Patent GB-1079003-A (GB)
Title: Distance measuring method and systems
IPC: G01S13/84
Filed: 1965-07-07
Abstract: 1,079,003. Radar. CSF-COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. July 7, 1965 [July 20, 1964], No. 28904/65. Heading H4D. The invention relates to a method of eliminating instrument phase shifts in a CW secondary radar system, of the type wherein range is determined by measuring the phase difference between modulations of transmitted and received response signals. In the system of Fig. 2 low frequency oscillators 21, 29, 26 and 231 all produce the same frequency. The output of oscillator 21 in the interrogator station S 2  modulates the output of high frequency transmitter 22 and undergoes a phase shift of # 22  in the modulation process. During transmission periods the modulated transmitter output is transmitted to the responder station S<SP>1</SP> 2 , via contact E 1  of switch I 21 , and antenna AA<SP>1</SP> 21 . Also during transmission periods, part of the transmitter output is fed via capacitor C 21  to the receiver/detector 23, wherein the modulator undergoes a further phase shift of # 23 . The detected modulation has a total phase shift of # 22 +# 23 , and is applied to one input of a phase discriminator 24, the second input thereof being fed by the output of phase controlled oscillator 29. The output of the discriminator is fed, via contact E<SP>1</SP> 1  of switch I<SP>1</SP> 21  (ganged with switch <SP>1</SP> 21 ), to the control terminal of oscillator 29. The operation is such that the two discriminator input signals are in phase, and the output of oscillator 29 lags that of oscillator 21 by # 22 +# 23 . At the responder the interrogator transmission is received with the modulation having undergone a phase shift of #/2+# 22'  where #/2 is the phase shift occurring between the interrogator and responder and is proportional to the distance therebetween. During the time that transmissions from the interrogator are expected, ganged switches I 22 , I<SP>1</SP> 22  are in the positions shown. The received transmission is therefore fed to receiver/detector 25 to produce a detected modulation with a phase shift of #/2+# 22 +# 25 , which modulation is fed to one input of phase discriminator 28. By a process similar to that already described, the output of phase controlled oscillator 231 is caused to also have a phase shift of #/2 + # 22  + # 25 . Switches I 22  and I<SP>1</SP> 22  now switch to their other positions, such that the output of the discriminator 28 controls the phase of the output of oscillator 26, which output modulates the output of transmitter 27. Part of the modulated transmitter output is fed to the receiver/detector 25 via capacitor C22 and the detected modulation is phase compared with the output of oscillator 231. The outcome is that the modulation of the output of transmitter 27 has a phase shift of # 22 +#/2. This output is transmitted to the interrogator, and is received thereat with a modulation phase shift of # 22 +#. Switches I 21  and I<SP>1</SP> 21  are now in their other position, such that the received and detected modulation at the output of receiver 23 has a phase shift of #+# 22 +# 23 , and is phase compared with the output of oscillator 29, to give an output from the discriminator 24 representing the phase shift #, which is fed to indicator I 21  calibrated in range. Frequency amplitude modulation may be used.