Patent GB-1437611-A (GB)
Title: Navigational radio transponder means
Inventor: ISBISTER E
IPC: C07F15/02
Filed: 1973-05-23
Abstract: 1437611 Radio transponders SPERRY RAND CORP 23 May 1973 [26 June 1972] 24554/73 Heading H4L An interrogator/transponder, Figs. 2a, 2b, 2c for marine signalling, identification or range finding, is operable in different modes and employs a carrier pulse-modulated by a binary message (Figs. la to 1d, not shown) respective groups of bits of which identify the calling station and the operating mode, and are coded/ decoded by a delay line 100 to 114. The called station is identified by another bit group of the message, which also has start-stop elements (BR1, BR2). Transponder operation.-The interrogating signal received by aerial 4 at frequency f 1  is mixed at 3 with the local frequency f 2 , and from the resulting i.f. the binary message is extracted at 6-10 and fed to the delay line at 114, whose number of delay stages equals the number of bits per message. Upon arrival of a complete message, the start-stop bits (BR1, BR2) at 100, 112 enable AND 12, which in turn primes AND's 13, 14, 15, connected to recognize respectively the three possible mode codes carried by the next three bits appearing at 102, 103, 104. Whichever mode is recognized, an OR gate 37 causes a message-length pulse from 36 to open an AND 35 which passes the message in the delay line, from normally open AND 33, to be retransmitted (40, 43, 2, 44) to the interrogating station at f 2 . In MODE 100 (Fig. 1a), AND 13 responds and the transponder recognizes its own (called ship) code at 105, 106, 107 by means of AND 19, so that AND 39 conducts and simply acknowledges with the said retransmission. In MODE 010 (Fig. 1b), AND 14 responds and recognition of its own code at AND 19 causes AND 38 to ring the ship's bell as well as retransmitting. In MODE 001, where selfidentification of the called ship is requested (CQ) AND 15 causes flip-flop 42 to enable a secondary delay line 46 and OR 47, to insert the called ship's code at 105 as the retransmission proceeds. A simple beacon transponder having no interrogating facility and arranged to respond only to the CQ mode is described (Figs. 8, 9, not shown). Interrogator operation.-Operation of switch 48 causes flip-flop 130 to enable AND 56 to pass a pulse from 51 into the delay line at 100. As the pulse proceeds to 113, the taps 101 to 111 drive message generating circuits whose outputs are combined serially by OR 30 to be transmitted at f 1 . The mode code is selected by switch 53 from taps 102, 103, 104. The called ship code is selected by switches 151 to 156 and OR 24. The interrogator's own code is permanently wired at OR 25. The response is received at f 2 , and the interrogator confirms the response as correct by identifying its own code at AND 26, thus priming AND's 27, 28. The called ship code, set at 151-156, is recognized at AND 23 so that AND 27 can prime AND 66d, 66e, 66f and these pass the code to a display 68 for visual confirmation. If switches 151-156 had requested identification (CQ call 000), the received ship's code appearing at 105, 106, 107 goes to the display by way of AND's 66a, 66b, 66c which are enabled by AND 28, itself primed by the CQ code (001) from AND 15. Range measuring.-A known or unknown (CQ) station is interrogated as above, and in addition the time elapsed between end of transmission and end of reply is measured by a counter 62, which is started and stopped by AND's 57, 66 sensing the two end-times. A display 64 indicates the average count obtained from a cumulative count in 62 taken over a number of interrogations (e.g. 256) determined by another counter 73, which thereupon resets a flip-flop 50 to return to transponder mode. Interface with existing radar by simple circuits (Figs. 4, 5, 6, not shown) permits interrogation to be initiated, by operation of switch 48, when the p.p.i. scan passes over a selected target. In addition to the range indication at 64, a variable range marker pulse from the radar may be used at 142, 74 to enable AND's 66a, 66b, 66c to give an identifying code at 68 when the target is unknown (CQ). If the scanning radar aerial is used for the interrogation signals, only a limited number of interrogations during each sweep will be successful. These are identified at the output of AND 66 and used in the modification of Fig. 7 (not shown) to gate only the successful counts from the range counter (310) to a register (313). The successful interrogations are counted in a further counter (315) for averaging purposes, and when a total interrogations counter (305) reaches 256, a divider (314) takes the average and displays it (at 317).