Abstract: Apparatus for generating a rate of turn signal includes a gyroscopic device arranged to generate the output indicative of the rate of turn in a desire plane relative to a nominal predetermined direction. The rate of turn signal includes a drift error generated by a drift of the actual predetermined direction and the error is corrected by generating a first signal with the gyroscopic device in a first orientation and a second signal with the gyroscopic device in a second orientation inverted relative to the first orientation so that the readings will now be in the opposite direction, comparing the two signals to determine the drift error and using the drift error to correct the output.