Patent GB-731901-A (GB)
Title: Improvements in or relating to radio distance measuring systems
Inventor: PALMER WINSLOW
IPC: G01S13/84
Filed: 1953-10-05
Abstract: 731,901. Radar. SPERRY CORPORATION. Oct. 5, 1953, No. 27237/53. Addition to 671,037. Class 40 (7). Relates to a C.W. radar system similar to that disclosed in Specification 671,661 in which adjacent frequencies a and b radiated respectively from spaced interrogator and responder stations are heterodyned at the interrogator to give a beat frequency (a-b) which is compared in phase with another signal of like frequency, the resultant phase difference #1 being dependent on the frequency b and providing an accurate but ambiguous " fine " indication of the distance D between the stations. According to the present invention the principles disclosed in Specification 671,037 are applied to such a system to resolve the phase ambiguity by transmitting a second pair of frequencies c and d from the interrogated and responder respectively such that (a-b) = (c-d) to give a second " fine " phase difference #2 which is dependent on frequency b, a less ambiguous " medium " phase indication being obtained by taking the difference of 61 and 62 which is dependent on a lower frequency (b-c). A still less ambiguous " coarse " indication 63 dependent on a lower frequency (a-b)N is obtained by generating at the interrogator a signal of such frequency and comparing it in phase with such signal after transmission to the responder and back by modulation on frequencies a and d respectively. The frequencies a, b, c, and d are preferably chosen so that the ratio of b: (b-c): (a-b)N is equal to 400: 20: 1. The relative velocity of the two stations may be determined by measuring the rate of change of the phase difference 61. In the drawings Figs. 2 and 3 illustrate the interrogator and responder respectively, reference numerals under 50 referring to Fig. 2. Fine phase difference #1. The frequency a modulated by the low frequency (a-b)N from oscillator 13 is radiated from transmitter 12 and the frequency b is radiated from transmitter 50. These signals produce in receiver 51 a beat frequency (a-b) and a modulation frequency (a-b)/N which are separated by filters 53 and 52 respectively and applied to phase detector 55, the modulation signal being first frequency multiplied at 54 by a factor N, and the detector output being applied to automatically regulate the frequency and phase of the b signal generated at transmitter 50 to maintain the two inputs to detector 55 equal in frequency and phase. The Specification shows that such control ensures that the phase constant # of the transmitted frequency b is equal to -bR where R = D/C, D being the distance between the two stations and C the velocity of propagation of radio waves. At the interrogator the frequencies a and b from transmitters 10 and 50 are heterodyned in receiver 11 and the beat frequency (a-b) separated by filter 12 is applied to phase meter 16 together with the output from oscillator 13 after frequency multiplication by the factor N at 15, the resultant " fine " phase difference 61 = 2bR being displayed on dial 16<SP>1</SP>. Medium phase difference (#1-#2). The frequencies c and d are radiated from transmitters 20 and 10 respectively and the phase constant # of frequency d is automatically maintained equal to -dR by a control system similar to that controlling frequency b described above and comprising filter 62, detector 64 and controller C3 ; the modulation frequency output (a-b)/N from filter 52 is also applied to C5 to modulate the frequency d. At the interrogator the frequencies c and d from transmitters 20 and 60 are heterodyned in receiver 21 and the resultant beat frequency (c-d) separated by filter 27 is applied to phase meter 26 together with the modulation signal (a-b)/N from receiver 21 after frequency multiplication at 23 by the factor N to produce an output proportional to #2 = 2cR. The outputs from phase meters 16 and 26 are applied to a mechanical differential 30 giving the medium phase difference (#1 - #2) = 2R(b-c) which is displayed on dial 30<SP>1</SP>. The " coarse " phase difference 63 is determined by applying the (a-b)/N signals from oscillator 13 and filter 22 to a phase meter 25 whose output 63 is displayed on dial 25<SP>1</SP>. The relative velocity between the two stations is indicated by a meter 16<SP>11</SP> which measures the rate of change of the phase difference 61.