I mainly fly in GPS mode but last night decided to give ATTI mode a try as i like the sound of filming whilst drifting there was no wind a very calm evening, i let the phantom hover in a stationary position then flipped the switch to ATTI mode and it began moving backwards on its own .Now I know its supposed to drift but i was under the impression it would drift in the position you moved. Every time i tried to fly forwards or left/right it kept trying to go backwards. Does it need a IMU calibration? In GPS it hovers perfectly no slight drift and it yaws without any drifting problems either, really not sure whats causing this?