Patent GB-903912-A (GB)
Title: Radio distance measuring system providing interrogation rate control
IPC: G01S13/76
Filed: 1958-12-19
Abstract: 903,912. Pulse radar; automatic phasecontrol systems. STANDARD TELEPHONES & CABLES Ltd. Dec. 19, 1958 [Dec. 23, 1957], No. 41150/58. Classes 40 (5) and 40 (7). Relates to a pulse interrogator-transponder radar-ranging system in which each interrogator comprises a locally generated strobe pulse which automatically searches for and locks on to a reply pulse, means being provided for automatically reducing the pulse recurrence frequency of the interrogator when such locking is effected thereby making it possible for a large number of interrogators to receive reply pulses from a single transponder. According to the present invention an output from a 4046 c/s. crystal oscillator 1, Fig. 1, is applied through a pulse generator 2 to a blocking oscillator count-down circuit 3 to give trigger pulses of recurrence frequency 150 c/s. during the search mode. The strobe pulse comprises an early gate pulse 47, Fig. 2 (A), having a sloping trailing edge and a late gate pulse 49, Fig. 2 (B), having a sloping leading edge, the two sloping edges being partially coincident so that the composite pulse 47, 49 has a central notch as shown in Fig. 2 (D) and (E). The reply pulse 51 and the gate pulses 47 and 49 are applied to a coincidence circuit 11 such that when a reply pulse 51 is coincident with the gate pulses 47, 49, the peak amplitude of the composite output, Fig. 2 (D) and (E), from the coincidence circuit 11 due to the reply pulse component 51 is a minimum when the reply pulse 51 is at the centre of the composite gate pulse 47, 49, Fig. 2 (D), and increases to a maximum if the reply pulse 51 moves to the flat-topped portions of the composite pulse 47, 49, Fig. 2 (E), so that the composite output when integrated at 20, Fig. 1, gives a control voltage which is a function of the acceleration of the transponder, said control voltage being applied via a relay 12 (track position) and amplifier 21 to vary the bias of the count-down circuit 3 thereby reducing the recurrence frequency of the transmitter trigger pulses to 1-30 c/s. during the search mode, the recurrence frequency being proportional to the acceleration of the transponder. The gate pulses 47 and 49 are generated by blocking oscillators 6 and 7 respectively and the early gate pulse 47 is initiated by the trailing edge of a variable-duration pulse from a phantastron 4 which is triggered by the trigger pulses from the count-down circuit 3 and terminated by the combined effect of a D.C. clamping voltage applied to its anode from a range potentiometer 5 and 4046 c/s. pulses applied to its suppressor grid and produced by applying an output from the crystal oscillator 1 to a pulse generator 30 through a phaseshifter 26 ganged to the range potentiometer 5 through a 10 : 1 gearing 31. The range potentiometer 5 is such that one complete rotation of its control shaft would change the duration of the output pulse from the phantastron 4 by an amount corresponding to 200 nautical miles and the period of the oscillator 1 corresponds to 20 nautical miles so that the timing of the gate pulses 47, 49 is variable over a range of 200 nautical miles with an accuracy determined by the phase shifter 26. The range potentiometer 5 and phase shifter 26 are controlled by a motor 23 energized by the control voltage from the integrator 20 as modified by a second integrator 19 and drive unit 18 thereby causing the gate pulses 47, 49 to track the reply pulse 51, the range of the transponder being shown on an indicator 25 driven by the motor 23. A rate feedback is applied from the motor 23 to the drive unit 18 through a generator 24 to prevent hunting. During the search mode the duration of the output pulse from the phantastron 4 varies cyclically. Memory operation. The relay 12 is set in its track position by the composite output from the coincidence circuit 11 but if the reply signal fails the relay 12 is maintained in its track position for a predetermined memory period by an output from a sum amplifier 15 coupled to the coincidence circuit 11 and a further output from the amplifier 15 is applied to a memory circuit 17 coupled to the integrator 19 which causes the tracking to continue during the memory period. The pulses transmitted by the interrogator and transponder are preferably coded, a single reply pulse 51 being derived from each group of reply pulses by a decoder 38.