Abstract: A VOR monitoring system responds to the phase difference between two signals to determine the bearing of an aircraft with reference to a transmitting station. Even harmonic distortion interference in the monitored signals produces a bearing error and when the bearing error exceeds a preset limit the monitor of the present invention energizes an alarm. The two monitored signals are identified as a 30 Hz reference and 30 Hz variable signal, each of which is doubled in frequency to 60 Hz and divided by three to produce two 20 Hz signals. One of the 20 Hz signals is identified as a positive error signal and the second as a negative error signal. The negative error signal is phase delayed through a programmable delay by an amount related to the bearing of an aircraft with respect to a transmitting field station. The two error signals are then phase compared and the difference tested against a preset bearing error limit.