GB1300698A - Improvements in or relating to interrogation-reply systems - Google Patents

Abstract

1300698 Radar SIEMENS AG 19 April 1971 [6 Feb 1970] 24111/71 Headings H4D and H4L An aircraft secondary radar system and responder protected against interference from an enemy, is interrogated by a friendly station by means of a code which changes from one interrogation to the next (2nd row) Fig. 6 accordingtoapattern known to both station and responder and replies to the correct code with a coded response, the code of which (3rd row) Fig. 6 also changes according to a mutually known pattern. A crypto computer 3 Fig. 1 supplies the series of interrogation codes to stores S2Z, S1Z, SG, S1V, S2V; a code passing from one store to the next at the end of an interrogation period. Store SG holds the code expected at the present time, stores S2Z, S1Z respectively hold the codes expected two and one interrogation periods in the future; and stores S1V, S2V respectively hold the codes which should have been received one and two interrogation periods previously. The computer 3 similarly supplies the response codes to "in the future" stores to present store SG<SP>1</SP> and to immediately past store S1V<SP>1</SP>. The contents of the interrogation code stores S2Z ... S2V are compared with the incoming code at 4 in comparators K22 ... K2V An agreement causes the respective one of the comparators to feed an enabling signal to the corresponding AND gate UG1Z ... UG1V and the corresponding response code is transmitted at 5. If the computer 3 is in advance of the corresponding computer in the interrogating station, the incoming interrogation code will possibly be the same as that in stores S1V or S2V. The resultant signal from comparator K1Y or K2V is fed to a correction unit 2 for resynchronism of the computer 3. The connection of comparator K1V to AND gate UG1V esures that the interrogator receives the response code he is expecting. A similar computer correction takes place if the computer 3 is lagging either comparator K1Z or K2Z giving the correction output. In an alternative embodiment Fig. 2 (not shown) a monitor unit (14) receiving the outputs of all the comparators determines the average by or lead over a period and avoids interference by an enemy retransmitting interrogation codes he has received. An analysis of the allowable deviation is synchronism between interrogator and responder is given, Figs. 3a, 3b, 4, 5 (not shown). The interrogation pulse repetition frequency may be varied to further avoid enemy interference or jamming.