Patent GB-862485-A (GB)
Title: Distance measuring system utilising radiant energy
IPC: G01S13/84
Filed: 1959-04-28
Abstract: 862,485. Radar; transponder apparatus. BENDIX CORPORATION. April 28, 1959 [May 12, 1958], No. 14523/59. Classes 40(5) and 40(7) Relates to an amplitude-modulated C W secondary radar system in which the distance between the interrogator and transponder is determined at the interrogator by phase comparison of the transmitted and received modulation signals. According to the present invention the interrogator and transponder transmit at the same carrier frequency for alternate periods each of approximately 0À2 secs. (see Fig. 4), the modulation signal at the interrogator is derived from a tuning-fork oscillator 3, Fig. 2, and at the transponder the received modulation signal drives a tuning fork oscillator 43, Fig. 3, having a normally closed negative feedback path which is opened near the end of the reception period thereby increasing the Q of the oscillator. A single interrogator transmission period may be initiated manually or such periods may be initiated cyclically and automatically. Interrogator, Fig. 2. The output from the tuning fork oscillator 3 is applied through a gate 16 to the transmitter 1 and to a pulse generator 21. The reply signal from the transponder is received at 2 and the modulation component is selected by a filter 23 and applied to a pulse generator 24, the outputs from the pulse generators 21 and 24 being applied to a bistable circuit 22 to give a pulse of duration proportional to the required distance which is applied through an integration network 25, switch 26, integration network 27 and a valve bridge circuit 28 to give a corresponding indication on a meter 30. In operation the gate 16 is opened and the transmitter 1 is keyed on for the transmission period by switches 18 and 20 controlled by the output from a monostable multivibrator 11 triggered by a push-button 7 or by a free-running multivibrator 6 as selected by a switch 8 and the switch 26 is closed during the "meter phase difference" period, Fig. 4, by the output from a monostable multivibrator 14 triggered from the multivibrator 11 via a further monostable multivibrator 13. Transponder, Fig. 3. The modulation component of the received signal drives the tuning fork oscillator 43 which includes a negative feedback path comprising amplifiers 45, 34 and a normally closed switch 41 which reduces the effective Q of the oscillator thereby enabling it to lock more quickly to the applied signal. The received modulation signal is also applied through a filter 32 to a detector 35 whose output triggers two series-connected monostable multivibrators 37, 38, the output from the latter being applied to open the feedback switch 41 and to energise the transmitter 1 ; as shown in Fig. 4 the transmitter does not radiate until after the termination of the received signal due to its warm-up time. Alternatively the input to the detector 35 may be derived from the tuning-fork oscillator 43 as indicated by the dashed connection 49. In Figs. 2 and 3 phase shifters 15, 33, 36 are inserted to compensate for fixed phase delays in the system. Fig. 5 (not shown) illustrates an apparatus comprising a single tuning fork oscillator which may be used selectively as an interrogator or transponder according to the setting of a plurality of switches. When operating as an interrogator the output from the tuning fork oscillator 3 is applied to the pulse generator 24 when the switch 26 is open in order to establish the output level of the integration network 25 at a value equivalent to zero range during the period before a reply signal is received from the transponder and a switch is provided for adjusting the sensitivity of the meter 30 in accordance with the setting of the selector switch 8.