Patent GB-1099011-A (GB)
Title: Improvements relating to radar automatic tracking apparatus
Inventor: STILWELL PETER FREDERIC THOMAS
IPC: G01S13/68
Filed: 1958-11-22
Abstract: 1,099,011. Pulse radar. ELECTRIC & MUSICAL INDUSTRIES Ltd. Nov. 16, 1959 [Nov. 22, 1958], No. 37681/58. Heading H4D. In radar automatic tracking apparatus the error signal for an automatic tracking circuit is weighted in response to the amplitude of a received signal in such a way that falsification of the error signal due to " glint " is reduced. When an echo source changes its altitude the magnitude of the echo exhibits periodic cusps corresponding to relatively sudden reductions in amplitude due to destructive interference, and this is believed to cause the error known as " glint." In Fig. 1 the echoes received from four aerials A, B, C, D are processed to produce a sum signal S and two difference signals d 1 , d 2  dependent on the angular misalignment of the aerial about two axes; the circuit 1 produces four outputs d 1 ŒS and d 2 ŒS which are amplified by I.F. amplifier 2 and demodulated by detector 4. The discriminator circuit 5 produces three outputs, by adding and subtracking the signals, corresponding to 2d, G, 2d 2 G and 4SG where G is the gain of the amplifier 2, and supplies an output to an automatic range-tracking circuit 7. The signal 4SG is supplied as automatic gain to amplifier 2 and on this condition G tends to a value proportional to I/S so that the signals 2d 1 G, 2d 2 G are proportional to d 1 /S and d 2 /S and are substantially S S independent of range. These two signals are fed to angle tracking circuits comprising switches 8a, 8b, amplifiers 9a, 9b, and servo motors 10a, 10b which displace the aerial about the respective axes. The switches 8a, 8b are normally closed but can be opened by a relay coil 12 in the anode circuit of a valve 13 which has a common cathode impedance with a valve 15. The A.G.C. signal from discriminator 5 is fed through an amplifier 6 and condenser 16 to the grid of valve 15 which is normally conducting and holds valve 13 in the non-conducting state. The time constant of condenser 16 and resistance 17 in such that slow variations of the gain control signal produce no change in the conducting state of valve 15 but if a cusp occurs due to the glint effect the conducting states of the valves are interchanged and switches 8a, 8b are opened for a period longer than the normal duration of the cusps. Whilst the switches are open the tracking circuits operate in response to the voltages stored on condensers lla, 1 lb so that the servomotors will continue to displace the aerials on the basis of the pre-glint values of the error signals. In another arrangement, Fig. 2, the switches 8a, 8b are omitted and a resistance 20, rectifier 21 and condenser 22 are connected as shown in the A.G.C. circuit between unit 5 and amplifier 2. The conducting direction of the rectifier is such that the leading edge of a cusp produces only a slight change in the potential across condenser 22; the trailing edge can produce a rapid change but as the potential on condenser 22 has not departed materially from its pre-cusp value the change is only small. Thus the A.G.C. circuit is less sensitive to rapid reductions in the amplitude of the sum signal and the desired weighting of the error signals is achieved.