Patent GB-1272745-A (GB)
Title: A radio navigational system
Inventor: MARRIOTT PAUL ALLEN
IPC: G01S1/22
Filed: 1968-01-03
Abstract: 1,272,745. Radio navigation. LIGHTVALE ELECTRICS Ltd. 3 April, 1969 [3 Jan., 1968], No. 41338/67. Heading H4D. In a radio navigation system wherein intersecting position lines for a vehicle (e.g. a ship) are established by measuring the difference in time of travel of signals from discrete pairs of radio transmitters, the signals from each transmitter pair each have a frequency repetitively increasing and decreasing with reference to a basic frequency e.g. in sawtooth manner at equal repetition rates, such that the instantaneous frequency difference has a predetermined value at any instant. The basic frequencies differ, and a receiver is tuned to receive simultaneously both signals, which beat to produce a measured heterodyne frequency which indicates the difference in time of arrival, and thus the difference in distance between the vehicular receiver and the transmitter to establish a specific hyperbolic position line; the heterodyne frequency being preferably indicated on a scale calibrated to show directly the appropriate position line, which may be overprinted on a chart. Transmitters A, B (Fig. 1A) are located at positions A, B spaced e.g. 186 miles and respectively transmit frequencies repetitively varying in linear sawtooth manner from e.g. 100 KHz to 100À5 KHz and from e.g. 99À2 KHz to 99À7 KHz, over periods of 0À005 sees. (Fig. 1B) so that an equidistant receiver on line CD receives signals differing by 800 Hz over each period, while a receiver at A or B receives difference signals of 900 Hz and 700 Hz; intermediate differences defining hyperbolic positional lines for the receiver station; e.g. Rx 93 miles nearer A than B for 850 Hz difference. The input signals are converted to lower frequencies at equivalent level by AVC controlled superheterodyne receivers with stable oscillators, and frequency multiplication is introduced before final heterodyning. Each transmitter produces alternately a constant frequency signal and a sawtooth FM signal over successive periods of e.g. ¢ second, and the transmitters may be those of standard navigational beacons spaced e.g. 186 miles in pairs of master and synchronized slave stations; the slave being delayed e.g. 0À001 seconds relative to the master and the frequencies being respectively 308 KHz steady and 307À9 to 308À1 KHz FM over 0À005 secs., and 300 KHz steady and 299À9 KHz to 300À1 KHz FM over 0À005 secs. The receiver (Fig. 2A) comprises identical master and slave AGC controlled superheterodyne channels with xtal oscillators 3, 7; the master output being applied to a continuously variable ultrasonic delay line 9, while both outputs are frequency multiplied by 32 at 11, 12 and heterodyne mixed at 13; the output being amplified and applied to a loudspeaker and frequency meter 16 marked at 1.5 KHz, and calibrated to designate specific chart position lines. During non-FM transmissions the output frequency is e.g. 1À5 KHz and during the sequential FM transmissions the frequency depends on the receiver position, relative to the transmitters, and the delay is initially adjusted until the loudspeaker output and/or frequency meter/indication is invariable at 1À5 KHz (Fig. 2B) over alternate e.g. ¢ second periods. Alternatively, the frequency sweeps are slowly and cyclically varied in phase displacement over a comparatively long period; the instantaneous frequency difference between the signals having a predetermined value at any instant; the heterodyne frequency varying to represent the receiver position and signal phase difference at that instant. Short constant frequency reference signals are transmitted before phase variation commences, and the time interval between the end thereof and the production of a predetermined heterodyne frequency depends on the receiver position; a trigger timer starting on cessation of the reference signal and arrested on inception of the heterodyne measuring this interval and designating the position line. Each signal may have, over alternate periods, a constant frequency differing from that of the other, with the varying phase displacement mentioned, and an output loudspeaker indicating differences of the heterodyne frequency in successive periods; the number of discrete signals counted from a reference instant until zero frequency difference indicating the position line. For accurate distinction between successive heterodyne frequencies, a frequency meter may be used, and the fractional part of the last count may be estimated therefrom if the heterodyne of the non-FM signals is made to alternate between two different frequencies whose difference is the frequency step of the heterodyne between successive FM signals (Figs. 3A to 3C, not shown). Coarse/fine operation may be obtained using low and high repetition frequencies. The transmitter signals may be derived photomechanically from synchronously rotated discs obturating illumination of a photo-cell, or electronically, and periodic synchronization of waveform commencement between master and slave transmitters is obtained by independent transmissions or alternatively by receiving signals from one transmitter and phasing that of the other for correct position signal reception at a point adjacent to the second transmitter.