flight mode manager: fix terrain hold

[AlexKlimaj]

Continuation of #21885 and should fix #22221

Flew under the deck which caused the GNSS fusion to stop then flew back out.
https://review.px4.io/plot_app?log=d6be638f-4ca1-4ed9-ae6b-8969ea724a9a

On a 650
https://review.px4.io/plot_app?log=de32ab72-e175-4ea4-8d6b-a76fae956b7e

[dakejahl]

This fixes terrain hold 🎉 Hovering very close to the ground, the drone maintains its altitude. Previously it would sink due to ground effect on the baro.
https://review.px4.io/plot_app?log=96c9a5e8-2b36-484b-8ccf-c9de20a5030b

[DronecodeBot]

This pull request has been mentioned on Discussion Forum for PX4, Pixhawk, QGroundControl, MAVSDK, MAVLink. There might be relevant details there:

https://discuss.px4.io/t/height-estimation-error/34674/2

[bresch]

Looks good. Should we also set the default value of MPC_ALT_MODE to "hold"?
What's the behavior if there is no range-finder?

[AlexKlimaj]

Looks good. Should we also set the default value of MPC_ALT_MODE to "hold"? What's the behavior if there is no range-finder?

It depends on _dist_to_bottom being valid. Looks like it shouldn't use terrain hold without a distance sensor. Needs testing though.

[bresch]

It depends on _dist_to_bottom being valid. Looks like it shouldn't use terrain hold without a distance sensor. Needs testing though.

Looks good, yes, _dist_to_bottom is dist_bottom when dist_bottom_valid and NAN otherwise. dist_bottom_valid is only true when there is constant range finder fusion.

[DronecodeBot]

This pull request has been mentioned on Discussion Forum for PX4, Pixhawk, QGroundControl, MAVSDK, MAVLink. There might be relevant details there:

https://discuss.px4.io/t/px4-maintainers-call-november-07-2023/35031/4