Abstract:
In a phase comparison radio navigation system in which spaced transmitters radiate phase-locked signals in a time-shared sequence, in order to enable a receiver to be operated on any selected one of a large number of different frequencies, a phase memory in the receiver for giving a continuous output representative of the phase of an intermittently received signal, comprises a controlled oscillator giving an output of a frequency much higher than the received frequency, a frequency divider providing a first sub-multiple of the oscillator frequency, a frequency synthesizer providing a heterodyne signal at a second sub-multiple and a phase comparator providing an output controlling the oscillator in accordance with the phase relation between the divider output and the output of a mixer mixing the received signal with a heterodyne signal.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to phase comparison radio navigation systems of the kind in which spaced transmitting stations radiate phase-locked signals in a time-shared sequence on one or more different frequencies and in which, at a receiver, the signals from the spaced stations are compared in phase. 
     If the radiated signals are interrupted or otherwise radiated in such a manner that signals to be compared are not continuously available when required for comparison purposes, it is necessary to provide, in the receiver, phase memories. Such phase memories consist essentially of oscillators which are phase-locked to the appropriate received signals when the latter are available and which serve to make available a continuous output representative of the phases of the intermittently available signals. 
     SUMMARY OF THE INVENTION 
     According to this invention, in a receiver for a phase comparison radio navigation system, there is provided a phase memory for giving a continuous output signal having a phase locked to the phase of a received radio frequency signal, which phase memory comprises a controllable oscillator at a frequency much greater than the radio frequency to be received, a frequency synthesiser fed from said oscillator to provide a selected heterodyne frequency at a frequency which is a multiple of a first sub-multiple of the oscillator frequency, a frequency divider coupled to said oscillator to provide an output which is a second sub-multiple of the oscillator frequency, said second sub-multiple being another multiple of said first sub-multiple, a mixer for mixing the heterodyne frequency from the synthesiser with the received signal to give an intermediate frequency signal, and a phase comparator for comparing the phase of the intermediate frequency signal with the phase of the divider output to provide a control signal which is applied as a frequency control to said oscillator to phase-lock the oscillator with respect to the received signal. 
     Using the arrangement described above, the frequency synthesiser can be set to give a heterodyne frequency such that the intermediate frequency output of the mixer is of the same frequency as the divider output. The phase discriminator provides control of the oscillator to bring the oscillator to the appropriate frequency. This arrangement permits the intermediate frequency being chosen quite independently of the radio frequency or, commonly, the radio frequency of the system can be selected independently of the intermediate frequency apart from the requirement that they are both multiples of a common sub-multiple. This common sub-multiple is the frequency step in the frequency synthesiser and may be very small. In a typical system with radio frequencies of the order of 1600-5000 kHz, steps of 100 Hz are employed. The arrangement of the present invention therefore has great advantages in a system in which it is required to select any radio frequency over a relatively wide band. 
     The receiver for a phase-comparison radio navigation system compares signals received from spaced stations. Separate phase memories may be provided for each signal to be stored but, for all signals of the same frequency, the same synthesiser, mixer and phase discriminator may be employed, with, for each signal, a separate oscillator and divider. The connections may be switched by a timing circuit, in synchronism with the sequence of transmissions from the stations so that each oscillator is phase-locked during the appropriate transmission. Preferably the frequency synthesiser is always driven from the same oscillator. It is thus possible to have in the receiver a number of oscillators, each forming phase memories for separate transmissions which may be radiated only intermittently, e.g. in a time sequence. 
     If more than one frequency is radiated, the synthesising means must provide a further heterodyne frequency or frequencies but, provided the received signals are available in a time sequence, only a single mixer and phase discriminator is required. 
     Unlike earlier types of receiver having separate heterodyne channels for separate signals and receivers having separate phase discriminators for each phase memory, the use of common circuits avoids differential phase shifts and thus avoids any need for referencing. By making the frequency changing within the phase lock loop, there is avoided any need for high frequency stability of the heterodyne oscillator and balancing of phase shifts due to the heterodyne circuits. 
