Patent GB-1021551-A (GB)
Title: Radio navigation system
Inventor: EARP CHARLES WILLIAM
IPC: G01S1/40
Filed: 1961-11-10
Abstract: 1,021,551. Radio navigation and directionfinding. STANDARD TELEPHONES & CABLES Ltd. Nov. 9, 1962 [Nov. 10, 1961], No. 40393/61. Heading H4D. In a radio navigation beacon system (and mutatis mutandis for a radio direction-finder) energy is cyclically commutated to verticallyspaced aerials 2, 3 . . . 6, Fig. 1, and at an aircraft 8 the received signals are amplitudedetected and give rise to a beat signal of mean frequency representative of the angle of elevation α; this effect arises from the differing phase at the aircraft of the direct and ground-reflected waves. The aerials may be disposed to give a sinusoidal modulation of the signal phase but in the particular embodiments a linear saw-tooth modulation of signal phase is provided by commutation to successive uniformly spaced aerials at a constant rate, thereby giving a received signal having a comparatively narrow spectrum and affording a mean beat signal frequency proportional to sin α. For glide-slope applications the receiver may incorporate a frequency discriminator tuned to the frequency representative of the glide slope. In an embodiment which maintains the linearity of steep glide-paths close to the ground a beacon aerial array 110, Fig. 2, comprises twenty stacked aerials 101 . . . 120, the lowest aerial being mounted at a height #/4 and the spacings between aerials being #/4; the array axis is inclined at a small angle #. Radio signals from the output of transmitter 142 are fed via a commutator switch 141 to the aerials of the array and the repetition frequency of the commutation process is determined by low-frequency signals from a generator 143; a suitable commutator switch is described in Specification 594,530. Point 108 represents an aircraft on a glide path in a vertical plane, represented by line 109, perpendicular to the plane of the drawing. In addition to the signal radiated by the commutated aerials an unmodulated carrier wave F 1 , of frequency 7 kc/s. away from the frequency of transmitter 142, is radiated. Aircraft receiver (Fig. 3).-In the receiver amplitude detector 200 wave F 1  beats with the phase-modulated signals received from the commutated aerials to produce signals having a centre frequency of 7 kc/s., which is amplitudemodulated at a frequency representative of elevation angle. The output of a filter 201, tuned to 7 kc/s., is connected to an amplitude detector 202 having a square-law characteristic and to an amplitude limiter 204; a frequency discriminator 205 feeds an amplitude detector 207 which is connected to an additive combiner 209, the other input of which arrives from a frequency responsive network 210 connected to an amplitude limiter 203. Combiner 209 output is connected to the fixed coils of a dynamometertype moving coil indicator 212, the moving coil of which is connected to an output circuit of network 210. To maintain the 7 kc/s. frequency difference constant the signal fed to the commutated aerials is a sideband of the carrier wave signal. Network 210 has an approximately linear relationship between the frequency of its input signal and the magnitude of its output D.C. signal. The current flowing in the moving coil of indicator 212 is dependent on aircraft elevation angle α and that in the fixed coil is dependent partly on α and partly on the proximity of the craft to the aerial array. It is stated that the combination of a fraction of " elevation current " with " proximity current " in combiner 209 leads to an indicator deflexion proportional to α<SP>2</SP>; and that by combination of proximity current with a constant direct current a linear scale can be achieved. It is also stated that the proximity current can be used to obtain azimuth bearing data when the aircraft is near touch-down. Commutation arrangements, Fig. 2.-It is stated that aerial energization in the sequence 120, 119 . . . 101, blank, 101, . . . 120 &c. minimizes the bandwidth occupied by the beat frequency obtained at the receiver. Aerial arrangements.-It is stated that twenty aerials give a glide slope usable range of 3 to 60 degrees but that, if the maximum glide slope required is 30 degrees, ten aerials suffice. The elevation snesitivity is increased by increasing the total aerial array height, e.g. to 10#; but, if glide slopes up to 90 degrees are to be provided, the vertical separation between adjacent aerials must not exceed #/4. In lieu of tilting of the aerial array a second and horizontal commutated array may be employed; in this embodiment, Fig. 4 (not shown), two parallel and horizontal aerial arrays are connected through respective commutation switches to separate outputs of a transmitter at respective side-band frequencies F + f 1 , F + f 2 , an aerial radiates a carrier wave of frequency F, and horizontal guidance is afforded by beating the side-band and carrier signals in the aircraft receiver and determining the sum of, and the difference between, the frequency modulations which ensue. A third aerial array, as in Fig. 2, but vertical, is fed from a source of frequency F + f 3 , and is off-set from the runway. Direction-finding, Fig. 2.-To measure the elevation angle of a transmitting aircraft unit 142 is a receiver as described above with reference to Fig. 3.