Security system with randomly modulated probe signal

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. For example, the correlation requirement may be controlled by a code that is generated (even randomly) at the intrusion sensor; or the correlation check may be made insensitive to yet further superimposed variations in signal level, frequency, or delay. Such further variations may, for instance, convey specific information about conditions at the remote secured facility--such as motion, sound or vibration there. Preferably the signals in both directions are optical signals transmitted by optic fibers. To make deception as difficult as possible (at least in the context of field operations) even for an intruder who knows exactly how the system works, the probe signal is of very low amplitude and the reply signal of very high amplitude.