Abstract: The radiated signal of an instrument landing system (ILS) has several monitor probes for the radiation. For each probe the RF signal is demodulated to obtain a composite signal, from which information is extracted by sampling at specific points of one cycle. Sample voltages are combined by addition, subtraction, and division to obtain relative RF level, total percentage modulation, and difference of the depth of modulation (DMM). A digital monitor is the preferred embodiment, utilizing a microprocessor for converting the sampled voltage into binary data. The monitor accepts inputs from eight probes and is self synchronized. Data is compared with established limits to indicate an alarm condition. In another embodiment, an analog monitor has a processor board for each input to be monitored, and a common control board.