
https://github.com/PX4/Firmware/blob/95dc522b99688a08402ac6462ad28f5dfd338eda/src/modules/land_detector/MulticopterLandDetector.cpp#L151-L154

One observation of the descend case is that the velocity setpoint jumps to the land speed instead of gradually reaching it like in Land (with GPS) mode.

To Reproduce
This is hard to reproduce in SITL because the drone seems to follow the setpoint quicker.

Expected behavior
It should not detect ground contact until actually landed.

Log Files and Screenshots

This happened in a failsafe descend test:
https://review.px4.io/plot_app?log=3104d449-dd29-473f-9055-354984ff8afa

This does not happen in normal landing:
https://review.px4.io/plot_app?log=2ea6423f-b232-45c9-8c40-32e775118359

More info in #13143 (comment).

FYI @bresch @MaEtUgR

[MaEtUgR]

Related: #13124, #11683

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[LorenzMeier]

@MaEtUgR Can you think of a simple solution?

[LorenzMeier]

This has been reworked since, closing.