EKF2: constant position aiding

[bresch]

Solved Problem

When on a catapult, the vehicle might need to throttle up before triggering the catapult. This makes the "at rest" flag false (and this is correct) and the ekf goes into inertial dead-reckoning if no other aiding source is active.

Solution

Since we know that the vehicle doesn't move while on a catapult, tell the ekf that that the vehicle stays at a constant position. The EKF can then start a "valid" fake position fusion. Then, if a valid fake position is fused and that we expect to have airspeed fusion in air, the local position can remain valid too.

Changelog Entry

For release notes:

New parameter: -
Documentation: -

Test coverage

unit test

[haumarco]

Do we need constant_pos and vehicle_at_rest?
Why dont we include the constant_pos condition in the land detector and make at_rest also dependent on the launch_detection_state?

[bresch]

Why dont we include the constant_pos condition in the land detector and make at_rest also dependent on the launch_detection_state?

Because when the vehicle is armed and throttling up on a catapult it's not exactly at rest; it shakes and fusing the zero gyro updates or zero velocity updates could be harmful

Do we need constant_pos and vehicle_at_rest?

"At rest" means that the vehicle is not moving at all, not rotating, nothing. "constant pos" is a bit more relaxed, it just means that the vehicle stays in the same location; it's not an issue if moved by a couple of cm or rotated.

[dagar]

Overall looks good, but why do we need both valid_fake_pos and constant_pos? To be honest valid_fake_pos sounds like an oxymoron.

[bresch]

Overall looks good, but why do we need both valid_fake_pos and constant_pos? To be honest valid_fake_pos sounds like a oxymoron.

constant_pos is an input status while valid_fake_pos is when the fusion is active. I called it valid_fake_pos because it's using all the functions of fake_pos and the same aid_src message. I agree that the name sounds a bit weird but maybe the best option would be to change the name "fake pos" to something else (constant position?).

[bresch]

I'll merge it as it works nicely in practice but we can still discuss about the name and I'll be happy to change it.

[dagar]

Yes renaming to "constant position" (and "constant height") sounds much clearer.