Patent GB-1024242-A (GB)
Title: Radio navigation system
Inventor: EARP CHARLES WILLIAM
IPC: G01S1/40
Filed: 1962-11-29
Abstract: 1,024,242. Radio navigation; frequency discriminators. STANDARD TELEPHONES & CABLES Ltd. Nov. 29, 1962, No. 45113/62. Headings H3A and H4D. In a Doppler beacon or direction-finding radio system in which a direction-dependent frequency modulation signal of frequency f produced by the real or simulated cyclic movement of an aerial around a circular path at a repetition rate f is compared in phase with a reference signal of frequency f synchronized with the cyclic movement, an additional more precise but ambiguous direction indication is obtained by phase comparison of a harmonic direction-dependent signal Nf (derived from the frequency modulated signal) with a harmonic reference signal n Nf where n and N are integers greater than one. In the examples given f= 30 c/s., N=3 or 5 and n=2, 3 or 5. Beacon systems.-In Fig. 1 (not shown) the beacon comprises (1) 18 aerials 1 equally spaced around a circle 2 and sequentially energized from a transmitter 5 of frequency F 1  = F - 99 60 c/s. and (2) a central aerial 3 energized from a transmitter 4 with a carrier frequency F amplitude modulated with reference signals f and 6f. At the receiver, Fig. 2 (not shown); the reference signals f and 6f are selected from the output of the amplitude detector 8 by filters 9 and 10 respectively and the frequency modulated 99 60 c/s. beat signal selected by filter 11 is applied (1) to a conventional frequency discriminator 12 to give the directiondependent signal f and (2) to a harmonic frequency discriminator 13 (see below) to give a direction-dependent signal 3f, the signals of frequency f being applied to a conventional phase comparator 14 and the harmonic signals 6f and 3f being applied to a phase comparator 15 to give the more precise indication; the comparators 14 and 15 may be coupled together so that the comparator 15 is inoperative until the comparator 14 gives its indication. Alternatively the reference signal 6f may be derived in the receiver from the reference signal f. In Fig. 4 the beacon comprises (1) a central aerial 3 energized from a transmitter 4 with a carrier F amplitude modulated only with the fundamental reference signal f and (2) a circular array of nine aerials 1 with a dummy load 27 between each pair of aerials and two transmitters 5 and 28 operating at F 1  = F - 99 60 c/s. and F 2  = F + 99 60 c/s. respectively connected to diametrically opposite points on the circle 2 by electronic, inductive, capacitative or mechanical distributers 6 and 29 so that the frequency radiated by the aerials 1 is switched between F 1  and F 2  at a repetition rate 9f which constitutes the harmonic reference frequency; the switching periods for F 1  and F 2  may overlap slightly. Instead of providing dummy loads 27, the transmitters 5 and 28 may be switched sequentially at the frequency 9f. At the receiver, Fig. 5 (not shown), the reference signal 9f is obtained by applying the IF output to filters 31 and 32 tuned to cover bandwidths of IF to IF-10,710 c/s. and IF to IF+10,710 c/s. respectively. The outputs from the filters 31 and 32 are amplitude detected at 33, 34 to give 99 60 c/s. beat signals each amplitude modulated at 9f which are selected by 99 60 c/s. filters 35, 36 and amplitude detected 37, 38 to give 9f outputs which are combined in phase in a common load. The reference signal f and the direction-dependent signals f and 3f are obtained as in Fig. 2. In a preferred arrangement using 25 aerials giving a harmonic reference signal 25f, a 5f direction-dependent signal is derived from the harmonic discriminator; alternatively the 25f reference signal could be transmitted as an amplitude modulation signal either with the reference signal f as in Fig. 1 or from a separate aerial. Direction - finding systems. - The aerial arrangements shown in Figs. 1 and 4 may be used at a receiver to determine the direction of a station transmitting a signal of frequency F, the reference signals being generated directly at the receiver and the frequency modulated 99 60 c/s. beat signal being produced by shifting the frequency of the signal received at the central aerial 3 by 99 60 c/s. and combining the resultant (FŒ99 60) signal with the signal from the circular array of aerials. When using the Fig. 4 aerial arrangement the inner ends of the distributers 6 and 29 are connected to a single receiver. Harmonic frequency discriminator, Fig. 3 (not shown).-This comprises a sum and difference type of circuit in which the frequency modulated input is applied in push-pull and in parallel to diodes 23, 24 by the secondary windings 18 and 19 respectively of the transformer 17, a delay circuit 20 being included in the circuit of the secondary winding 18 which is adjusted to increase the amplitude of the desired harmonic, e.g. third or fifth, of the modulation frequency appearing at the output 25, 26.