Patent GB-1503978-A (GB)
Title: System for identifying objects equipped with an automatic transponder
IPC: G01S7/36
Filed: 1975-12-15
Abstract: 1503978 Secondary radar THOMSON-CSF 15 Dec 1975 [18 Dec 1974] 51344/75 Headings H4D and H4L This is concerned with the modification of a secondary surveillance radar system of the type wherein an aircraft transponder is interrogated by a ground station by means of a pair of pulses having a pulse spacing indicative of the mode in which the respond is to reply, i.e. its identity or its altitude. The modification being for the purpose of stopping an enemy aircraft from monitoring the responses of the friendly aircraft and duplicating the same, whereby said modification consists in scrambling the responses in accordance with preset rules communicated up to the responder in the ground station interrogation. An unmodified responder comprises Fig. 3 a pilot control 10 in which the response code word comprising of twelve consecutive bits is set up together with the mode, a multiplexer 11 for receiving said twelve bits in parallel and serially reading out the same on a single output in accordance with a set of addresses from a memory 12. The twelve bit serial response word is fed to the responder transmitter 16 via a gate 15 which is enabled if a side load suppressor circuit 13 detects that the main load is interrogating the aircraft and if a mode detector 8 determines from the spacing of the interrogating pulse pair that the mode required from the ground station is that stored in the pilot control panel 10. The invention thus sets out to substitute in possible enemy territory a random access memory 20, Fig. 5, for memory 12. The random access memory 20 contains a plurality of sets of scrambling addresses each set having a corresponding indexing address. The indexing address is indicated to the transponder by the interrogation signal from the ground station comprising four pulses Fig. 4 made up of two pairs of pulses, the pulses of each pair being separated by the mode indicating duration and the two pairs being separated by a duration indicating said indexing address. The ground station includes a random access memory identical to that in the transponder however the indexing address used therein and communicated up to the transponder in the interrogation signal is randomly selected. The selected set of addresses in the random access memory 20 are read out at a clock pulse H rate by means of an addressing circuit 21. The addresses of the pulses and of the centre position of the response word are stored in a fixed address circuit 23 which addresses are also fed to the random access memory via a switch 22 controlled by a word sequencing circuit 24. To further scramble the responses a further memory may be provided in which is stored a law governing the changes in the response codes with time, the output of this further memory after temporary storage is combined with the output of a random code generator and the combination fed for a succession of cycles to the parallel inputs of the multiplexer instead of the bit output of the pilot control 10. The border as to whether the word in the pilot control or the random cogenerator output should be fed to the inputs of the multiplexer is conveyed up to the responder by various pulse intervals in the interrogation signal. In this case then the transponder transmits a response formed by a cycle of n response words comprising n codes whose bits are in different order from one response word to the next.