     According to a further aspect of the present invention, in a phase comparison radio navigation system in which phase-locked radio frequency signals are transmitted from spaced stations, one of the stations, hereinafter referred to as the prime station, has at least one frequency synthesiser and a oscillator providing an input signal to said synthesiser or to each synthesiser at a frequency many times greater than the required radiated frequency or frequencies and means for radiating a signal or signals at a radio frequency or frequencies determined by the oscillator and synthesiser or synthesisers, and two or more secondary stations have means for receiving signals rediated by the prime station and for transmitting signals of the same frequencies or at least one of the same frequencies phase-locked to the corresponding frequency signals from the prime station and the or each receiver has means for receiving the radiated signals, a controllable oscillator, referred to hereinafter as the prime oscillator, a frequency synthesiser for each frequency to be received, the or each synthesiser having the prime oscillator as a signal input source, a divider coupled to said prime oscillator to divide the frequency thereof down to a sub-multiple thereof, a mixer for mixing the output of the or each synthesiser with received signals and a phase comparator for comparing the phase of the output of the mixer with the output of the mixer with the output of said divider, means for setting the or each synthesiser so that the two inputs to the phase comparator for each received signal are of the same frequency, namely that of the divider output, means for applying the output of the phase discriminator as a frequency control signal to said prime oscillator when signals of one frequency from the prime station are received, a plurality of other oscillators and associated dividers corresponding to each of the other transmissions from the spaced transmitting stations and switching means for applying the output of the discriminator as a phase control signal to the appropriate oscillators during periods of reception of each of said other transmissions whereby the oscillators in the receiver are each phase-locked with the respective transmission. 
     In the arrangement described above, the receiver has only a single phase discriminator which is utilised for all the received signals. The phase-lock loop includes the heterodyning system and thus the various oscillators, although operating at frequencies completely different from and preferably many times greater than the radiated frequencies form phase memories for each of the radiated signals. These phase memories can be and preferably are at the same nominal frequency but the frequencies will differ slightly due to the different doppler effects caused by the differing rates of relative movement of the receiver with respect to the various transmitters. Indicator means may be provided for comparing and displaying phase relationship between any pair of oscillators forming memories of radiated signals of the same frequency so as to provide positional information. 
     In a multi-frequency system wherein the transmitting stations are arranged to radiate signals of more than one frequency in a time sequence, preferably the prime station synthesisers are arranged to provide outputs which are mixed with a divided down output from the oscillator in the prime station to provide the required radio frequencies. In each of the secondary stations, there may be provided a frequency synthesiser for each frequency to be radiated, a phase memory oscillator corresponding to the oscillator at the prime station, a transmission oscillator, a divider for dividing the transmission oscillator frequency and a mixer for mixing the divided down frequency with the synthesiser output to provide the required radiated frequencies. The phase control of the sequential transmissions may be effected by controlling the transmission oscillator sequentially in accordance with a phase comparison between the radiated signal and a signal from an appropriate memory oscillator. If the system is operated in a hyperbolic mode with fixed transmitting stations then only a single memory oscillator is required in the secondary stations. For operation in a circular mode, where one of the transmitting stations is on a movable craft or vehicle, there will be doppler phase shifts between the transmissions, and, for this reason, there are preferably provided further memory oscillators in each secondary station corresponding to each of the further frequencies radiated from the prime station with phase-lock means for maintaining each of the oscillators in phase-lock with the appropriate received signal and phase-lock means for maintaining the radiated frequencies in predetermined phase relations with the appropriate oscillator output signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a timing diagram illustrating the timing of transmissions from a number of transmitting stations of a phase comparison radio navigation system; 
     FIG. 2 is a block diagram illustrating a prime transmitting station; 
     FIGS. 3A and 3B together are a block diagram illustrating one of the secondary transmitting stations; and 
     FIGS. 4A and 4B together are a block diagram illustrating a receiver. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The particular navigation system to be described with reference to the drawings has a prime transmitting station which radiates a synchronising signal for a period of 20 milliseconds followed by a transmission at a frequency f1 for 20 milliseconds and then a frequency f2 for 20 milliseconds. Each of the secondary stations radiates the frequency f1 for 20 milliseconds followed by f2 for 20 milliseconds. The timing cycle permits of up to five secondary stations. With one secondary station only, the system will provide position line information. A single set of position lines is used in some circumstances; for example a single set of concentric position lines may be employed for a sea-search. The two stations might be used as fixed stations with a receiver on a vehicle so that the two stations form the base line of a single hyperbolic pattern or one of the stations may be on a vehicle so that the system gives range information. More generally two or more secondary stations would be used to give a position fix. Any pair of stations can form the base line of a pattern of position lines if, as in one form of receiver, means are provided for comparing the phase of any two received signals of the same frequency. Thus, in a hyperbolic navigation system, four stations can provide six sets of hyperbolic position lines while six stations can provide fifteen sets of hyperbolic position lines. 
