tuning noise levels to the exact chip, tuning for bias of the gyro by removing offset

[jerabaul29]

I cut corners on this specific point (by contrast with the other points, which I hope I got right ^^), but that should be a "smaller perturbation" compared with getting units / signs right:

• the Kalman filter works by comparing the measurements it gets with the dynamic system evolution it expects from its internal model
• to decide what to trust more or less, the Kalman filter needs error bars / uncertainties
• these are from 2 sources: comparing the mismatch dynamic mode vs measurement (ie dynamic, from looking at how well the filter does), and a priori based on the expected noise level of the sensors
• so, to do things really right, one should provide to the Kalman filter the expected noise characterisitcs of the exact sensor one is using, following the datasheet and or experimental measurements / tests

I did not do this and just used the typical default values (that probably correspond to a given set of accel / gyro / mag). But to do it really right, these parameters should be tuned:

https://github.com/adafruit/Adafruit_AHRS/blob/69b3a854be83d8b6c6d897d801882a222d1ae510/src/Adafruit_AHRS_NXPFusion.cpp#L41-L65

also if using gyro these should be adapted to the lat / lon:

https://github.com/adafruit/Adafruit_AHRS/blob/69b3a854be83d8b6c6d897d801882a222d1ae510/src/Adafruit_AHRS_NXPFusion.cpp#L66-L69

That may not play an as big role as the other effects, and may be a bit tricky to get right though...