Mk II IMU Calibration Background

The Mk II Receiver contains an IMU that has internal magnetometers, accelerometers and gyros. These sensors are imperfect and can have manufacturing bias and linearity errors. Additionally, external magnetic disturbances can cause the magnetometers to make a bad estimate of magnetic north. The IMU has a built-in processing system that attempts to compensate for drift and offsets in the IMU’s sensors and for external magnetic influences. Calibrating the IMU can speed up the internal error correction process.

You should only invalidate the stored calibration set while the IMU is happy and thinks it is self-calibrated (i.e., IMU status is stable at "3333"). This is so you can be confident a new calibration set gets stored right away. If your receiver is not mounted on an ROV, you should only need to get one good calibration set stored and then leave it alone. When you power up the receiver, restoring the IMU to fully calibrated should be simple – like turning the receiver upside down a couple of times and waving it around once or twice.

We recommend you do the initial Mk II IMU dance (see following sections) on your boat or on the dock next to your boat rather than back in the shop, as this is likely to be the least magnetically-disturbed environment and most similar to where it is used in the water. Remember your boat engines are probably big cast iron chunks so try not to do it right on top of them.

Leaving the system sit for a long time doesn’t really improve calibration. The Mk II IMU’s algorithm wants to see motion and change in orientation so it can continually self-calibrate. It starts at “zero” and after enough motion, it determines a set of self-calibration constants. These are what we store in the receiver’s flash memory. When the system starts up again, we re-load this set of self-calibration constants back into the IMU, and it then only needs to see enough motion for the IMU to say “oh yeah, the calibration is reasonable, let’s go.” It’s not starting the calibration from zero again.

The IMU normally goes up and down in calibration confidence as the system moves around.