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Occasionally EKF3 estimations going haywire on ground

Issue details
I was just checking GPS RTK mode status in open space without flying with multiple quadcopters. When checking the logs, i found something going really wrong with EKF3 estimations after some time, please have a look of POS log, position and altitude estimations almost going haywire after some time since boot up. But these kind of EKF3 inaccuracies does not arise everytime on ground, it happens very rarely, but in this case it has been observed in 2 quadcopters at same time. I have never seen these kind of POS log when i fly these quadcopters.

From the outside it seems like some trivial issue but I have been through some series of events where i am suspecting some serious underlying problem here, i don’t know what causing EKF3 estimations going haywire like this, it can also possibly due to some faulty hardware/component too but i am seeing this problem on multiple quadcopters. I can clearly see that EKF3 innovations are not happy, but i don’t know what will happen if this kind of behaviour happen in air cause there is no warning or failsafe event happened during the time when drones are on the ground.

But on the other hand, i have never seen these kind of EKF3 estmations inaccuracies when i fly multiple quadcopter except in one case which is mentioned in detail here Continuous uncommanded climb, suspecting EKF issues, before this happened, on this particular quadcopter i was facing much more serious EKF3 inaccuracies which is mentioned here
Too large deviations between raw GPS & EKF3 POS data, i don’t know whether my current issue is having some relationship with it or not.

ArduCopter v4.0.4

[ ] All
[ ] AntennaTracker
[ * ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type

Hardware type
Omnibusf4pro (Omnibus F4 SD)

Drone1 -
Drone2 -

Servers by jDrones