Patent GB-626938-A (GB)
Title: Improvements in or relating to radio navigation systems
IPC: G01S1/30
Filed: 1946-02-05
Abstract: 626,938. Radio - navigation. STANDARD TELEPHONES & CABLES, Ltd., EARP, C. W., and STRONG, C. E. Feb. 5, 1946, No. 3619. [Class 40 (vii)] In a system for determining the bearing of a mobile receiver with respect to a beacon comprising spaced transmitting aerials, the signal radiated from each aerial is modulated sinusoidally with a frequency greater than the upper limit of the audio-frequency spectrum, and the bearing of the receiver is obtained by determining the phase relationship between the received modulation signals. Any form of modulation may be employed but in order to minimise interference from adjacent stations frequency or phase-modulation is preferred. The invention is first described as applied to a system with two spaced aerials with a spacing of the same order as the modulation wavelength. The aerials are energized alternately at the same wavelength, the exciting currents being modulated with the same frequency and phase. of modulation and the two transmissions being identified by additional low-frequency modulation tones or by pulses of different amplitude or width radiated at the beginning of each transmission; alternatively the aerials could be energized continuously with different carrier frequencies. At the receiver, the phase difference between the modulation frequencies is determined by a dynamometer type phase meter or by a cathode-ray tube. Such a system could be used as an omni-directional beacon or to provide a fixed course line and additional course lines could be produced with the same aerial system by modulating with additional frequencies. In order to facilitate the measurement of phase difference, the different modulated transmissions should be operated with the same depth of modulation and in the case of F.M. or P.M. this ensures the most effective use of receiver bandwidth. In a practical embodiment, Fig. 1, means are shown for producing a constant-phase course line normal to the line of the three aerials A, B, C, the outer aerials A and C being spaced apart somewhat less than a wavelength of the modulation frequency and the three aerials being energized sequentially at the same radio frequency for equal periods under the control of the commutator 5 which also controls the synchronized modulating frequency sources 3 and 4 so that the signals from B are modulated at 250 kc/s. and the signals from A and C are modulated at 500 kc/s. The lengths of the transmission lines 6, 7 and 8 feeding the aerials are such that the modulation frequencies transmitted from A and C are in phase opposition with respect to each other and in phase quadrature with respect to the modulation frequency transmitted from B. For low-speed switching, the commutator may comprise a mechanical switch, but for high-speed switching, e.g. 3 kc/s., an electronic commutator system of the type disclosed in Specification 596,699 may be employed and in this case additional modulation, eg. identification signals and/or telephony, may be applied when the reference aerial B is energized. At. the receiver the intermittent wave trains obtained on demodulation are converted into continuous wave trains of 200 c/s., the relative phasing of which correspond to the relative phasing of the intermittent modulation wave trains. The B signal 200 c/s. wave train is taken as the reference and the A and C 200 c/s. wave trains are combined to produce a resultant wave train which is of zero amplitude when the receiver is on course and which is either in phase or phase opposition with the reference-wave train when the receiver Is displaced to one side or other of the course line. As shown in Fig. 2, the intermittent wave trains of 500 kc/s. and 250 kc/s. produced by the receiver 31-35 are separated by the filters 37 and 39, and the 250 kc/s. signal is frequencydoubled at 40, the two sets of 500 kc/s. signals being then heterodyned in separate mixers 38 and 41 with an oscillation 43 of frequency 500.2 kc/s. giving intermittent 200 c/s. wave trains which are converted by means of 200 c/s. filters included in the mixers 38 and 41 into continuous 200 c/s. wave trains for application to the two coils of a dynamometer type leftright indicator 44. According to the Provisional Specification, the signals received from the aerials A, B and C could be separated by a synchronized gating system and pulse width discriminator. A monitoring receiver may be mounted on the course line and any output from this receiver indicating that the course line is not in correct alignment may be applied for automatic correction, e.g. by modifying the intervals during which the aerials A and C are energized. Figs. 3 and 6 (not shown) illustrate an extension of the above principles to an omni-directional beacon comprising four aerials located at the corners of a square together with a central reference aerial, the aerials being energized sequentially at the same radio frequency, but the operative period of the central aerial being twice that of the outer aerials. As before the reference aerial is modulated at 250 k/cs. and the outer aerials are modulated at 500 kc/s. with a relative phase of 0 degrees, 90 degrees, 180 degrees and 270 degrees. Phase-modulation is employed with an initial phase deviation of 45 degrees which is increased by applying the modulated carrier to the aerials through two frequency-doubling stages in tandem. Provision is also made for superimposing speech modulation on all the aerials. At the receiver, Fig. 4, the speech signals are separated out from the output from the receiver proper 81-85 by means of the low-pass filter 88, and the 250 kc/s. signals are filtered out and frequencydoubled at 86 and 89 and then heterodyned with the crystal-controlled 500.1 kc/s. oscillation 93 in the mixer 92 giving an intermittent 100 c/s. wave train which is applied to the filter 95 giving a continuous reference 100 c/s. wave train which is applied to the rotor 99 of a dynamometer 98. The 500 kc/s signals are converted into a continuous 100 c/s. wave train by similar means 87, 90 and 96, and applied to one field coil 100 of the dynamometer and it is shown that the phase of this wave train is directly proportional to the bearing angle. In order to produce a phase quadrature component of this variable phase wave train for the other field coil 101, the 500.1 kc/s. oscillation is fed to a second mixer 91 through a 90 degrees phase shifter 94, the output from the mixer being then applied to the coil 101. Alternatively the phase quadrature component could be derived directly from the component applied to the coil 100 by means of a 90 degrees phase-shifting circuit, but this would impose stringent frequency stability requirements on both the beacon modulation source and the receiver oscillator 93. In any of the embodiments of the invention the omni-directional aerials may be replaced by directional arrays in order to limit the beacon signals to a given sector. Specifications 452,235, 575,164, 577,824, 579,346, 582,085, 584,727 and U.S.A. Specification 2,198,113 also are referred to.