Patent GB-1429743-A (GB)
Title: Apparatus for transmitting bearing information
IPC: G01S1/44
Filed: 1974-03-14
Abstract: 1429743 Radio navigation STANDARD TELEPHONES & CABLES Ltd 14 March 1974 11336/74 Heading H4D Apparatus for transmitting bearing information comprises two fixed omnidirectional aerials A1, A2, Fig. 1, spaced a predetermined distance apart, means for transmitting from one aerial (A2) a composite signal of a VHF carrier frequency signal modulated by an audio frequency signal and the unmodulated carrier frequency signal, the said two signals being in antiphase, means for transmitting from the other aerial (A1) the modulated signal only, means for cyclically varying the relative phase between the two transmitted signals, and means for distinctively altering the audio frequency modulation of the carrier frequency signal at the commmencement of each said cyclic variation of relative phase. As described, the output from a VHF oscillator is divided by a power splitting network P and part is fed to a modulator M, where it is modulated by a audio frequency tone. The modulated carrier frequency signal is then fed to a hybrid circuit H1, where it is divided equally to feed two further hybrid circuits H2, H3. The hybrid circuit H2 also receives an unmodulated carrier frequency signal from the network P. The modulated and the unmodulated signals are combined in the hybrid circuit H2, the respective path lengths being arranged so that the two signals are in anti-phase. The outputs of the hybrid circuits H3, H2 are fed to the aerials A1, A2, which are spaced #/4 apart (#=wavelength at the carrier frequency). The path lengths H1 to A1 and H1 to A2 are arranged so that at the aerials the transmitted carrier frequency signals are initially in quadrature phase relationship, i.e., # 1 =# 2 +#/2 (# 1 =phase at the aerial A1, # 2 = phase at the aerial A2). The feed line from the hybrid circuit H3 to the aerial A1 also includes a line strecher or variable phase shifter circuit L, which varies the phase of the signal to the aerial A1 at a slow rate, e.g., over a period of one minute, under the control of a unit C. The total phase variation introduced is such that at the end of a cycle # 1 =# 2  + 3#/2. Therefore, at time t, # 1 =# 2 +#/ 2 +(t/T).# ,where T is the duration of the phase varying cycle. The control unit C is also operative upon the modulator M to key it initially with a station identification signal, e.g., a Morse code group, and thereafter, following a five to ten seconds period of uninterrupted audio frequency modulation, to key it in successive pulses of, for example, 0À9 seconds audio frequency modulation separated by 0À1 seconds intervals without modulation. A receiving station on a particular bearing from the transmitting station observes minimum modulation of the signal therefrom at a unique instant during the cycle of phase change. Hence, by counting the number of modulation pulses from the initial identification signal until that minimum, an indication of the bearing angle is obtained. In order to improve resolution three aerials may be provided at the transmitting station, which are spaced in a straight line at equal intervals of, for example, #/4. In a first cycle of transmission, the centre aerial and one outer aerial are used, followed by a second similar cycle of transmission using the outer two aerials, and so on.