Patent GB-1038084-A (GB)
Title: Improvements in or relating to radar systems
Inventor: PLAISTOWE DONALD LEOPOLD
IPC: G01S13/78
Filed: 1961-11-03
Abstract: 1,038,084. Radar. MARCONI CO. Ltd. Sept. 18, 1962 [Nov. 3, 1961], No. 39423/61. Heading H4D. A radar system comprises means for scanning space with a pulsed directional radio beam, means for receiving echo signals from reflecting targets encountered by the beam, means for displaying received echo signals from reflecting targets to indicate the direction and range of the targets, means for receiving responder pulses and means, selectively responsive to received responder pulses, for eliminating or substantially reducing the display of echo signals when responder pulses are received. Such a system is described as being used to display aircraft provided with responders as dark markings on a bright background display of enveloping cloud or other clutter producing media. The invention is described with references to five broadly described embodiments (Figs. 1 to 5, all not shown) and one detailed embodiment (Fig. 6). The system of Fig. 1 may comprise a transmitter 1 feeding pulses via a T-R cell 2, to a scanning aerial 3. The pulses at a frequency F1 encounter for example, an aircraft A within a cloud C. Normal echoes at a frequency F1 are produced by the aircraft and cloud. In addition the aircraft A is provided with responder means comprising a receiver RR receiving the pulses from the aerial 3 and a transmitter RT retransmitting the pulses to the aerial 3 at a frequency F2. The echo pulses are passed through filter 5, tuned to frequency F1, are amplified, and are used to control the first grid of amplifier tube 8. The responder pulses are passed through filter 6, tuned to frequency F2 and are used to control the second grid of tube 8. The output of tube 8 is used via amplifier 10 to intensity modulate the beam of a C.R.T. display tube 11. Pulses from amplifier 7 produce a brightening of the trace, whilst responder pulses from amplifier 9, are opposite in polarity and produce a darkening of the trace. The aircraft is therefore displayed as a dark spot on a bright " cloud " background. Once outside the cloud the aircraft would not normally be displayed due to its echo pulse and responder pulse cancelling out at the tube 8. For this reason the pilot of the aircraft is informed of his leaving the cloud, either from his own instrumentation or from ground, and switches off the responder. A storage tube, Fig. 2 (not shown), may be provided between the filter 6 and amplifier 9 to produce a display of the aircraft's track in the form of a plurality of previous positions. In another embodiment, Fig. 4, the necessity for the pilot to switch the responder off and on is removed by preventing responder pulses from reaching the tube 8 when the aircraft is in open air, by means of a normally closed gate 14. The gate 14 is opened by an echo pulse from a time constant circuit 15, which only passes echo pulses having a duration too long to be associated with anything else but cloud. The tube 8 may be replaced, Fig. 3 (not shown), by a normally open gate passing the echo pulses and shut by the responder pulses. In another variation of the embodiment, of Fig. 4, Fig. 5 (not shown), the responder pulses pass through a normally open gate 13 when the aircraft is in open air, the gate being closed by the pulse from circuit 15. In a detailed embodiment, Fig. 6, airborne equipment comprises a transmitter T, triggered by pulser P, and feeding pulses at a frequency F1 via a T-R cell, to an aerial A, for radiation thereby. Echo signals from cloud and aircraft are received back in receiver R and passed to one control grid G1 of a double gun direct viewing storage tube CR1 to produce a B-type display. Tube CR1 is constructed and biased such that a signal on grid G1 produces a brightening of the trace, whilst a signal on grid G2 darkens the trace. Responder pulses at frequency F2 are received at R2 and are passed to a control grid of a second storage tube CR2 where a B-type display of four successive positions of the aircraft is produced. This display is viewed by television camera CT in sync. with the two storage tubes, and the output pulse thereof is applied to the second control grid G2 of storage tube CR1, thus producing a display showing the four aircraft positions as four dark spots on a bright cloud background. By moving ganged switches S1, S2 and S3 to their righthand positions the equipment can act as a responder to the signals of another aircraft, a signal at a frequency F1 from the other aircraft causing, via switch S1, the responder transmitter RP to transmit a pulse at a frequency F2. Amplitude separation of the echo and responder pulse signal may be used instead of frequency separation. When carried by aircraft the system may be used for anticollision purposes.