Patent GB-601536-A (GB)
Title: Radio locating systems
IPC: G01S13/78
Filed: 1945-09-11
Abstract: 601,536. Radiolocation ; radio signalling. STANDARD TELEPHONES & CABLES, Ltd. Sept. 11, 1945, No. 23453. Convention date, Oct. 26, 1943. [Classes 40 (v), 40 (vi) and 40 (vii)] A radar system operates in conjunction with fixed transponder beacons which respond only to signals radiated on particular frequencies and which retransmit signals with given characteristics identifying the beacon. The operating frequency of the radar system is varied continuously, thus improving secrecy, and the transponders are differently tuned so that not more than one will be triggered by a signal of the same frequency. The radar system 10, Fig. 1, which may operate with beacons X, Y and Z, comprises a transmitter 14, and a receiver 38 associated with a directive aerial 30 which may be rotated manually or by motor 32. The transmitter pulsing and receiving blocking are controlled by a synchronizing unit 18 which also controls the time-base circuit 44 for the type-A display on the cathode-ray tube 42. The tuning of the transmitter and receiver, which comprises R.F. and variable frequency I.F. stages, is controlled by a unit 16 which may comprise motor-driven mechanical means such as that described in Specification 590,469 or, Fig. 2 (not shown), thermionic-reactance modulators which are controlled by saw-tooth- or sinusoidal-wave generators locked to the synchronizing signal so that a number of pulses at definite frequencies are transmitted during each cycle of frequency variation. The signal of special form required to trigger the beacons may be a plural pulse arrangement or a pulse of given width and is produced from the output pulse 19 of the synchronizer by the unit 20 shown as containing a delay device 24 which when connected in circuit by switch 26 causes the double pulse 21, 23 of spacing controlled by the delay device to be fed to modulate the transmitter. If the signals returned from beacons are coded in width or by the number and spacing of pulses, they may be identified visually on the oscillograph or a pulse width discriminator 64 such as that disclosed in Specification 592,789 may be inserted after the receiver detector 50. Identification of the beacons in terms of operating frequency may be made by connecting a tunable narrow band filter 62 in circuit prior to the detector 50. The transponder beacon, Fig. 3, comprises a transmitting oscillator 98 and receiver unit 84 interconnected by a blocking unit 99 and coupled to a common aerial system 80. The received triggering signal comprising the double pulse 21, 23 is fed directly to one grid and through an appropriate delay device 86 to the other grid of a coincidence valve 88 to produce the single output pulse 95 which will only be produced by the correctly spaced double pulse. The pulse 95 is fed to modulate the oscillator 98 directly or through a signal generator 96 if a specially characterised return pulse signal is required. Composite pulses comprising a plurality of critically spaced pulses, Fig. 5 (not shown), may be produced by signal generator arrangements, Fig. 4 (not shown), in which the input pulse 95 is fed directly to an output connection and also through parallel paths, each comprising a delay device and pulse.reshaping circuit. Alternatively, Fig. 6 (not shown), the pulse 95 may trigger a number of multivibrators producing square waves of differing duration which are differentiated by a common inductance to produce short spaced pulses corresponding to the trailing edges of the square waves. The return pulses may be characterised by width coding, in which case the signal generator 96 may comprise a simple multivibrator. In order to economize power, the transponder beacon may be arranged as disclosed in Specification 588,777 so that the transmitting circuits are only energized from the supply unit 104 when triggering signals are received. A control unit 100 comprises a gas discharge tube 110 which when fired by the triggering signal is maintained conducting for a sufficient time to operate a slow release relay 112, the contacts 102 of which close the power supply circuit for the oscillator 98. Ganged switches 105, 106 put the control unit out of operation and close the supply circuits. Specifications 302,602, [Class 40 (v)], and 387,643 also are referred to.