This is how to perform the DJI Phantom 3 IMU Calibration within the DJI Go App.  Many people were unable to find the “calibrate IMU” option within the DJI GO App but there is where to find it.  I calibrate my DJI Phantom 3’s IMU after every firmware update.  From my experience, performing a Phantom 3 IMU calibration helps to better optimize flight stability, as well as camera performance.  If you’ve ever encountered a “tilted horizon” issue with the Phantom 3’s camera, then there is a good possibility that performing a Phantom 3 IMU calibration may resolve that issue.