Patent GB-1045310-A (GB)
Title: Improvements in radio navigation systems
IPC: C09B47/10
Filed: 1963-08-20
Abstract: 1,045,310. Radio navigation. CSF-COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. Aug. 20, 1963 [Aug. 23, 1962], No. 32991/63. Heading H4D. In a radio navigation system in which a surface S 0  is defined by moving beams one at least of which has a characteristic which is a function of its instantaneous angular position, the corresponding receiver stores a continuous signal which is a function of the ratios of the characteristics of the beams at surface S 0 . The direction-characterizing signals may be carrier wave amplitude, frequency or phase, or those of a signal modulating the carrier wave. In the ground installation of a landing system, Figs. 4a, 4b, line OA is the plan projection of a vertical plane Pa in which an aircraft M has to descend; this vertical plane is defined by transmitters D<SP>1</SP>, G<SP>1</SP> symmetrically placed with respect to line OA and radiating contrarotating scanning beams (S<SP>1</SP> 1,  S<SP>1</SP> 2 ) about vertical axes, the instantaneous positions of the beams being defined by their azimuths # 1 , # 2 , Fig. 2b, as functions E 1 <SP>1</SP> (# 1 ), E<SP>1</SP> 2  (# 2 ). Plane Pa is defined by corresponding received voltage ratios e<SP>1</SP> 1 , e<SP>1</SP> 2 . The elevation of M from O is defined by two scanning beams (S 1 , S 2 ) of the same origin S, Fig. 2a; these beams have their respective instantaneous positions defined by angles # 1 , # 2 : a characteristic of the first beam varies as E 1 (# 1 )= log a#, where a is a constant and the corresponding characteristic of the second beam has a constant value E 2 . Distance measurement is afforded by a transmitter at a point D<SP>11</SP> from which a beam (S<SP>11</SP>) rotates in azimuth with a characterizing function E 1 <SP>11</SP>(α 1 ) = log pb/tan α, where p = D<SP>11</SP>O, b is a constant and α 1  is azimuthal angle relative to a line through D<SP>11</SP> and parallel to OA. Since OM#Om = r, E<SP>11</SP> 1 #log br. A further beam, rotating similarly, may be added at D<SP>11</SP>, its characterizing function being E<SP>11</SP> 2  = constant. Transmitters, Fig. 5.-The corresponding transmitting arrangement utilizes serials 101 ... 105 to produce corresponding characterizing functions E 2 , E 1 (# 1 ), E<SP>11</SP> 1 (α 1 ), E<SP>1</SP> 1 (# 1 ), E<SP>1</SP> 2 (# 2 ); the carrier wave frequencies are identical and the five beams are transmitted consecutively. The characteristic used is the amplitude of a signal proportional to the value to be reproduced and modulates in amplitude a lowfrequency (L.F.) wave which itself frequencymodulates the carrier wave. The frequency of the L.F. wave is different for each beam. The aerials are fed and driven by similar systems: e.g. system 192 comprises an electro-mechanical rotator 122, a backward-wave oscillator 112 of frequency varying linearly as a function of voltage applied by a potentiometer 132 fed from a D.C. source 142 and controlled by a rotator 122, via an amplitude modulator 152 also fed from a L.F. oscillator 162. The various aerial displacements are synchronized by a device 230: for example beam S<SP>11</SP> sweeps through a 90-degree sector, S 1  and S 2  sweep through a common -20-degree sector, and S<SP>1</SP> 1  and S<SP>1</SP> 2  sweep respectively through two 230-degree sectors to cover up to 50 degrees on either side of plane Pa, a complete five-beam cycle lasting 1/10 second, and beams being blanked during rotation or oscillation outside the desired sectors. The L.F. oscillators have frequencies f 1  . . . f 5  corresponding to aerials 101 ... 105. A monitoring receiver 200 is fed from a fixed aerial 210 of known spatial co-ordinates and generates error signals to modify the D.C. voltages fed to the potentiometers in units 191 ... 195. Aircraft receivers. Fig. 6.-The aircraft receiver comprises a super-heterodyne receiver 10, the output signals at I.F. passing via a limiter 11, to remove any amplitude modulation of the carrier wave, to a linear discriminator 12 supplying the L.F. signals which are filtered by units 21 . . . 25 centred on respective frequencies f 1  . . . f 5 . The filtered L.F. signals F are detected (31 ... 