Patent GB-754397-A (GB)
Title: Improvements in or relating to frequency modulated radar systems
Inventor: BRANDON PERCY SAMUEL
IPC: G01S1/02
Filed: 1954-08-06
Abstract: 754,397. Radar. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 23, 1955 [Aug. 6, 1954], No. 22885/54. Class 40 (7). A frequency-modulated radar system is arranged to provide a display of a remotely situated area with a better resolution that can be obtained with a conventional system. In the modified radar beat frequencies representative of the ranges of targets are derived by mixing echo signals from the targets with signals returned by an active or passive responder located in the area to be examined with good resolution. The responder may be stationary or mounted on a mobile craft such as a ferry boat. The transmitted signals are preferably directed to the responder and received back from the responder by a separate aerial or by separate aerials to those used for the targets and these aerials made directive towards the responder. In the case of a mobile responder they may be directed by means of an auto-follow system. In an embodiment of the invention using separate aerials for transmission and reception the transmitter 1 is frequency-modulated according to a saw-tooth law by modulator 2 and feeds aerial 3 scanning in azimuth and aerial 4 directed towards responder 6 (assumed stationary). Signals returned by the responder are passed from receiving aerial 8 through amplifier 10 to balanced modulator 14 also fed with I.F. signals from oscillator 11. Signals of I.F. frequency Œ received signal frequency are passed from the modulator to two mixers 17 and 18 over channels 15 and 16 of which channel 16 is longer by a quarter of a wavelength. Mixers 17 and 18 are also fed over channels of equal length from receiving aerial 9 scanned in synchronism with aerial 3 and receiving echo signals from the targets. Each mixer feeds its own detector-mixer 23 or 24 through an I.F. amplifier and the detectormixers are also fed from the oscillator 11. The output beat frequency signals are respectively fed through band-pass networks 25 and 26 which provide phase delays differing by 90 degrees for all frequencies in the pass band (network 26 having the greater delay). The filter outputs are added at 27, to give an output consisting of signals due to targets nearer the radar than the responder, and subtracted at 28 to give an output of signals due to targets further away than the responder. These outputs are applied respectively through gate circuits 30 and 31, opened alternately, to panoramic receiver 29 whose output intensity modulates cathode-ray display tube 38. An oscillator 32 whose output is limited and shaped at 33 provides the switching waveforms for the gates 30 and 31 and also a further rectangular waveform which is integrated at 34 to provide the waveform controlling the sweep of the oscillator in the panoramic receiver. The further rectangular waveform is also differentiated at 35 and the positive pulses, derived by circuit 36, used to trigger the range timebase circuit 37 providing the Y co-ordinate deflection scan of the cathode-ray tube 38. The X direction scan for the tube is derived from the azimuth scan variations of the aerials 3 and 9 by conventional means represented by chain line 39.