Perimeter Security System

A perimeter security system for detecting physical intrusion in a monitored area comprising

• a plurality of narrow-band, low data rate, low power radio units arranged in a network, each of said radio units having a signal transmitter, a signal receiver, a circuit coupled with the receiver to measure the signal strength of signals received by the receiver and produce an output representative of received signal strength, a controller responsive to said circuit output to cause said transmitter to transmit a transmitted signal representative of the strength of the signal received by said receiver and to provide said radio unit with a receive mode and a transmit mode such that said receiver and said transmitter do not operate simultaneously, said radio units being positioned such that each radio unit is within communication range of at least one other radio unit;
• and
• a base station positioned to receive said transmitted signal from one of said radio units and to provide an indication of intrusion into the monitored area.