Patent GB-1524679-A (GB)
Title: Radar station identification systems
IPC: G01S13/78
Filed: 1975-04-01
Abstract: 1524679 Radar transponder; secondary radar systems DEL NORTE TECHNOLOGY Inc 1 April 1975 [1 April 1974] 51196/74 Headings H4L and H4D In a radar system of the kind disclosed in U.S. 3,810,179 and comprising a number of responder stations each individually identifiable by a different pulse repetition rate of received interrogation pulses, each station incorporates an arrangement for generating clock pulses and for counting a predetermined number of these in response to an interrogation pulse received when the station is in a first, armed, state. Receipt of the pulse puts the station into a second, test, state wherein pulses arriving at the identifying frequency, as set by the predetermined clock pulse count, are counted. When a predetermined count is reached the station switches to a third state in which a succession of pulses indicating the receipt of a valid succession of interrogation pulses is generated. (These generated pulses will be returned to the interrogation station so that range can be determined). If while the station is in the third state a predetermined number of interrogation pulses are missed the station reverts to the first state. In the second and third states, interrogation pulses not at the station identifying rate are rejected. The responder described incorporates three counters-a sequence counter 15 which in response to clock pulses from an oscillator 10 and its divider 11 and the count from the main counter provides a number of timed outputs for controlling various operations in the arrangement; a main counter 12, which counts the pulses derived from a clock 10 and divided at 11, and provides an output to a decoder which responds to a particular count indentifying the station to control operation of the main counter and sequence counter; and a lock counter 26 which counts successively received interrogation pulses. The lock counter 26 and latch 14 are first reset, and the resync latch 25 is set by a pulse on line 24, to put the detector into a ready state. The resync latch acts to reset the divider 11 and sequence counter 15. As soon as an interrogate puke is received on line 28, it passes AND gate 30 to reset the latch 25, allowing the divider 11 and sequence counter 15 to operate. When the sequence counter reaches 7, its "7" output sets the latch 14 to enable the main counter 11 and stop the sequence count. When the main counter reaches the count defined in decoder 13 (corresponding to the particular station) the latch 14 is reset, the main counter stops and the sequence counter continues until it reaches "0". Its "0" output opens the AND gate 33 allowing any interrogation pulse appearing during the time of the "0" output to set the latch 32, thus enabling the counter 26. The next "6" output from the sequence counter clocks the counter 26 by 1 and the next "7" resets the latch 32. Thus successive interrogation pulses occurring at the right time enable the lock counter and counting continues until "6" is reached in the lock counter, whereat the AND gate 33 is closed by the output from inverter 38 connected to the lock counter "6" output. This output also opens AND gate 37 so that signals derived from gate latch 40 controlled by the sequence counter 15 can be transmitted by the responder. Output pulses from the counter 26 are also used to trip a single shot multivibrator (not shown in the figure). If this times out, due to the absence of a predetermined number of interrogation pulses, a reset pulse is applied to line 24 and the responder is returned to its initial armed state.