     With a single frequency, the system can be arranged as a multi-user hyperbolic system with between three and six stations according to the coverage required or to provide two circular patterns for up to four users. By employing two or three secondary stations depending on the area to be covered, multi-user position fixing is provided and further secondary stations can be used for giving circular patterns for individual users, thereby providing for operation in both hyperbolic and circular modes together. 
     The two frequencies f1 and f2, in this embodiment are in the range of 1600 to 5000 kHz. Use of one frequency only provides accurate positional information but, as is well known, ambiguities arise. The second frequency f2 is made a small percentage less than f1 and the difference between phase angle determinations at frequencies f1 and f2 provides a coarse pattern of position lines for resolving these ambiguities. With circular patterns, the second frequency may be employed by further users if coarse information is unnecessary. 
     The frequencies f1 and f2 need not be harmonically related. As will be described later, each of these frequencies may be separately selected from frequency synthesisers providing frequencies in 100Hz steps. Conveniently the two frequencies differ by multiples of 100 Hz. 
     The synchronising transmission from the prime station is used for controlling timing of the secondary station transmissions and also in the receiver. The synchronising transmission comprises a phase modulation, the phase of the f1 transmission being shifted by +72° for two periods of 5 milliseconds during the 20 millisecond period of this transmission. 
     The prime station, shown in FIG. 2, has frequency synthesisers 10, 11 for producing frequencies F1 and F2 respectively where F1 = f1 + 100 kHz and F2 = f2 + 100 kHz, the frequencies being selected by thumbwheel controls 12, 13 respectively in 100 Hz steps. A signal generator unit comprising a 10 MHz temperature-stabilised crystal-controlled oscillator 14 and a 100 to 1 frequency divider 15 provides a 10 MHz signal for the synthesisers 10, 11 and a 100 kHz signal which is fed via a phase shifting unit 16 to balanced modulators 17, 18 where the 100 kHz signal is combined with the F1 and F2 signals respectively from the frequency synthesisers 10, 11 to produce signals of frequencies f1 and f2. Narrow band filters are provided in the 100 kHz inputs to both balanced modulators 17, 18 in order to avoid second harmonic breakthrough and to shape the transient introduced by the phase modulation. The output signals from the two balanced modulators 17, 18 are amplified by an amplifier 19, passed through a filter 20 and further amplified at 21 before being fed to a power amplifier 22 to provide signals which fed via an antenna matching unit 23 to an antenna 24. The phase shift unit 16 includes separate controls 25, 26 for digital switched phase shifting of the f1 and f2 transmissions. A 10 MHz input from the oscillator 14 to the phase shift unit 16 enables the phase shifting to be controlled digitally in steps of 1/100th of a cycle. 
     The two frequency synthesisers 10, 11 enable selection of the frequencies F1 and F2 to be made independently; each frequency can be selected in 100 Hz steps. The synthesiser 10 also provides a 4 kHz signal which is fed to a timing control unit 30 controlling the timing cycle. 
     Outputs from the timing control unit 30 are applied to the electronic switches 31, 32 which control respectively the inputs to the balanced modulators 17, 18. The switch 31 is closed during the synchronising period and during the periods of transmissions at frequency f1 whilst the switch 32 is closed during the periods of transmission of frequency f2. The timing control unit 30 also applies a signal to the phase shifter 24 during the synchronising transmissions to introduce the 72° phase shift for two periods of 5 milliseconds in each synchronising transmission. 
