Patent GB-624407-A (GB)
Title: Improvements in or relating to radio target locators
IPC: G01S1/02
Filed: 1944-04-25
Abstract: 624,407. Radiolocation ; valve circuits. SPERRY GYROSCOPE CO., Inc. April 25, 1944, No. 7745. Convention date, April 12, 1943. [Classes 40 (vi) and 40 (vii)] [Also in Group XXXVIII] In a pulse radar system in which the radio beam is scanned throughout a solid angle, means are provided for automatically varying the effective range of the system in accordance with the radial distance to the ground along the beam, whereby responses due to ground reflection are eliminated. In the airborne equipment shown in Fig. 3, the axis of the beam 13 produced by the reflector 12 is scanned spirally by rotating the reflector about a spin axis 14 and simultaneously moving it about a " nod " axis 15 as described in Specification 621,485. The responses are presented by intensity modulating a cathode-ray tube having a synchronized spiral scan so that the direction, but not the range of a target is indicated by the position of a corresponding spot on the screen of the tube (type C indication). In this system, responses from targets in the same direction but at differing ranges are superimposed ; thus, when the beam is directed towards the ground, the -responses from targets between the aircraft and ground are obscured by the ground signals. According to the invention this is avoided by generating a variable gate pulse, the duration of which is proportional to the instantaneous distance to ground along the axis of the beam ; the response signals are normally suppressed in the receiver 45 or indicator 42 and the gate pulse is applied to remove the suppression, thus passing signals up to a range equal to that of the ground. Computer-type system.-When the spin axis 14 is horizontal, the relation between the altitude of the aircraft h and the distance r to ground along the axis of the beam is given by r=h/sin#cos# where # is the spin angle measured from a point vertically under the spin axis and # is the nod angle measured positively downwards from the axis 14. In one embodiment an alternating voltage of amplitude proportional to h is produced by a known form of altimeter 38, the alternating voltages with amplitudes proportional to cos 0 and sin # are produced by the electrical transmitters 26 and 31 (see Group XXXVIII). These three voltages are fed over lines 39, 35, and 37 into the computer 34 and through the demodulator 49 to give a " radius " voltage of amplitude proportional to r which is fed to the converter 48 to produce the variable gate pulse. The converter 48 may comprise a cathode-coupled flip-flop, as described with respect to Figs. 6 and 7 (not shown). In a modification, the voltage functions cos 6 and sin # from the transmitters 26 and 31 are converted into reciprocal functions sec 6 and cosec # so that the computer has to perform a product operation r=h sec # cosec #. This is accomplished by separately demodulating the voltages from lines 39, 35 and 37 and feeding the sinusoidal voltages through amplifier distorters, Fig. 13 (not shown). Each amplifier may comprise a pentode with a sharp cut-off characteristic, the anode load resistance being two or three times the value for maximum linearity in order to effect the necessary shaping and phase reversal. The valves are so biassed as to provide a constant output for values of 6 between 90 and 270 degrees and for values of # between 90 and 180 degrees and between - 90 and -180 degrees, since for these values the beam never strikes the ground and the variable gate should have a constant length greater than R the maximum effective range of the equipment. When the spin axis is parallel to the fore-and-aft line of the aircraft, errors in the computation of r due to roll-andpitch of the aircraft may be corrected by adjusting the angular position of the stators of transmitters 26 and 31 in accordance with the roll-and-pitch as determined by the gyroscope 53, the adjustment being effected by a selsyn control (see Group XXXVIII). In the general case in which the scanner is mounted in any position, the computer may be such as to solve an equation expressing r in terms of h, #, #, band g, where band g are the angles of the scanner supporting frame with respect to the ground plane. Ground echo controlled system, Figs. 18 and 19. In this system the duration of the variable gate is determined by the time of reception of signals due to ground reflection. The variable gate flip-flop generator 122 is fed with a synchronizing pulse 198 to produce a positive gate pulse 202 with a variable trailing edge 91, the position of which is controlled by the bias applied to grid 146, and in the absence of ground signals this trailing edge corresponds to the maximum effective range R. The variable gate pulse is fed through output terminals 47 to the indicator and through the cathode follower 124 and peaking condenser 153 to trigger off the blocking oscillator 123 which produces a short-range gate 203 in synchronism with the trailing edge of the variable gate. This short-range gate output is fed through the peaking condenser 166 to trigger off the blocking oscillator 125 which produces a second shortrange gate 204, the leading edge being coincident with the trailing edge of the first short-range gate 203. The two range gates are also fed to one set of control grids 168, 172 of the coincidence valves 126, 127 and signals of positive polarity are fed from the video stage of the receiver through terminals 119 to the other set of control grids 173, 174. These valves are only conducting when coincident positive signals are present on both control grids and in such cases positive signals are produced in the secondaries 178 and 179 of the output transformers and are fed to the cathode followers 128, 129, the time constants of the R.C. cathode loads of these valves being such that signal variations occurring during a time comparable with the duration of the range gates 203, 204 are smoothed out, but the condensers are completely discharged during the pulse repetition period, giving an output waveform 207, Fig. 191. The outputs from the valves 128, 129 are fed to the valves 131, 132, which are connected in series to form a potential divider so that the potential at the point 187 depends upon the relative amplitude of the outputs from valves 128, 129. The potential at the point 187 is limited by the diode 133 and controls the cathode-follower 134, the constantcurrent pentode 136 forming the cathode load of this valve. The output from the cathodefollower is fed along the line 135 to bias the generator 122 and hence a gate pulse is produced, the duration of which is proportional to the potential at the point 187. In order to produce a gate pulse of constant maximum duration in the absence of signals from the receiver, a large resistance 193 is connected between the grid and anode of valve 134, and in order to produce a bias for the valve 131 which is independent of the voltage fluctuations of its cathode, the grid of the valve is connected to a tapping 170 on a resistance 180 in the anode circuit of the constant-current pentode 136. The automatic control may be made more sensitive by shaping the ground echo pulse 205 and the range gates 203 and 204, as shown by the dotted lines.