Patent GB-1488757-A (GB)
Title: Apparatus for transmitting bearing information
IPC: G01S1/44
Filed: 1975-07-29
Abstract: 1488757 Radio navigation STANDARD TELEPHONES & CABLES Ltd 29 July 1975 31663/75 Addition to 1429743 Heading H4D In a radio beacon designed to give the user of a simple V.H.F. radio receiver an indication of his bearing with respect thereto, two fixed omnidirectional aerials 5, 9, Fig. 1, are spaced a predetermined distance apart, means 1 producing V.H.F. carrier with said carrier being power split (2) to provide a fraction which is amplitude modulated (6) at an audio frequency (a.f.) with the a.m. carrier (XY) being fed to one of the aerials 5 together with the unmodulated carrier fraction OX, the a.m. carrier also being periodically phase reversed (11) and the resultant signal YZ being fed to both the aerials 5 and 9, with a plane shifter 10 cyclically varying the phase of the signals transmitted from one of the aerials relative to that of the signals from the other aerial, the start of the cycle being defined by the transmission of a distinctive a.f. modulated carrier. The a.f. may be 1 kHz and the phase reversal means 11 being in one state for <SP>2</SP>/ 3  sec. and the antiphase state for <SP>1</SP>/ 3  sec. The phase shift from 10 varies in step fashion 0-# 360 degrees remaining constant during each 1 sec. cycle of phase reversal means 11. During, e.g. the <SP>2</SP>/ 3  sec. part of the phase reversal cycle, the vector diagram of Fig. 3 pertains for the circuit of Fig. 1 at the instance t 1  of maximum amplitude 3 of the amplitude modulated carrier XY, whilst Fig. 2 pertains for instance t 2 =t 1 +0À5 msec. of minimum amplitude 1. During the <SP>1</SP>/ 3  sec. part of the phase reversal cycle (YZ output of 11 and ZV output of 10 moved through 180 degrees), vector diagrams of Fig. 5 (t 1 ) and Fig. 4 (t 2 ) pertain. At any point in space the field vector #ZV from aerial 9 and field vectors (#OX + #XY + #YZ) from   aerial 5 sum to give a resultant vector #OV having an amplitude R varying from Rα to R# at 1kHz and a phase varying through an angle (#-α) at 1 kM 3 . The manner in which (#-α) varies as the phase shift # is increased with time, for both parts of the phase reversal cycle is shown in Fig. 6, and the cross-overs at 90 degrees and about 230 degrees indicate that at these values of # the phase or frequency modulation intensity heard by the user should not change at the change over from the <SP>2</SP>/ 3  sec. part to the <SP>1</SP>/ 3  sec. part of the phase reversal cycle; Fig. 7 shows the manner in which Rα and R# vary with # for both parts of the phase reversal cycle and it is seen that for # = 230 degrees a large variation of R is evident, whereas for # = 90 degrees the variation is negligable. The combined effects of the phase modulation (Fig. 6) and the amplitude modulation (Fig. 7) causes the user to experience no change in modulation intensity at said change over, only for # = 90 degrees. The phase # as "seen" by the user will differ from that produced by shifter 10 due to the path difference to the two aerials, such that the time that the user experiences said no change in modulation intensity relative to the time of reception of the start of cycle distinctive modulation varies with his bearing and is indicative thereof. Fig. 9 (not shown) shows a modification of the arrangement of Fig. 1 wherein the combining hybrids 3 and 4, Fig. 1, are replaced by two hybrids (12), (14) for the combination of the   vectors #OX, #XY and #YZ in the stages (1) #XY + #YZ and (2) (#XY + #YZ) + #OX instead of the   stages (1) #OX + #XY and (2) (#OX + #XY)+   #YZ as in Fig. 1.