fixed altitude estimator error estimation

[RomanLut]

ISSUE:

The signed difference is currently used to update the error estimation in the altitude estimator. A large negative difference incorrectly reduces the error, while it should actually increase the error.

Explanation: The difference gpsAltResudual should be passed through fabsf() to ensure that the error magnitude is always positive, preventing incorrect reduction of the error when the difference is negative.

IS:

const float gpsAltResudual = posEstimator.gps.pos.z - posEstimator.est.pos.z;
ctx->newEPV = updateEPE(posEstimator.est.epv, ctx->dt, MAX(posEstimator.gps.epv, gpsAltResudual), positionEstimationConfig()->w_z_gps_p);

SHOULD:

ctx->newEPV = updateEPE(posEstimator.est.epv, ctx->dt, MAX(posEstimator.gps.epv, fabsf(gpsAltResudual)), positionEstimationConfig()->w_z_gps_p);

[Jetrell]

@RomanLut I ran some tests with a copter in Poshold, using this commit against the old method. At the same location, and all tests done within 5 minutes of each other.

I ran your change first... The first test was armed with a HDOP of 1.4.. It held the target rather well. With minimal vertical drift, based on line of sight reference. Maybe 0.2m drift over a period of 4 minutes.
Then I loaded another battery and armed with a HDOP of 1.0.. It too held the same position a little better again.

Then I loaded the old method, before this was merged. It was armed with a HDOP of 0.8.. It started out looking okay, by holding its altitude within the same variance.. But after around 80 seconds, it slowly started to rise in altitude, to a maximum of 0.9m above what it was when I entered Poshold.. Which was typical of what I had seen previously in other testing.. This too was based on line of sight reference, from the position above the ground I enabled Poshold.
So your fix appears to have made a noticeable improvement. 👍

The interesting part about testing this stuff, is the vertical drift can't be reliably referenced with the logs alone, due to the Target Altitude rarely being correct, based on the ground arming reference.