For carrying out an interrogation a position modulated laser light wave is used. A key word is displayed in an interrogator and coded in accordance with a cyclic code, for example Golay's code. This coded key word is used for modulating the laser wave. In the responder the same key word is displayed and coded in the same way. After reception and demodulation each message is compared in a correlation circuit with the local reference word. The correlation method used allows parasite pulses to be taken into account and the word received to be validated with a given security threshold.

A communication system is provided for identifying the position of aircraft in a surveillance area. The communication system includes a dual-frequency interrogator which produces two signals at different frequencies. These signals are transmitted over a narrow beam, scanning antenna, the antenna being located within the surveillance area. A transponder is provided on the aircraft with an auxiliary antenna assembly tuned to a frequency equivalent to the difference between the dual frequencies. The transponder produces a signal containing aircraft identification information. A processing unit is co-located on the aircraft with the transponder to receive the two signals transmitted by the interrogator and produce a signal equivalent to the difference between those signals. The difference signal produced by the processing unit is coupled with the transponder and is used to identify the position of aircraft on the ground. A stationary antenna and receiver are tuned to the frequency of the transponder response and receive the transponder response. The stationary receiver transmits information to a position-estimating computer regarding the time of arrival of the aircraft signal. The position-estimating computer also receives information from the interrogator regarding the time of interrogation and the position of the antenna at the interrogation time. The position-estimating computer processes this information to identify the position of the aircraft on the airport surface.

The conventional way of achieving an identification, friend or foe (IFF) function for a moveable subject requires the subject to carry a transponder that emits a coded return when a radar pulse is received by its receiver. In the present invention, an IFF system is disclosed which departs from the conventional transponder method and employs no radio-frequency power source. Instead, it uses the scattering or reflecting properties of an antenna to modulate, in a distinctive manner, the radar return from a moveable subject. A binary sequence code generator modulates the impedance of a receiver antenna, with the reflected signal being detected and correlated with a synchronized code sequence at the radar.

A top mounted buoy signaling device for marine navigation which is selectively activated in response to a coded address signal. The buoy signaling device includes a buoy having a top mounted receiver assembly which is activated by a transmitter assembly. The receiver circuit, transmitter circuit and antennas therefor are printed on circuit boards. The transmitter circuit includes an encoder chip which is preset to provide a coded address. The coded address is utilized to modulate a carrier signal to provide a coded address signal to activate the receiver circuit. The receiver circuit includes a decoder chip which is preset to the coded address of the corresponding transmitter. A lamp driver subcircuit is enabled in response to the coded address signal. The lamp driver subcircuit includes a master timer subcircuit which maintains the driver subcircuit in an operative condition for a predetermined period of time and a flash timer subcircuit which periodically energizes a lamp to provide a flashed output. The receiver circuit may also include a power timer subcircuit to periodically energize the receiver circuit for short-interval operation and a sweep circuit to allow the receiver circuit to receive coded address signals within a predetermined bandwidth.

A method of communicating to a RF tag having a low power mode and a scan mode with a radar and an interrogator. The method comprises the steps of alternating the RF tag between the low power mode and the scan mode and then transmitting a wake-up call to the RF tag with the radar. Next, the wake-up call is received from the radar by the RF tag when the RF tag is in the scan mode. Once the wake-up call has been received, the radar will transmit a downlink message to the RF tag. Upon receipt of the downlink message, the RF tag will send an uplinked message to the radar. After the uplink message has been sent to the radar, the RF tag will return to the low power mode. By alternating the RF tag between the low power mode and the scan mode, the power consumption of the RF tag is greatly reduced thereby increasing the battery life thereof.