Abstract: This security system monitors a remote intrusion-sensing unit by probing it with a probe signal that is randomly modulated. The intrusion-sensing unit replies with a composite signal in which "secure" or "alarm" status information is superimposed on the random modulation of the probe signal. (The system may if desired be elaborated to accept and utilize other status information, such as "access" or "reverse correlation.")A master unit checks the correlation of the reply-signal modulation with the probe-signal modulation, and generates a special "deception" alarm if the correlation is not in accordance with an established pattern--such as positive correlation, reverse correlation, or correlation varying in some way that is systematic or otherwise determinable by the master unit.