In a quadcopter that only has an accelerometer and gyro, and no GPS, how is the drift in pitch/roll corrected for? From what I can understand, the EKFs use GPS measurements for its innovations which flows through to correct pitch/roll, but does not use the gravity vector for innovations. So would flights modes like stabilise and altitude hold use DCM instead of the EKF when flying without GPS, or can the EKF still manage somehow?