Patent GB-683689-A (GB)
Title: Improvements in or relating to navigation aiding radio systems
Inventor: COCKERELL CHRISTOPHER SYDNEY
IPC: G01S1/02
Filed: 1949-01-27
Abstract: 683,689. Radio-navigation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 25, 1950 [Jan. 27, 1949], No. 2311/49. Class 40 (vii). In a C.W. hyperbolic navigation system each of a plurality of spaced stations transmit at least four phasesynchronized frequencies half of which are above and half below the mean frequency and being so chosen that all the frequencies are different and two adjacent upper or lower frequencies differ from each other by the same amount and any two adjacent frequencies in the upper, or in the lower, group from any one station differ by a multiple of that amount and the difference between the highest upper frequency and the lowest lower frequency being small in relation to the mean of those frequencies. The required frequencies are preferably sidebands derived at each transmitter by modulating a carrier which is the same for all the transmitters. The carrier and modulating signals in this case, and in general the radiated signals, must be phase synchronized. In carrying out the invention it can be so arranged that fine medium and coarse positional determination can be carried out at a co-operating receiver by the selection and addition or subtraction of the received frequencies. In one embodiment of the invention comprising a system of four stations A, B, C and D the transmitted signal frequencies may be as shown diagrammatically in Fig. 3. This Figure shows station A frequencies of fo+fm, fo-fm, fo +fm + 4fl, and fo-fm-4fl, station B frequencies of fo+fm+fl, fo-fm-fl, fo+ fm+ 5fl and fo - fm - 5fl, and so on. Preferably fm is considerably greater than fl and the difference between the highest and lowest frequencies used (2fm+14fl) is made small compared with the mean frequency fo. At the receiver for fine positional indication the sum of corresponding upper and lower signals from the same station is compared in phase with the sum of similar signals from another station. For example 1A+A1 may be compared with 1B+B1 at the common sum frequency of 2fo. For coarse positional indication phase comparison may be made at a frequency of 4fl obtained by frequency subtraction of signals from the same stations such as 1A - 2A and 1B - 2B. In an embodiment giving fine, medium and coarse positional information the frequencies in a four station system may be as shown in Fig. 4. In this embodiment the frequencies below fo are the same as in the previous embodiment but the upper frequency " band " commencing with A1, B1, C1, D1 as before is completed by only 3 further signals C2, B2 and A2 in that order separated by frequency intervals of fl. For fine positional indications at a co-operating receiver sum frequencies 1A+A1, 1B+B1, 1C+C1, 1D+D1, are compared in phase in pairs as necessary. For medium positional indications comparison is made at a frequency of 2fm+6fl between difference frequencies (A2 - 1A) ... (D1 - 1D) or alternatively at 2fm+10fl between (A2 - 2A) and so on. Finally, for coarse indication comparison is made between difference frequencies of 4fl taken from the series (1A - 2A), (1B - 2B) and so on. A system is described similar to the embodiment of Fig. 4 but having ten stations stated when suitably sited to be capable of world coverage (Fig. 5, not shown). Numerical frequency values of from 11,625 c/s. to 13,305 c/s. with a frequency fo of 12,500 c/s. are suggested. A suitable receiver for such a system is also described, this receiver is capable of receiving the signals from any four stations at the same time and the filters are provided with exchangeable plug-in crystals so that it may be used in conjunction with any selected stations. The receiver comprises a broad band amplifier stage followed by the crystal filter units separating the individual signals. The outputs of the filters are fed through switch means in pairs to combining means where they added or subtracted as the case may be and the combined signals from different stations compared in phase in pairs in two separate indicators having fine medium and coarse indication scales and pointers, Fig. 6 (not shown). Specification 683,688 is referred to.