Patent GB-1308147-A (GB)
Title: Phase comparison radio navigation systems
Inventor: PACE B
IPC: G01S1/30
Filed: 1969-11-10
Abstract: 1308147 Radio navigation DECCA Ltd 11 Nov 1970 [10 Nov 1969] 54970/69 Heading H4D Relates to a radio navigation system in which spaced transmitters normally radiate phasedlocked signals of different but harmonically related frequencies from fixed stations (normal transmission periods) and in which periodically for a short duration each station in turn radiates signals of all the frequencies in fixed phase relation, the normal radiations from the other stations being interrupted. According to the present invention a receiver for such a system comprises a number of oscillators 68 ... 71, Fig. 3, corresponding to the number of different received frequencies, means to phase-lock the oscillators to the respective received signals during the normal transmission periods, the oscillators having frequencies either equal to the respective received frequencies or being in the same ratio as them, phase discriminators 83, 85, 90, 91, 94, 95 coupled to the oscillators to provide at least two sets of sine and cosine signals, a separate rotary indicator 14, 12, 13 for each such set, a frequency divider 98 dividing the output of an oscillator down to the fundamental frequency F of the oscillators, means for producing from the multiple-frequency signal transmissions from the various stations signals of the fundamental frequency and digital phase indicator means including a digital counter and a clock source for indicating 174, 175, 176, Fig. 5, the phase relation between the divided-down oscillator output and the successive signals of the same frequency derived from the multiplefrequency transmissions from the various stations in turn. DETAILED OPERATION In the embodiment the transmissions on frequencies 5f ("purple slave "), 8f ("Red slave "), 9f (" Green slave ") and 6f (" master ") from different chains of stations are heterodyned to fixed frequencies 5F, 8F, 9F, 6F by mixing with harmonics of a signal of frequency # = f + f, Fig. 2, signal # being derived from a matrix 122, Fig. 3, associated with a divider train 121 and a stable oscillator 120. Oscillators 68 ... 71, Fig. 3, are phase locked by utilizing respective phase discriminators 64 ... 67 of the kind described in Specification 1,258,117. The use of locked oscillators for the meters 12, 13, 14, Fig. 3, affords noise-free drive and bridging of interruptions in the normal transmissions. Each meter has mechanical integrators for lane and zone indications. The output from a phase discriminator 100, Fig. 2 is fed to a lamp 18, Fig. 4 and to a detector 111; if oscillator 71, Fig. 3, is not correctly phase locked the lamp " blinks ". Detector 111, Fig. 4, gives an output indicating the start of each multifrequency (" multipulse ") transmission from each station. Referencing.-The referencing oscillator 130, Fig. 4, produces a signal at frequency 8F phase locked from divider 98, Fig. 3. Pulses at frequency f from a shaper 135, Fig. 4, are applied to the input channels of the receiver via a gate 136, Fig. 2, controlled by a switch 27 to replace the received signal inputs. Meters 12, 13, 14, Fig. 3, are set to zero using respective controls 24, 25, 26, Fig. 2. (The stability of referencing oscillator 130, Fig. 4, is sufficiently high). Lane identification is obtained by measuring the phase lag of an effective F signal from each slave station compared with an F signal from the master station as provided by oscillator 71, Fig. 3, via divider 98. An arrangement incorporating a 300 F pulse generator 143, Fig. 5, and gates 142, 145 produces a stored count in register 144 proportional to the complement of the master-to-multipulse delay. The capacity of store 144 corresponds to one cycle at F (i.e. a count of 300 at 300 F.) and lane identification is accomplished by determining the quantity required to fill the store until it overflows in units dependent on the slave concerned. The pulses fed into display register 150 are in units of <SP>1</SP>/ 240 th of a zone for red, <SP>1</SP>/ 180 th of a zone for green and <SP>1</SP>/ 300 th of a zone for purple, i.e. they correspond to one tenth of a lane for each of the patterns. For stated reasons register 150 is set to zero at the beginning of the red lane identification count into it, to 30 for the green count and to 50 for the purple count, by pulses from a circuit 147. In order to correct for a sixfold possible ambiguity in the phase of the F output from divider 98 with respect to the F multipulse from the master station (the multiple transmissions peak at F), it is arranged that a store 170 in conjunction with a bi-stable 171 operates on register 144 to bring register 150 to zero on the master multipulse transmission. To avoid confusion between master and red readings, the display on numerical indicators 174, 175, 176 is made to blink during the master indication.