Safety/GPS failure params need to clarify position loss

[hamishwillee]

From PX4/PX4-Autopilot#10906 (comment)

I'm noticing that the documentation on this isn't very good: Safety, GPS Failure Navigation (params)
Multiple terms are used "loss of position", "loss of global position", and "loss of GPS". They all have slightly different meanings are are not used consistently. ( @hamishwillee maybe something for a separate discussion)

@Antiheavy Historically GPS and global position loss have been synonymous. Yes, it would be good to make it clear which cases we're actually talking about estimated position loss vs GPS.

My understanding is that for global position EVERYTHING, things that say "GPS" actually mean "estimated global position loss" (ie from EKF).

• This includes NAV_GPSF_LT which refers to "GPS recovery"
• The only exception is GF_SOURCE which allows you to explicitly set the source.

Is that correct?

[Note, so if GPS falls out that is not the trigger for these failsafe actions - that comes a little later after the EKF no longer trusts its position estimate]

We don't actually talk about local position and position failsafe. Is the behaviour the same for multicopter if local position from estimator is lost? (FW does not support local estimation, so ignoring that case).