     FIGS. 3A and 3B together illustrate in block diagram form a secondary transmitter. This transmitter has an antenna 40 which is coupled via a matching unit 41 both to a power amplifier 42 for transmission and to a receiver unit 43 for reception. The receiver unit 43 includes a broad band amplifier 44 feeding a mixer unit 45 where the receiver signals are heterodyned with signals of frequencies F1 and F2 from frequency synthesisers 46, 47 respectively to provide an output at 100 kHz to an amplifier 48. The sinusoidal output from the amplifier 48 is passed through a squarer 49 to a phase discriminator 50 where it is compared, during appropriate time periods, with one or other of the 100 kHz signals from two local signal generators referred to hereinafter as the φ1.1 and φ1.2 signal generators. The φ1.1 signal generator comprises a 10 MHz varactor controlled oscillator 51 and a 100 to 1 frequency divider 52. Similarly the φ1.2 signal generator comprises a 10 MHz varactor controlled oscillator 53 and a 100 to 1 frequency divider 54. These two signal generators are frequency and phase-locked as will hereinafter be described, to the received f1 and f2 transmissions respectively. During the period of transmissions from the prime station, the f1 and f2 received signals are the signals received from the prime station. When the secondary station is transmitting, the output from the power amplifier 42 fed to the antenna 40 provides, from the matching unit 41, received signals which are compared by the discriminator with the outputs of the φ1.1 and φ1.2 signal generators. The 10 MHz oscillator 51 provides a signal of that frequency not only to the frequency divider 52 but also to the F1 synthesiser 46 and to the F2 synthesiser 47. These synthesisers 46, 47 have thumbwheel controls 55, 56 respectively permitting of selection of any frequency in the required bandwidth in steps of 100 Hz. 
     The outputs of the 100 to 1 dividers 52, 54 are fed to a control switch 57 enabling selection of one or other of these two 100 kHz signals for application via a phase shifter 58 to the aforementioned phase discriminator 50. The phase shifter 58 has timing control for a timing control unit 72 and includes manually adjustable control means 59, 60 for adjusting respectively the phase shift during the transmissions from the secondary station at frequencies f1 and f2. 
     The output from the phase discriminator 50 representative of the phase difference of its two inputs is applied to a switching unit 61 which switching unit causes this control signal from discriminator 50 to be applied at the appropriate times to the appropriate one of four separate output leads, the switching unit 61 being controlled for this purpose by the timing control unit 72. Two of these leads 62, 63 apply the analogue output from the phase discriminator 50 respectively to the 10 MHz oscillators 51, 53 as frequency control inputs for frequency and phase locking of the φ1.1 and φ1.2 signal generators to the received signals from the prime station. The other two outputs from the switching unit 61 on leads 64, 65 are applied respectively to two further signal generators, referred to hereinafter as the φx1 and φx2 signal generators which constitute the signal sources used for transmissions from the secondary station during the periods of transmission on frequencies f1 and f2 respectively. The φx1 signal generator comprises a varactor controlled 10 MHz oscillator 66 and a 100 to 1 frequency divider 67 whilst the φx2 signal source comprises a varactor controlled 10 MHz signal generator 68 and a 100 to 1 frequency divider 60. The 100 kHz outputs from the dividers 67, 69 are fed to a radio frequency output unit 70 to which are also fed the synthesised F1 and F2 frequencies from the synthesisers 46, 47. The RF unit 70 is identical with the RF unit in the prime transmitter (comprising switches 31, 32, balanced modulators 17, 18, amplifier 19, filter 20 and further amplifier 21) and provides the required outputs at frequencies f1 and f2 to the power amplifier 42. 
     The phase shifter 58 is a digital phase shifter and, in order to enable the controls 59, 60 to give steps of phase of 100th of the cycle, it requires an input signal at exactly 100 times the frequency of the signal being controlled. These inputs are provided directly from the aforementioned oscillators 51, 53 via a switch 71. 
