Patent GB-977418-A (GB)
Title: Determining the distance between two spaced points by means of the transit time of waves
IPC: G01S13/84
Filed: 1961-06-13
Abstract: 977,418. Radio distance - measuring systems. SOUTH AFRICAN COUNCIL FOR SCIENTIFIC & INDUSTRIAL RESEARCH. June 13, 1961 [June 14, 1960], No. 21324/61. Heading H4D. Relates to a C.W. phase comparison system for determining at a first radio station T1 the distance of said station from a second radio station T2 by transmitting from the station T1 two unrelated frequencies #1, f2 and transmitting from the station T2 a frequency f3 which is related in frequency and phase in a predetermined manner with the frequencies #1 and #2 as received from the station T1, the frequency f3 as received at the station T1 being combined with the local frequencies #1 and f2 to give two comparison signals which are compared in phase to give the required distance; at the station T1 the frequencies #1 and f2 are produced by free-running generators and at the station T2 the frequency f3 is produced by a generator automatically controlled in frequency and phase in accordance with the frequency and phase comparison of two comparison signals corresponding to those produced at the station T1. In a first embodiment, Fig. 1, the frequencies of the two comparison frequencies are (#3 - #1) and (f2-f3), where 2(#3 -#1) = (#2 - #3) and the "pattern" " frequency, i.e. the frequency determining the phase sensitivity of the system, is 3#3 = (2#1 + f2). In a second embodiment, Fig. 2, f3 = #1 + f2, the frequencies of the comparison signals are (#3 - #1) and f2 where f2 = (#3 - #1) and the " pattern " frequency is f3 = #1 + f2. In the first embodiment the frequencies #1 f2 and f3 are transmitted as radio frequencies and in the second embodiment they are transmitted by FM as modulation frequencies, #1 and #3 being of the same order of magnitude and f2 being much less than #1 and f3. First embodiment, Fig. 1. As shown the frequencies #1, f2 and f3 are produced by generators 1, 2 and 8 respectively. At T2 the comparison frequencies (#3 - #1 and (#2 - #3) produced by a detector 9 are amplified at 10 and applied to a phase discriminator 11 which produces an output which is a function of the phase of a fundamental frequency relative to its second harmonic, e.g. two opposed peak rectifiers each detecting the peak voltage of each polarity of the wave to give zero output when the peaks of each polarity are equal. The output of the discriminator 11 is applied to a circuit 12 controlling the frequency and phase of the generator 8 to maintain the frequency relationship (#2 - #3) = 2(f3-f1). At T1 the comparison frequencies (#3 - #1) and (#2 - #3) produced by a detector 3 are separated by filters 5 and 6 and applied to a phase indicator 7; alternatively the comparison frequencies may be applied to a discriminator of the type used at T2 to give a cycle count signal. Second embodiment, Fig. 2. In this the modulation frequencies #1, f2 and f3 are produced by modulators 13, 14 and 20 respectively and carrier waves of different frequencies F1 and F2 are produced by generators 15 and 21 respectively. At T2 the frequency modulated carriers are mutually heterodyned in a detector 22 and applied to a bandpass amplifier 23 whose centre pass frequency is equal to (F1 - F2), the pass band being less than #1 or f3 and greater than #2, to give I.F. signals of frequency (F1-F2) which are amplitude and frequency modulated respectively at the frequencies (f3-f1) and f2. These modulation signals are detected by the AM and FM detectors 24 and 25 respectively and applied to a phase discriminator 26 controlling the modulator 20 to establish the frequency relationship (#3 - #1) = f2. At T1 the comparison frequency (#3 - #1) is derived in a corresponding manner by corresponding components 16, 17, 18 but the comparison frequency f2 is derived directly from the modulator 14. Phase ambiguities may be resolved by providing a number of different values for #1 and f3 and retaining f2 constant.