After the sensor achived calibration level 3, you can read out the calibration parameters via sensor register then stored on the host MCU side.
Then next time when sensor is powering on, you can restore those parameters to sensor register.
Then BNO055 sensor will use those parameters as start point.
But actually, because of changing magnetic environment, we are always recommend user to do calibration after re power on. Only exception is that there is no position changing and no distorting applied to sensor environment during power off and re-power on.
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