     Timing of the operation of the various switches is controlled by the aforementioned timing control unit 72 which has a 4 kHz input on lead 73 from the synthesiser 46. This timing control unit provides outputs at 74 to control the aforementioned switches 57, 71 for selecting inputs to the phase discriminator 58 during the appropriate period. It also provides a control signal on a lead 75 to the phase shifter 58 for selection of the appropriate phase shift from the manual controls 59, 60. On a lead 76, the timing control unit 72 provides a signal to a gating control 77 controlling the switching unit 61 for selection of the appropriate leads to which the output of the phase discriminator 50 is to be fed. The timing control unit also provides signals on leads 78 to the Rf output unit 70 for controlling the switches therein corresponding to the switches 31, 32 of FIG. 2. 
     The cyclic sequence of the timing control unit has to be initiated in synchronism with the cycle of transmissions from the prime station. To do this there is provided a synchronising discriminator unit 80 which includes, as will hereinafter be described, the necessary circuits to detect the phase modulation on the signals received from the prime station during the synchronising transmission and to generate from this the required timing pulse for initiating operation of the timing control unit. The timing control unit essentially is a counter providing outputs at appropriate time intervals after the initiating pulse. During the synchronising transmission from the prime station, the phase modulated signal is received at the secondary station on the antenna 40 and passes through the receiving channel 44, 45, 48 to give a phase modulated 100 kHz input to the squarer 49. The output from the squarer 49 is applied to one of the inputs of the synchronising discriminator 80, the other input being a 100 kHz reference input from the divider 52. The phase modulated input to the synchronising discriminator results in a square wave output from the synchronising discriminator 80. Before the secondary station is locked to transmissions from the prime station, there may be a few cycles difference between the interediate frequency at amplifier 48 and the reference 100 kHz derived from the divider 52. However, between leading edges, the 5 millisecond wide synchronising phase shift pulses effectively are one cycle of a 100 Hz signal which, notwithstainding the possible few cycles out of lock, is detected by the synchronising discriminator and fed to a digital recognition circuit comprising a shift register clocked at 1 kHz pulse rate in the discriminator unit 80. The two 5 millisecond phase shift pulses are 10 milliseconds apart if measured either on their leading edges or their trailing edges. Conveniently the positive edges are selected, shaped and fed into this shift register which is arranged to detect when there are pulses 10 milliseconds apart to provide signals to an AND gate which generates the second and active edge of the trigger condition. Any random pulses occuring during a 10 millisecond correlation period inhibit the trigger function. Thus the synchronising detector unit 80 provides an output signal timed in accordance with the received synchronising transmission from the prime station. This signal is fed via a fast monostable to the reset line of the timing control 72 which latter consists of a modulo 80 divider for the 4 kHz input from the synthesiser 46 driving a shift register generating the required 20 millisecond timing intervals. Thus, although at the commencement of the transmission of a chain, the local signal generators φ1.1 and φ1.2, which constitute phase memories for received signals, are not frequency and phase-locked to the received signals, chain timing is locked from the synchronising phase modulation on the f1 transmission from the prime station. The initial frequency difference between the intermediate frequency from the prime station f1 and the 100 kHz from the φ1.1 signal generator 51, 52 is not significant. The timing control unit 72 can thus now control the operation of the various switches during the appropriate periods of the various transmissions from the prime and secondary stations in the transmitting cycle. The phase-lock discriminator 50 receives a 100 kHz input continuously from the receiver but the appropriate reference signal is switched to the second input during the required periods and the output from the phase discriminator is selected only during the required periods to give the appropriate controls. In this particular embodiment, the phase-locked discriminator is made active during a 5 millisecond period which commences 100 milliseconds after the leading edge of the relevant 20  millisecond timing pulse. During the period of the f1 transmission from the prime station, the phase-lock discriminator 50 locks the 10 MHz oscillator 51 in frequency and phase so that the 100 kHz output from the divider 52 corresponds in frequency and phase to the 100 kHz output from the shaper unit 49 subject however to the phase shift introduced by control 59. During the f2 transmissions from the prime station, the discriminator 50 locks the oscillator 53 so that the output of the divider 54 has the required