Patent GB-1459266-A (GB)
Title: Radar systems
IPC: G01S13/87
Filed: 1974-03-14
Abstract: 1459266 Secondary radar system SIEMENS AG 14 March 1974 [21 May 1973] 11309/74 Heading H4D The Specification describes a system for differentiating between desired (synchronous) and undesired (asynchronous) target response signals received by a secondary radar device, which is operably associated with a primary radar device, there being a plurality of such associated devices in a vicinity, and a particular target within said vicinity, equipped with a transponder activated by pulsed interrogation signals received from said secondary radar devices, the interrogation signals transmitted by each of which being obtained by means of integral frequency division of the transmitted pulse repetition frequency of the primary radar devices which have an identical or similar P.R.F. to one another. The system includes an interference blanking device which operates as a "defruiter", by suppressing undesired response signals produced by the transponder due to reception of interrogation signals from other secondary radar devices, and which is based on differing frequency division rates for a particular interrogation signal sequence for each secondary radar device, the division ratios in recurring sequences sequences changing in a time dependent fashion, so that each device recognizes its own interrogation signal sequences and suppresses target response signals to other interrogation signals. The interrogation signal pulses are generated so that primary radar echo pulses and target response signals are received co-incidentally so that they may be displayed together on, e.g. a P.P.I. screen 5, Fig. 2. Each radar station comprises a primary radar device Fig. 2, and a secondary radar device Fig. 3. Primary radar comprises antenna 1, T-R switch 2, transmitting section 3, pulse central control 4, display screen 5, receiving section 6 and analysis circuit 7. Pulse control centre 4 controls the primary radar p.r.f., and the deflecting beam of screen 5, which also receives the secondary radar display signal through line 10. Terminals q, r, s in Figs. 2 and 3 are interconnected. In Fig. 3, the secondary radar system comprises a rotating antenna 11 with T-R switch 12, and a secondary lobe suppressor (SLS) comprising omni-directional antenna 14a, T-R switch 14b and SLS switch 14c. Secondary radar Fig. 3 further comprises modulator 15, coder 16, circuit for trigger preparation 17, a circuit for pretrigger generation 18, and a controllable divider circuit for dividing the pulse repetition rate of the primary radar and which is connected to a statistical or pseudostatistical random generator for defining the divisors to be set in the divider circuit 19. The average divider ratio is set to correspond to an internationally agreed "maximum permissible" divider ratio. In other embodiments, instead of random generator 20, means may be provided for controlling the divider ratios to be a series symmetrical about the maximum permissible ratio value. Alternatively the successive lengths of divider ratios may successively increase, and then subsequently decrease over a plurality of time intervals. The number of possible divider ratios may be chosen to correspond to the expected target response per sweep at the "maximum permissible" interrogation frequency. The receiving section comprises receiver 21, decoder 22, adder 23, store 24, which may comprise shift registers, threshold circuit 25, control circuit 26 for store 24 and a circuit 27 which generates channel pulse trains to construct within each gated reception period a plurality of individual range gate channels R1, R2 &e. (see line m, Figs. 1a and 1b, not shown). Store 24 stores individually per channel the number of previous response signals within each range gate channel. Threshold circuit 25 is set at a specific count acceptance threshold, which when reached, a display signal is forwarded over the line 10 to the display apparatus screen 5, Fig. 2, where it arrives at the correct time with the relevant primary radar echo signal and can be displayed or analysed. The acceptance threshold may be adjustable depending, e.g. on the antenna sweep rate. In another embodiment Fig. 4, instead of range gate channel discrimination for clutter suppression, the interference blanking circuit comprises a five stage shift register 30, which is clocked from trigger preparation circuit at the interrogation frequency and operates as a coincidence detector for each divider ratio present in stage 17. For example, for signals shown in Figs. 1a and 1b (not shown) a response signal AA0<SP>1</SP> which has been shifted through four stages of register 30 corresponding to the contemporaneous divider ratio, coincides via AND gate 32 with the next target response signal AA4 thus triggering an output on line 10 on occurrence of a trigger pulse from circuit 17, thereby cancelling all received signals, non- coincident with the relevant divider ratio. Fig. 5 (not shown) shows how the random generation of divider ratio reduces interference between neighbouring secondary radar systems of approximately the same interrogation frequency.