Remove mode_req_local_alt from Descend mode and handle it in FW position controller

[sfuhrer]

Solved Problem

I recently crashed a VTOL vehicle in FW after disabling both GPS and baro fusion in air, which made it switch from AUTO to TERMINATE mode.

Solution

• remove mode requirement for a valid altitude estimate from DESCEND
• similar to what is done for MC handle invalid v/vz in the FW position controller. For FW we in case of invalid v/vz measurements would likely just want the thrust to be 0, such that the vehicle glides down to the ground while still tracking the loiter (assuming it doesn't stall).

Changelog Entry

For release notes:

Improvement: Remove mode_req_local_alt from Descend mode and handle it in FW position controller

Test coverage

Can be easily tested in SITL: Disable Baro fusion (EKF2_BARO_CTRL), then takeoff in auto mode, then disable GPS fusion --> with current main it goes to Termination mode, with this PR to Descend.

[Jaeyoung-Lim]

Sorry for the crash.

LGTM, just one minor point that you can decide whether to incorporate

[MaEtUgR]

It looks good but I gave it a spin on multicopter and even though it does the right thing in the end the flight task produces invalid setpoints I assume because the wrong one is ran. And the only thing preventing a worse outcome is the emergency failsafe inside the controller 👀 Let me check, there must be a simple fix.

[MaEtUgR]

I'm on it. I think it's because I broke the order here 0770478#diff-2ece9759fa20e22b2e0af81c3f3b5892b934c8535b65d0eea103aa342fe997fdL43 🤦‍♂️

[MaEtUgR]

I found the mistake I made in #21192
a) The big one is that I changed the order of the horizontal and vertical acceleration setpoint and in the case you don't have a vertical velocity estimate (like here) it broke.
b) The smaller but not negligible one was that I reduced the downwards acceleration to a too low value in hindsight. It's too risky that the hover thrust is not perfectly accurate and this results in an ascent instead.

I fixed both in the one last commit, please quickly double-check.

[MaEtUgR]

@sfuhrer I readded my commit (only) 👍

[MaEtUgR]

Thanks for looking into that before other people accidentally crash even though they don't have to 🙏