Patent GB-1468016-A (GB)
Title: Phase comparison radio navigation systems
IPC: G01S1/30
Filed: 1973-11-16
Abstract: 1468016 Radio navigation DECCA Ltd 18 Nov 1974 [16 Nov 1973] 53183/73 Heading H4D A phase comparison radio navigation system features a plurality of spaced apart stations transmitting signals in a time shared sequence which are of the same frequency and which are phase-locked together. Said plurality of stations comprises a prime transmitting station and at least one secondary transmitting station. As is shown in Fig. 1, in each sequence the prime station first transmits for 60 msecs., and then each secondary station in turn transmits for 40 msecs. The prime station signal consists of three successive 20 msec. periods of a synchronizing signal, a signal of frequency f1, and a signal of frequency f2. A secondary station signal consists of two successive 20 msec. periods of signals of respective frequencies f1, f2. The said frequencies are in the range 1À6 to 5À0 MHz. The frequency f2 is made slightly less than f1, and is provided so that phase angle differences between determinations at the two frequencies provide a coarse pattern of position lines for ambiguity resolution. The synchronizing signal is of frequency f1, with a phase shift of + 72 degrees introduced during two 5 msec, portions thereof. A common aerial 40, Fig. 3, is used at a secondary station for receiving signals from the prime station, and for transmission. It is coupled via a matching unit to a receiver unit 43 and to a power amplifier 42. The receiver unit comprises a mixer 45 having heterodyning inputs of frequencies F1, F2 which are respectively displaced by 100 kHz from the frequencies f1, f2. Hence, the mixer provides a 100 kHz output whichever frequency is being received from the prime station. This output is passed through a squarer 49 to a phase discriminator 50, where it is compared during appropriate time periods with one or other of 100 kHz signals from two local signal generators #1À1, #1À2. These generators comprise respective varactor controlled 10 MHz oscillators 51, 53, which act as phase memories for the received signals, and respective 100/1 frequency dividers 52, 54. The signals of frequencies F1, F2 are developed by synthesizers 46, 47 which receive inputs from the oscillator 51. The outputs of the dividers 52, 54 are fed to a control switch 57 enabling selection of one or the other for application via a digital type phase shifter 58 to the phase discriminator 50. Manually adjustable controls 59, 60 are provided and, to enable steps of phase in 100th of a cycle, an input from either the oscillator 51 or the oscillator 53 is coupled via a switch 71. The output from the phase discriminator 50 is applied to a switching unit 61 which at appropriate times connects it to one of four output leads 62 to 65. Of these, the leads 62, 63 apply the output from the phase discriminator 50 respectively to the oscillators 51, 53 as frequency control inputs for frequency and phase locking of the signal generators #1À1, #1À2 to the signals being received from the prime station. At the appropriate times for transmissions from the secondary station, the leads 64, 65 apply the output from the phase discriminator 50 respectively to two 10 MHz varactor controlled oscillators 66, 68. These oscillators are associated with respective 100/1 frequency dividers 67, 69 to constitute 100 kHz signal generators # x 1, # x 2 which feed an RF output unit 70. Said unit 70 also receives inputs of frequencies F1, F2 from the synthesizers 46, 47, and it provides inputs at frequencies f1, f2 to the power amplifier 42. During transmission from the secondary station, sufficient power is coupled via the matching unit 41 to provide an input to the phase discriminator 50 for com. parison with the reference input from the signal generator #1À1 or #1À2. At such times, the control signal from the phase discriminator 50 is utilized to obtain frequency and phase locking of the signal generators # x 1, # x 2 to the signal generators #1À1, #1À2. Timing of the operation of the various switches is controlled by a unit 72 which receives one input of frequency 4 kHz from the synthesizer 46 and another from a discriminator unit 80, which initiates the operation of the cyclic sequence of the timing control unit 72 in synchronism with the cycle of transmissions from the prime station. The unit 80 receives one input from the squarer 49 and another input from the signal generator #1À1. It comprises circuits to detect the phase modulation on the signals received from the prime station during the synchronizing period and to generate an initiating pulse. The timing control unit 72 essentially is a counter providing outputs at appropriate time intervals after the initiating pulse. Specifications 1,468,017, 1,468,018 and 1,468,019 are mentioned.