GB1485880A - Secondary radar systems - Google Patents

Abstract

1485880 Secondary radar systems; interrogator-responder systems FERRANTI Ltd 27 Aug 1975 [28 Aug 1974] 37507/74 Headings H4D and H4L A secondary radar system includes an interrogator station, Fig. 2, and at least one cooperating transponder station, Fig. 1, which transmits a specially coded response signal which is detected in the interrogator station. A pulsed interrogation signal is generated at 21, Fig. 2, and transmitted from 20. A radar receiver 10 in the transponder is selectively responsive to this signal to trigger a signal generator 11 to generate, e.g. a 20 Ás burst of r.f. oscillations, or, may in an alternative embodiment (not shown) generate a D.C. level which is subsequently modulated. This signal is fed to a surface acoustic delay line 12 from which, for example, 20 taps are taken whereby twenty 1 Ás bursts of r.f. signal are fed through separate channels in phase shifter 13, wherein, according to a code word CW, arranged to have an even number of "ones" and "zeros" a # radians phase shift is introduced respectively to those channels corresponding to a "one" in code word CW. These signals are combined in amplifier 14 to be transmitted sequentially in a 20 Ás pulsed signal from transponder transmitter 15. The pulsed signal is received in interrogator receiver 23 and fed to an identical tapped delay line 24, which on complete receipt of a 20 Ás pulse provides at its tapping points signals corresponding to the phase shifted 1 Ás signals. These are applied simultaneously to two phase shifters 25 and 26 identical to 13. In 25, the code word CW is again applied to the signals whereby all the phase-changed bits have their phases reversed again and, on summing these signals co-temporaneously in amplifier 27, an output signal is produced and fed to threshold detector 29. This signal only exceeds the predetermined signal when all the 1 Ás bits add in phase corresponding to detection of the transponder response signal. In phase-shifter 26, a code word CW<SP>1</SP> is applied which corresponds to CW and in addition reverses the phase in every other channel, whereby on the instantaneous presentation of the transponder signal 1 Ás bits to the twenty channels of phase-shifter 26 an equal number (ten) of zero shifted and # radians shifted signals are produced, which, when summed in amplifier 28 mutually cancel. Threshold detector 31 responds to this zero output, and comparator 30 is arranged to give an indication, when response signals are simultaneously supplied by detectors 29 and 31, of receipt of a transponder response signal. It is stated that the code word CW may be changed at will to increase the security of the system.