phase relation with the received f2 signals. It will thus be seen that the local signal generator φ1.1 now stores the prime phase datum for the f1 transmissions. The oscillator 51 is now at the correct frequency and phase. It drives the heterodyne frequency synthesisers for the f1 and f2 transmissions from the secondary stations. It will be noted that any change in the frequency or phase of the oscillator 51 causes a corresponding change in the input to the mixer 45 and hence into the signal input to the phase discriminator 50 by an amount depending on the setting of the synthesiser. Any frequency or phase shift on the oscillator 51 will produce 100th of the frequency or phase change on the reference side of the discriminator 50 and thus the degree of control of the oscillator is reduced approximately in proportion to the ratio of the synthesiser frequency to 100 kHz. The analogue output from the discriminator 50 is applied via the lead 62 to the varactor control input of the oscillator 51 in such a sense as to pull the frequency in phase to that represented by the received f1 transmissions. In this way, over a few system cycles, frequency and phase of the phase datum memory oscillator 51 is ensured. In a similar way the discriminator 50 provides an analogue output on lead 63 to lock the 10 MHz oscillator 53 to the frequency and phase represented by the received f2 transmission. It will be noted in this case that any change in the frequency or phase of the oscillator 53 is applied only to the reference side of the discriminator 50 and not to the signal input side since the synthesiser 47 is driven by the oscillator 51. 
     When locking the φ1.1 oscillator 51 the reference is 100 kHz derived from φ1.1. That is, the incoming f1.1 transmission is heterodyned with a frequency synthesised from φ1.1, and any difference between the 100 kHz i.f. and the 100 kHz derived from φ1.1 generates a control output from the synchronising discriminator to correct frequency and phase of φ1.1. 
     When locking phase memory ocillator φ1.2 the reference is 100 kHz derived by heterodyning a frequency synthesised from phase memory oscillator φ1.1 (already locked to f1 transmission) with the incoming f2 signal. This 100 kHz intermediate frequency is compared with 100 kHz derived from phase memory oscillator φ1.2 and output from the phase-lock discriminator corrects the frequency and phase of φ1.2. 
     It can be seen that in the case described for φ1.1 the reference effectively is on the φ1.1 side of the discriminator because the φ1.1 oscillator is providing both 100 kHz to the discriminator and the synthesised heterodyne frequency for generating the i.f. In the case described for φ1.2 the reference is on the received signal side of the discriminator because the heterodyne frequency is generated from phase memory oscillator φ1.1, whose frequency and phase already have been locked. 
     At this stage each secondary station control unit has its timing locked to the stores f1 phase datum transmitted from the prime station. It also has stored a f2 phase datum. If the stations are all fixed, these phase data will be locked to the received transmissions, and stable. If a secondary station is moving, the stored data will be plus or minus the doppler generated by the vessel moving towards or away from the prime station. The same will apply if the prime station is moving and the secondary stations are fixed. 
     Once the timing control unit 72 has been synchronised, a switch 81 may be operated. This is effective to disconnect the feed from the synchronising discriminator 80 but only after reception of the next trigger pulse. The timing control unit 72 is now controlled by the 4 kHz input on lead 73 derived from the oscillator 51 via the synthesiser 46. The update of the timing is thus now dependent on the narrow band filtering to which the prime station signals are subject in the φ1.1 control loop. In effect therefore the bandwidth for timing control purposes is narrowed with consequent improvement in the signal-to-noise ratio. 
     The phase discriminator 50 is also used to phase-lock the transmissions from the secondary station with the stored signals in the memories φ1.1 and φ1.2. To do this, a synthesised F1 and F2 is heterodyned in the unit 70 with the 100 kHz signal from the appropriate one of the dividers 67, 69 to provide the f1 and f2 frequencies. The synthesised heterodyne frequencies F1 and F2 are derived, using synthesisers 46, 47, from the transmission phase memory oscillator φ1.1 which is locked to the prime station phase datum. The phase discriminator 50 serves to compare the phase of the transmitted signal during the secondary station transmission with a reference signal derived from the memory φ1.1 or φ1.2 as appropriate and to control the transmission oscillators 66, 68 to maintain the required phase relationship. It will be noted that any phase shift introduced by the phase shifter 58 is thus added in during the locking and is effective on the phase of the transmitted signals from the secondary station. The varactor-controlled oscillators 66, 68 are phase-locked by means of the analogue outputs on leads 64, 65 respectively from the discriminator in a similar manner to the oscillators 51, 53. 
