Patent GB-654101-A (GB)
Title: Improvements in or relating to radio navigation aiding systems
IPC: G01S1/02
Filed: 1946-10-18
Abstract: 654,101. Radio navigation. SADIR-CARPENTIER. Oct. 18, 1946, No. 31125. Convention date, Oct. 22, 1945. [Class 40 (vii)] In a phase-comparison glide-path beacon offset from the landing path, sideband energy is radiated having a highly directional pattern in the vertical plane which has a sharp null whose direction defines the glide path and a carrier frequency is also radiated having a less directional pattern with a substantial radiation component in the direction of the said null so that the modulation signal obtained in the aircraft receiver is of zero amplitude when the craft is on the glide path and rapidly increases for deviations from the path, the phase of the deviation modulation signal being in phase or out of phase with a reference modulation signal transmitted by the beacon depending on whether the deviation is above or below the glide path. The reference modulation may be transmitted on a different carrier frequency or it may be amplitude or frequency modulated on to a sub-carrier which is modulated on to the same carrier. As shown in Fig. 3 the beacon comprises two vertically spaced aerials (a) and (b) whose directive patterns in the vertical plane are shown in cartesian from at I and II, Fig. 1, where # denotes the angle of elevation. A carrier frequency F and modulating frequency f1 from sources 1 and 2 are applied to a balanced modulator 14, the resultant sidebands (FŒ f1 ) being applied to the upper aerial (a) and the frequency f and a sub-carrier frequency f1<SP>1</SP> from 11 are applied to modulator 12, the output components f<SP>1</SP>, f1<SP>1</SP>, (f1<SP>1</SP>Œf1 being applied together with the carrier F to modulator 13 whose output components are applied to the lower aerial (b). The frequencies f1 applied to modulators 13 and 14 are arranged to be of opposite phase so that the sidebands (FŒf1) from the two aerials will cancel at the angle #o at which the directivity patterns of the two aerials intersect, the resultant sideband energy above and below #o being of opposite phase so that at the aircraft receiver the modulation signal f1 obtained by detecting the resultant sideband energy (FŒf1) and the carrier F radiated from (b) will be in phase or out of phase with the reference modulation signal f1 obtained by separating and demodulating the sub-carrier f1<SP>1</SP>. The resultant phase condition may be indicated by applying the two frequencies in phase and phase opposition to the two stators of a crossed-coil meter, Fig. 2 (not shown). Fig. 4 illustrates a glidepath beacon of the above type combined with a phase-comparision azimuth beacon. In the glide-path portion a carrier F and modulating frequency f1 from 28 and 27 respectively are applied to a balanced modulator 29 and a normal modulator 30, the outputs from the two modulators being applied to the upper and lower horizontal aerials (a) and (b). In the azimuth portion, a carrier F<SP>1</SP> and modulating frequency f2 from 22 and 24 are applied to a balanced modulator 23, the resultant sidebands (F<SP>1</SP>Œf2) being applied to a vertical loop aerial 21 and the carrier Fl is also applied via modulator 26 to an open aerial 25 so that the modulation signal f2 obtained in the aircraft receiver by demodulating the sidebands (F<SP>1</SP>Œf2) and carrier Fl will be of zero amplitude along the nulls of the loop aerial horizontal directivity pattern ; in any other direction the modulation signal will be in phase or phase opposition with a reference frequency f2 which is radiated as a modulation on the carrier F from the lower aerial (b) of the glide-path beacon. In this case the reference frequency f1 for the glide-path beacon is radiated as a modulation on the carrier F<SP>1</SP> from the open aerial of the azimuth beacon. In a modification the lower aerial (b) of the glide-path beacon is constituted by two separate parts radiating carriers of frequency F modulated respectively by frequencies f1 and f2. According to the Specification as open to inspection under Sect. 91 the carrier F radiated from the lower aerial (b) of the glide-path portion of the beacon shown in Fig. 4 may be modulated only by the azimuth reference modulation f2 in which case the glide-path is defined by the direction of the null of the radiation pattern of the upper aerial (a). An overlapping-beam equi-signal glide-path beacon is also described in which sidebands (FŒf) and a carrier and sidebands F and (FŒf) are radiated respectively from upper and lower vertically spaced horizontal aerials (a) and (b) having different vertical directivity patterns. In one embodiment the phase of the lower aerial modulation is periodically reversed in complementary sequence so that two overlapping resultant sideband directivity patterns are alternately produced, the equi-signal locus defining the glide path. In another embodiment the modulation on aerials (a) and (b) is in opposite phase and the modulation depth of the transmission from the lower aerial is varied in complementary sequence producing two different sideband directivity patterns which alternately combine with the sideband directivity patterns of the upper aerial to give two overlapping patterns defining the glide path. A..system is also described in which different frequencies f1 and f2 are transmitted simultaneously from the two aerials respectively. In any of the above systems the glide angle is a function of B/A and h where B and A are the amplitudes of the sidebands radiated from aerials (b) and (a) and h is the effective height of the upper aerial and the glide angle may be progressively reduced as the aircraft approaches the touch-down point by causing a progressive increase in B/A or h. This may be achieved by offsetting the beacon from the approach path and inclining the aerials (a) and (b) at different angles to the approach path, Fig. 3 (Cancelled, not shown), or by replacing the upper aerial by two orthogonal aerials at different heights, Fig. 4 (Cancelled, not shown). This subject-matter does not appear in the Specification as accepted.