Patent GB-1096452-A (GB)
Title: Electric signal transmission system
Inventor: EARP CHARLES WILLIAM
IPC: G01S5/00
Filed: 1964-05-08
Abstract: 1,096,452. Radio navigation. STANDARD TELEPHONES & CABLES Ltd. May 7, 1965 [May 8, 1964], No. 19217/64. Heading H4D. In a path difference, phase comparison, radio navigation system comprising a plurality of transmitting stations and one receiving station, each transmitting rotation transmits two waves having respective frequencies unilaterally displaced from a third frequency by harmonically related difference frequencies and having respective known phase relationships with a wave of said third frequency. The two waves are received at the receiving station to provide an indication of the phase said wave of said third frequency would have at the receiving station had it been transmitted. A phase indication so provided by one transmitting station can be compared with that provided by another transmitting station to define an isochrome. Each transmitter, Fig. 1 (not shown), comprises a single sideband transmitter wherein a carrier of frequency nf, e.g. 10,000 c/s., is modulated by harmonically related frequencies # 1  and 2#1, e.g. 50 c/s. and 100 c/s., to produce the two waves for transmission at frequencies of 10,050 c/s. and 10,100 c/s. The frequencies f 1  and 2f 1  are different for each transmitter. At the receiver, Fig. 2, the two waves are received and passed via predetector stages 11 to a mixer 12. The mixer is also fed with the output of a stable oscillator 13, operating at the frequency nf, via a variable phase shifter 14. Outputs at frequencies of f 1  and 2f 1  are selected from the mixer output, by filters 15 and 16, and are compared in phase-possibly after frequency doubling of the output at frequency # 1 -in a differential phase detection 19. The output of the phase detector controls a motor 18, which in turn varies the phase shifter 14, until the two mixer outputs are in phase. When this is accomplished, the signal at frequency nf at the output of phase shifter 14 has the same phase as the carrier would have at the receiver had it been transmitted. The output is therefore fed to a phase comparator 31 where its phase is compared with that of a similar signal derived using transmissions from another transmitting station. The isochrome so defined gives an ambiguous position and to overcome this ambiguity each transmitter also comprises a second single sideband modulator wherein a carrier of frequency mf e.g. 12,000 c/s., phase locked to the first carrier of frequency nf, is modulated by a frequency #1, to produce a signal for transmission at a frequency of 2000 c/s. Due to the difference in the frequencies of the 12050 c/s. signal and the 10,500 c/s. and 10,100 c/s. signals, the former signal may be transmitted with a different propagation time to the receiver, and to overcome resulting phase errors, a manual phase shifter 23 and an automatic phase shifter 24 adjust the phase of the 2000 c/s. signal. The output of the automatic phase shifter 24 is frequency multiplied five times in harmonic generator 25 and compared for phase with the output of phase shifter 14 in phase difference detector 26. The output of the detector controls a motor 27, controlling the phase shifter 24. The phase adjusted signal at 2000 c/s. is compared for phase with a similar signal derived from another transmitter station in phase comparator 32. The isochrome so defined is coarse but unambiguous.