Patent GB-671661-A (GB)
Title: Improvements in or relating to distance-measuring systems employing electromagnetic waves
IPC: G01S13/58
Filed: 1948-10-21
Abstract: 671,661. Radiolocation. SPERRY CORP ORATION. Oct. 21, 1948 [Oct. 21, 1947], No. 27386/48. Class 40 (vii). In a C.W. radar system comprising an interrogator-responsor 1 and a responder (transponder) 2 frequencies u and p are radiated continuously from transmitters 20 and 15 located respectively at 1 and 2. At the responder the frequencies p and u are received at 18 and heterodyned to give a beat frequency (p - u) which is selected by filter 19 and frequency-divided at 24, giving a frequency (p-u)/N which is applied to amplitudemodulate the transmitter 15 which accordingly radiates a carrier P and side-bands PŒ (p - u)/N which are received by receiver 23 at the interrogator together with the frequency u radiated by transmitter 20. The modulation signal (p - u)/N is selected by filter 26 and frequency multiplied at 37 to give a frequency (p - u) which is compared in phase with a signal of equal frequency obtained by heterodyning the carriers p and u in receiver 23 and selecting the resultant beat frequency by a filter 27. As shown, the outputs from 37 and 27 are respectively applied directly and via a rotary phase-shifter 30 to a phase discriminator 28 whose output controls a servo 29 which rotates phase-shifter 30 until the inputs to the discriminator are in phase, the resultant rotation as displayed on a counter 31 being a measure of the phase'difference 6 between the outputs from 37 and 27 and the Specification shows that #=2# u 2 D/c, where D is the distance between the interrogator and responder and c is the velocity of propagation; i.e. although the phase measurement is carried out at a low frequency (p - u), the phase angle is dependent on the carrier frequency u. If the distance D is changing, the frequencies of the outputs from 37 and 27 will differ due to the Doppler effect and the frequency difference is measured by meter 34 calibrated to indicate relative velocity. The velocity may also be indicated by gearing a calibrated rate device 35 to the range indicator 31. If it is desired to obtain distance indications at each end of the system, the interrogator is modified to function additionally as a responder by frequencydividing at 32 the beat-frequency (p - u) from 27 and applying the resultant frequency (p - u)/i to amplitude-modulate the transmitter 20. The responder is then replaced by a similar combined interrogator-responder, the filter corresponding to 26 having a pass-band (p - u)/i and the divider corresponding to 32 having a division factor N. The phase comparator is illustrated in more detail in Figs. 2-4 (not shown). The Specification as open to inspection under Sect. 91 describes a modification, Fig. 2 (Cancelled, not shown), providing " fine," " medium " and " coarse " phase shifts to resolve ambiguities in which a first " fine " phase difference #1 is determined as above utilizing frequencies a and b corresponding to p and u so that #1=kD and a second " fine " phase difference #2 is determined by additional similar apparatus utilizing frequencies c and d where (a -b)= (c - d) and b > c, so that #2=kD, the " medium " phase difference being given by 61 - #2= k(a - d)D and the " coarse " phase difference 63 being determined by observing the phase shift on the modulation frequency (a - - b) or (c - d), so that #3 = k(a -  b)D; the ratios of the frequencies (a - b), a - d) and a are preferably 1: 20: 400. This subject-matter does not appear in the Specification as accepted.