Patent GB-653784-A (GB)
Title: Radio survey system
IPC: G01S1/02
Filed: 1948-10-01
Abstract: 653,784. Radio navigation. SEISMOGRAPH SERVICE CORPORATION. Oct. 1, 1948, No. 25696. Convention date, Oct. 9, 1947. [Class 40 (vii)] The transmission system for a hyperbolic equi-phase contour radio navigation system comprises three spaced non-synchronous transmitters for radiating continuous waves of different frequencies, means responsive to the radiated waves for producing signals having frequencies respectively representative of the heterodyne frequencies between different pairs of said waves, and means for modulating at least two of the signals on a common carrier wave for radiation. The signals representative of the heterodyne frequencies may be modulated on a separate carrier of different frequency or on one of the radiated waves and may be the heterodyne frequencies themselves or signals obtained by frequency division therefrom. In a two channel system, that is one in which the reference signals are radiated on a separate carrier, the three spaced transmitters 10, 11 and 12 (Fig. 1), may radiate signals of frequencies 1601.75 kc, 1602 kc. and 1602.60 kc. respectively. At the link unit or reference signal generator 14 signals from the spaced transmitters are received by receiver 14a and heterodyne frequencies of 250 c/s., 600 c/s. and 850 c/s. are obtained in the receiver output. The 850 c/s. signal is removed by wave trap 15 and the 250 c/s. and 600 c/s. signals used to modulate the 1700 kc. carrier radiated by transmitter 14b. At the receiver whose position is to be determined reference signals from the link unit are received on 1700 kc. receiver 16 and the detected 250 and 600 cycle signals separated by filters 18 and 19 and applied respectively to phase meters 20 and 21. Signals from transmitters 10, 11 and 12 are received directly by receiver 17 tuned to 1602 kc. and the heterodyne frequencies passed through wave trap 24 to remove the 850 c/s. signal. The 250 c/s. and 600 c/s. heterodyne signals passed by trap 24 are separated by the filters 22 and 23 and applied respectively to the other inputs of phase meters 20 and 21. The phase meters are preferably of the integrating type and meter 20 indicates the receiver position with respect to a hyperbolic contour pattern having transmitters 10 and 11 as foci while meter 21 indicates position with respect to a similar pattern intersecting the first and having transmitters 11 and 12 as foci. In a single channel system (Fig. 2), the transmitter 11 and link unit 14 of Fig. 1 are combined. The central station radiates a 1602 kc. signal which is received together with the 1601.75 kc. and the 1602.60 kc. signals from spaced transmitters 10 and 12 by receiver 27. The 850 c/s. beat frequency is removed by wave trap 28 and the 250 c/s. and 600 c/s. frequency divided, for example by a factor of 10, in unit 29 and then used to modulate the 1602 kc. wave transmitted. At the receiver unit 13A two 1602 kc. receivers are used, the output of one (30) being multiplied in frequency by the same factor used in divider 29 so that the 250 c/s. and 600 c/s. reference waves may be separated and recovered for application through filters to meters 35 and 36 respectively. Receiver 31 obtains the direct heterodyne signals as in the first embodiment and required ones of these are applied to the other inputs of meters 20 and 21 which give indications as before. In a variation of this embodiment the divided reference frequencies from unit 29 may be used to phase modulate the 1602 ke. transmission and receiver 30 designed for phase detection instead of amplitude detection. In a further embodiment (Fig. 3, not shown), phase modulation of the 1602 kc. is alone relied on to obtain selective detection of the reference signals at a receiving unit and frequency divider 29 and the corresponding multiplier 39 in the receiver omitted. Specifications 452,235 and 607,371 are referred to.