Patent GB-630867-A (GB)
Title: Improvements in or relating to radio position-indicating systems
IPC: G01S1/30
Filed: 1946-08-21
Abstract: 630,867. Radio navigation. O'BRIEN, W. J. Aug. 21, 1946, No. 24982. Convention date, Aug. 27, 1945. [Class 40 (vii)] In a radio frequency navigation system comprising at least three spaced transmitters arranged to radiate radio frequency signals of different but harmonically related frequencies having a fixed multiple phase relation and so defining intersecting equi-phase position-determining patterns, means are provided for also radiating signals which either alone or in conjunction with the first-mentioned signals define a coarse geographical co-ordinate system. Three embodiments, in which different means of providing the less ambiguous positiondetermination system are described. In each case the more accurate but ambiguous system is that described in Specification 620,479. In the first embodiment the radio frequency signals of the system of this Specification are modulated by audio-frequency signals. Considering one pair of stations, say A and B, radiating 60 kc. and 80 kc. radio frequency signals which provide a hyperbolic field pattern in which there are 200 zero phase difference lines between the two stations at the comparison frequency of 240 kc., then to provide a system with only quadrant ambiguity the audio frequency used is a hundredth of the comparison frequency or 2400 cycles. This frequency may be derived by frequency division from the main oscillator at the master station and used to modulate the radiated signals. At the slave stations the signals received from the master station are modulated at 2400 cycles and this modulation is passed unchanged through the slave stations and modulates the slave station signals, also at 2400 cycles, Fig. 2 (not shown). At the mobile receiver signals from the spaced stations are received, separated and compared in pairs in the manner described in Specification 620,481, to provide the accurate determination of position. The output of each receiver is also fed to a detector and the phase of the audio frequency modulation signals compared in pairs to give indications which determine the position of the craft according to the less ambiguous system. Phase-shifters with calibrated scales inserted in one-input of each phase comparison indicator may be adjusted so that the indication is of zero phase difference. In a second embodiment the radiation from one station of each pair is periodically interrupted and radiation at the same radio frequency from a station spaced a difference of one wavelength at the comparison frequency and preferably in the same direction from the other station, which continues to radiate, substituted. The phase difference at the receiver between signals of the substitute station and those of the station that continues to radiate determines a position line on which the craft lies with only quadrant ambiguity, Fig. 4 (not shown). In the third embodiment, the radio frequency signals of the spaced stations are varied by a small amount so that for each pair the number of equi-phase lines at the comparison frequency is reduced or increased by one. Two stations A, B, forming one pair and radiating at frequencies of 60 kc. and 80 kc. normally may be separated by one hundred wavelengths at the comparison frequency of 240 kc., so that there are 200 equiphase lines 1. If periodically the frequencies of the two station are proportionately reduced so that their separation is ninety-nine and a half wavelengths at the new comparison frequency then there will be a new set of equi-phase lines 2 only coinciding with the original set 1 along the line 30. The phase difference between the phase comparison values on the original and the substitute system will vary progressively from zero along line 30 to 180 degrees along the line 31-32. At the master transmitter of such a system arrangements are made to feed the power amplifier alternately from two master oscillators tuned to 60 kc. and 60 kc.  #f. Only one oscillator may be used, the frequency of which is periodically switched between these two values, for example by changing the constants of the oscillatory circuits. As the apparatus of the slave transmitters operate to radiate frequencies proportional to that of the master station the transmitted frequency from the slave stations will automatically be shifted proportionally to the change in master transmission frequency. Specifications 620,480, [Group XXXVI], 628,125, and 628,747 also are referred to.