Low Magnetic Field region Here2 calibration issues


I use the Cube Black with 2 I2C Here2 GPS (in order to use Blending), Arducopter 4.0.3. At this moment we are flying 15 UAV´s with the exact same setup for crop spraying in Brazil. Brazil has one of the lowest magnetic fields on earth: 24 µT (1/2 of the strength than US for example). Adjusting compass in Brazil is a “sensitive” problem for most flight controllers. The automatic compass scale made the calibrating process better in 4.0.3 (but not as good as it was with the first Here GPS).

But I´m having problems with 4 units that were OK for 2 months and now are getting very difficult to adjust compass (one even impossible). Is it possible that the magnetometer used in Here2 gets “less sensible” over time? What kind of data should I send in order to identify better this problem?

When I change the problematic Here2 GPS for a new one, it works again. I checked interference from possible faulty motors, but have not found anything. What could/should I look for?

I appreciate your help.