35) and applied to normallyopen. electronic switches 41 ...45 controlled by a detector 14 the output from which is a pulse when a beam sweeps through the aircraft; that pulse controls receiver gain and, after differentiation, clipping and further differentiation, closes switches 41 ... 45 only at the peak of an incoming signal. The switch outputs are fed to devices 51 . . . 55 each consisting of a comparator followed by an integrator: the second inputs to the comparators are continuous voltages e 2  &c. and the comparator outputs are integrated over a few successive sweeps to afford error signals of the form F-e. The output of device 51 is applied as a gain control to a L.F. amplifier 13 to maintain the peak level of the signal F 2  at the output of switch 41 at a constant level equal to e 2 . The outputs of the comparators 52 ... 55 control the gains of respective amplifiers 72 ... 75 to adjust the voltages e 1 , e<SP>11</SP>, e<SP>1</SP> 1  and e<SP>1</SP> 2  according to the peak values of the corresponding signals F. For azimuthal guidance the outputs of amplifiers 74 and 75 are compared on a centrezero voltmeter 81. For a linear glide slope of angle # 3 , voltage e 1  is compared with a voltage e 2  log # 3 /E 2  where Eg is a constant. Asymptotic approach, Fig. 6.-For an asymptotic approach to landing the aircraft first follows part of a line of slope γ 0 , terminating at a point Q a distance d ahead of touchdown point O, and then follows a shallower line of slope #. Substantially log # + log r - log (r - d) - log γ 0  = 0 where r is the distance of the aircraft from point O. A.C. voltages of equal amplitudes v and in anti-phase are provided by sources 80, 84. Voltage vL0<SP>0</SP> modulates voltages e, and e<SP>11</SP> thereby affording voltages representing log # and log r at the outputs of modulators 82, 83 respectively. Sources 80, 84 feed identical logarithmic potentiometers 831, 832 the output of potentiometer 831 being proportional to log k<SP>1</SP># where k<SP>1</SP> is a constant and # is the angular position of its shaft reckoned from a reference position such that when k<SP>1</SP># =1 the output amplitude is zero. A servo-mechanism 834 supplies a signal log r - log k<SP>1</SP># to position the shaft of the potentiometer at #=r/k<SP>1</SP>. A differential mechanism 833 angularly displaces the shaft of potentiometer 832 by a quantity d with respect to that of potentiometer 831, and, k<SP>1</SP> being conveniently made unity, the angular position of the shaft of the former represents (r - d) and it supplies a voltage representing - log (r - d). A manually-operated potentiometer 821 supplies a voltage representing - log γ 0 . The four voltages representing log #, - log γ 0 , - log (r - d) and log r are respectively supplied to a summing amplifier 89 directly and via switches I 1 , I 2 , I 3  which when closed afford an amplifier output " error " voltage the phase and amplitude of which respectively indicate the sense and magnitude of error from the first part of the glide path and are determined by a detector 85 the output from which is read on a centre-zero D.C. voltmeter 86. When the aircraft's distance from the touchdown point falls to r 0 , corresponding to the beginning of the shallower glide path, switches I 1 , I 2 , I 3 , are opened and switch I, is closed by a circuit 88. fed with voltages representing log γ 0  from a potentiometer 87 and log from amplifier 73; amplifier 89 then supplies a signal representing log # - log (# desired). A path with further linear sections, or a continuouslycurved glide path, may be arranged, Fig. 9. To avoid the effects of possible variations in the characteristics of filters 21 ... 25 and possible differences between the characteristics of detectors 31 ... 35, the L.F. amplifier 13 may feed a single detector. Fig. 7. Calibration.-The monitoring receiver 200, Fig. 5, may be similar to the aircraft receiver of Fig. 6 or 7: however, the gain of amplifier 13 is constant and amplifiers 72... 75 are replaced by manually-adjustable potentiometers. It is stated that the invention reduces errors due to re-radiating objects other than those between transmitter and receiver.