Patent GB-805448-A (GB)
Title: Improvements in or relating to radar apparatus
Inventor: PARKER ERIC
IPC: G01S13/76
Filed: 1956-03-28
Abstract: 805,448. Pulse radar; radio signalling; aerials. DECCA RECORD CO. Ltd. April 9, 1957 [March 28, 1956; May 10, 1956], Nos. 9767/56 and 14643/56. Classes 40 (5) and 40 (7). In a secondary radar system for identifying responses from mobile craft (see Figs. 1 and 2), a crystal detector 31 on the latter provides pulse modulation of a communication transmitter 33, simultaneously fed with speech signals and with detected radar signals received on a different radio frequency, the communication receiver 20 at the ground radar station having its output fed (i) to a loud-speaker 22 via a band-pass filter, and (ii) to a pulse-shaping circuit 40 via a high pass filter 40 ; the shaped pulses are delayed by unit 43 so as to cause them to correspond on display 15 with the radar echo of the preceding pulse repetition period. The delay unit preferably comprises a multivibrator with a pulse duration equal to the required correcting delay and a differentiating circuit to produce from the trailing edges of the output pulses the short-duration pulses to be fed to the display; alternatively a delay line may be connected between filter 40 and unit 41. Switches 44 permit the display of echoes only, reply pulses only, or both. The reply pulses are arranged to have a greater amplitude at the display than the echo pulses. A two-colour cathoderay tube may be used for the display. The aircraft may carry a number of aerials such as 30, each having its own crystal detector, for the reception of signals of different polarization and frequency, switches between the detectors and a common amplifier 32 being provided, whereby the aircraft apparatus responds only to radar stations with the corresponding frequency and polarization characteristics. It may be arranged that emissions from the aircraft communication transmitter arise only during the periods of pulse re-transmission. To avoid crystal " burn-out " when the aircraft is near to the radar transmitter the associated aircraft receiving aerial is shielded by a movable shutter, e.g. coupled to an undercarriage control. A flashing lamp 38 indicates that radar pulses are being received. In a modification to the ground apparatus (see Fig. 3, not shown), the re-transmitted pulses are received by a directional aerial mounted on, and directed as, the radar aerial, and a separate receiver and aerial is used for the communication signals. In a further modification (see Fig. 4, not shown), the delayed received re-transmitted pulses are fed through a direct channel and a channel which includes a further delay unit, to a coincidence gate and thence to the display; the further delay unit introduces a delay equal to the pulse repetition period and may be a circuit comprising a monostable device triggered by each input pulse and is self-resetting, producing a short-duration output pulse after a predetermined delay. The pulse repetition frequency may be varied, the delay in the first delay unit being varied correspondingly. By arranging that different radar stations use different pulse repetition frequencies spurious responses are reduced. In the modification to the ground radar reception arrangements illustrated by Fig. 5, a unit 70 delays the output of the radar receiver by an amount equal to the delay in the aircraft equipment; the range markers and time-base control signals are also fed through that unit. The later Provisional Specification mentions the use of the same frequency for the re-transmitted pulses as the radar frequency.