Patent GB-1091903-A (GB)
Title: Radio navigation system
Inventor: EARP CHARLES WILLIAM
IPC: G01S5/00
Filed: 1964-02-20
Abstract: 1,091,903. Radio navigation. STANDARD TELEPHONES & CABLES Ltd. Feb. 19, 1965 [Feb. 20, 1964], No. 7129/64. Heading H4D. The Specification describes a radio navigation system wherein the distance of a mobile object from a fixed station is represented ambiguously by two signals and wherein means are provided to obtain from the two signals, a signal representative of the relative timing of the two signals, and to obtain from said representative signal an unambiguous indication of the distance between the mobile object and the fixed station. The object of the invention is to reduce the effect of phase errors due to the propagation of the two signals being different. In the simplest embodiment, Fig. 1, two signals at frequencies mf and nf, e.g. 11 kc/s. and 10 kc/s. are transmitted from a fixed station and received at the mobile station. The two signals are taken from the receiving stages 1 and are separated out by filters 3 and 4. The phase of the signal at mf is compared in phase meter 11 with that of a signal from a highly stable source 12 of like frequency, to provide a fine but ambiguous distance indication. The two signals are converted to corresponding pulse trains in pulse generators 5 and 6, and the pulse trains are fed to a coincidence gate, to give an output therefrom at a frequency mf-nf, e.g. 1 kc/s. This output, after passing through filter 10 is compared in phase, in phase meter 8, with the output of a highly stable source 9, of like frequency, to provide a course but unambiguous distance indication. A manual phase adjuster may be provided in the output of one of the filters 3 and 4 to enable any zeroing adjustments. If the phase of the signal at nf is in error by p, and if the width of the pulses generated by generators 5 and 6 is <SP>1</SP>/ 20 th of the spacing of the pulses from generator 6, then the resulting phase error measured by meter 8 is only p/20. In a second embodiment, Fig. 3 (not shown), the signals from the two filters 3 and 4 are frequency multiplied by factors n and m respectively to convert them to a common frequency mnf, e.g. 110 kc/s. These two signals of common frequency are phase compared and the result of the comparison produces a signal adjusting a phase shifter in the path of the filter 4 to the frequency multiplying means. If the phase of the signal at mf is in error by p, then the frequency multiplied signal will be in error by np. The output of the phase comparator adjusts phase shifter by amount k, which becomes mk at the input to the phase comparator, due to the m times frequency multiplying means, until the two signals of common frequency are in phase and k = 0. The phase shift introduced by the phase shifter is thus (n/m).p. The differential error   between the output of filter 3 and the phase-   shifted output of filter 4 is thus (p-(n/m)p, e.g. p(1 - 10/11)p/10. These two outputs are beat together in a detector, to produce a signal at (m-n)f, e.g. 1 kc/s., which is compared in phase with the output of a highly stable generator to give the unambiguous course distance. The fine, ambiguous distance indication is given as in the embodiment of Fig. 1. Where the position of the mobile station is to be found, three fixed transmitting stations may be used, radiating different pairs of signals, and the arrangements of Figs. 1 and 3 may be triplicated. Alternatively, the transmitting station may transmit the same signals on a timesharing basis. A course indication of the differential distance between the mobile station and two of the fixed stations may be obtained by subtracting the corresponding phase difference measurements on meter 8. If, due to extreme propogation distances, the phase error p is too large for reduction by the methods of Fig. 1 and Fig. 3, manual phase adjusters may be provided in one of the signal paths.