Surveillance method and system with electromagnetic carrier and plural range limiting signals
Patent 4139844 Issued on February 13, 1979. Estimated Expiration Date: 
October 7, 1997. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.
October 7, 1997. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.Patent References 3707711 3895368 InventorAssigneeApplicationNo. 05/840240 filed on 10/07/1977US Classes:340/572.2, Specified relationship between field and detection frequencies (e.g., nth order harmonics)342/42RADAR TRANSPONDER SYSTEMExaminersPrimary: Swann, III, Glen R.Attorney, Agent or FirmInternational ClassG08B 13/24 (20060101)AbstractA microwave carrier signal is transmitted throughout a surveillance field. Two low frequency signals are also radiated into the field from opposite sides to limit the field size. A miniature receptor-reradiator containing a nonlinear signal mixing device responds to the signals when in the field to return a modulated carrier signal to a receiver device for actuating an alarm. The alarm is actuated only when the receiver detects a signal having a carrier component corresponding to the microwave signal with sideband components corresponding to the sum of the frequencies of the two low frequency signals. | |
