GB1329902A - Automatic communication systems and transponders therefor - Google Patents

Abstract

1329902 Transponder systems A C COSSOR Ltd 22 June 1972 [30 July 1971] 36032/71 Heading H4L A transponder is provided with switching means adapted to place the transponder in a first mode in which it will respond only to a general call interrogation signal or in a second mode in which it will respond only to an interrogation signal selectively addressed thereto, and an automatic control circuit for the switching means operative in the first mode to set the transponder to the second mode when the transponder has made a predetermined number of replies to a general call interrogation and operative in the second mode to set the transponder to the first mode when the transponder fails to receive a predetermined number of interrogations selectively addressed thereto within a predetermined time. As shown, a shore station 1 tracks a plurality of ships each carrying a transponder 2. The shore station transmits at frequency f 1 and includes a computer 4 which stores the address codes of the vessels together with their co-ordinates in known manner and causes each vessel to be interrogated by its address code when the aerial 3 is on the correct bearing for that vessel. Periodically, the computer causes the selective address signals to be replaced by a general call to which none of the transponders which have already been acquired replies. Replies are received at the shore station via an omnidirectional aerial 15 and receiver 16. If the response is to a selective address signal the store is updated accordingly and if it is in response to a general call, this is registered in the store whereby the vessel responding is acquired for tracking. The transponder is provided with decoders 8 and 9 for the selective and general code respectively, whose outputs are fed via gate 10 or 11 respectively and via OR gate 12 to trigger the transmitter 12 to transmit the reply code on frequency f 2 . A bi-stable 17 determines which gate 10 or 11 is enabled and is controlled by monostables 18, 19. When the bi-stable 17 is set, the gate 11 is enabled and a counter 21 produces an output when a predetermined number of general calls have been received and this resets bi-stable 17 to enable gate 10. If no selective code arrives during the delay period of monostable 18, this circuit resets and after a further delay produced at 20 the bi-stable 17 is set for the general call. To prevent two vessels from replying to a general call simultaneously, the delay 20 is arranged to vary in a random manner.