| abstract |
A coherent frequency, velocity responsive electronic surveillance system detects the motion of a transponder relative to two receiving antennas on opposite ends of a surveillance zone positioned at the exit of a retail store. Motion in opposite directions relative to the two antennas indicates that the transponder is within the surveillance zone. As a means of reducing false alarms, motion of a transponder proximate to, but outside the zone is distinguished from motion inside the zone because motion outside the zone is in the same direction relative to both antennas. False alarms are reduced even further by requiring detection of motion through a selected distance before indicating an alarm. |