Position holding inaccurate

ArduCopter Traditional Helicopter
1

Hello again,

I did a test flight today and when I activated RTL at the end of the flight then the helicopter went to home location, but started to very strangely act in roll and pitch axis. It was sort of like “lurking” around the home point.
I have also noticed that in loiter the aircraft seems to be moving around more than it should. Especially today as there wasn’t any wind. The poorer position holding was noticed after I changed the GPS setup to two Zubax GNSS 2 units. Previous version of the aircraft used one 3DR GPS and one Zubax GNSS 2 and I haven’t noticed such poor location holding on that one.
Maybe somebody has some ideas what might be the issue and how to solve it?
Also, it’s not evident from this log as this was only hover, but when I have done Auto flights then it tends to be a bit off-course and when it needs to do a loiter unlimited at the end of the mission, then the helicopter starts to again “lurk around” in the vicinity and it also yaws with respect to the location. It kinda looks like a fixed wing loitering around a point and that is strange.
Log file of the flight is here

2

Sound like a so called “toilet bowl” effect. Try calibrating your compasses again.

3

I have done that actually several times by now and it doesn’t look like a classic toilet bowl as it sometimes is holding very well and other times it just drifts around. And it doesn’t go worse like a classic toilet bowl does, there isn’t any spiralling outwards motion. EKF seems to be very happy, at least according to the MissionPlanner EKF viewer. It might still be something to do with compasses, but I haven’t found any reason to suspect it. But thanks for the suggestion, I’ll look into it once more just in case