Abstract: Apparatus for calibrating the magnetic azimuth detector of a gyromagnetic compass system utilizes the system gyro to provide a plurality of heading references to which the corresponding heading outputs of the magnetic azimuth detector are compared to provide cardinal and inter-cardinal error data from which one cycle and two cycle compensation coefficients are derived. The gyro heading references are compensated for gyro drift in accordance with the proportionate drift occurring with respect to a particular heading reference with respect to the total gyro drift occurring during the calibration procedure in accordance with coincident initial and final heading reference points. The calibration is effected by orienting the aircraft in which the gyromagnetic compass system is installed at the plurality of headings to provide the gyro reference and magnetic azimuth detector data.