     The matching unit 41 constitutes in effect an attenuator serving to apply to the receiving channel 43 a small proportion of the output from the power amplifier 42. Conveniently this unit 41 has a low impedance circuit between the antenna and the receiver channel including, in series, the contacts of a relay. During periods when the secondary station is transmitting, the relay contacts are opened and the capacative impedance of the contacts forms a high impedance circuit connected between the antenna and the receiving channel. This high impedace is constituted by the capacitance of the contacts, typically of about half a picofarad, which will give a 90° phase shift substantially unaffected by temperature changes or by changes in frequency. 
     A single prime station can be used with up to 5 secondary stations employing a timing cycle as shown in FIG. 1. If fewer than 5 secondary stations are employed, the overall period of the timing cycle can readily be reduced as appropriate. The transmissions on frequencies f1 and f2 may be made from separate stations. It will be noted that a prime station essentially uses equipment which constitutes part of the equipment employed at a secondary station. The equipment may readily be made in modular form to facilitate assembly in various different combinations to form prime and secondary stations as required. 
     FIGS. 4A and 4B together illustrate the construction of one form of receiving equipment. This particular embodiment includes means for staticising up to 3 fine patterns on the frequency f1 selected out of any of those possible from a particular chain configuration. The display also provides for coarse positional information to be obtained utilising the f2 transmissions of the appropriate stations. The display is digital and typically provides a 6-digit readout with continuous display of the selected patterns in terms of whole and fraction lane counts. With a full complement of circuit boards, the receiver can memorise the received phase of all 12 possible chain transmissions and relate f1 and f2 of any station pair, in either station order, for each of the three displays. Position in patterns centred on base lines joining any pair of stations can be measured and displayed. An additional 2-digit readout with each position display gives receiver position in a difference pattern, that is to say a coarse pattern, derived by comparing the received phase of the f1 and f2 transmissions, from each selected station pair. A sequenced binary coded decimal output of the instantaneous values of each 6-digit readout together with the setting of the two related station selector switches is also avialable. 
     Referring to FIG. 4A there is a receiving antenna 100  connected to a receiving channel 101 comprising an amplifier 102, heterodyne mixer 103 and an output amplifier 104, leading to a squarer 105 providing one input of a phase-lock discriminator 106. The heterodyne signals are provided from two synthesisers 107, 108 forming the F1 and F2 synthesisers respectively which synthesisers can be set manually by thumbwheel controls 109, 110 to the selected frequencies in steps of 100 Hz. The input signal for the synthesiser is obtained from a varactor-controlled 10 MHz oscillator 111 which with 100 to 1 divider 112 constitutes a prime station memory referred to hereinafter as the φ1.1 memory. It will be immediately apparent that the receiver equipment described thus far is identical (apart from the absence of the matching unit 41) with the corresponding equipment in the secondary station control unit. For each received signal, the receiving station has an appropriate memory unit. In addition to the memory unit 111, 112 there are shown eleven further corresponding memory units each comprising a 10 MHz oscillator 113 to 123 with a 100 to 1 frequency divider 124-134. These units are referred to hereinafter as φ1.2, φ2.1 etc, the notation employed being that the first of the two figures indicates the transmitting station and the second of the two figures indicates the frequency. Thus φ3.2 is the memory oscillator for the f2 transmission from station No. 3. The memory units all give outputs of nominally the same frequency but the phases will differ due to the doppler effect of differing rates of relative movement of the receiver with respect to the various transmitting stations. 
     The outputs from the 12 memory units are applied to 12 inputs of a gating circuit 150, the gates of which are controlled by a timing control unit 151 to be described later. At the appropriate time instants, the outputs from the various memory oscillators are applied as reference inputs to the aforementioned phase-lock discriminator 106. This discriminator thus compares the phase of the selected reference oscillator with the phase of the incoming received signals and provides an output on a lead 152 which is applied via a switching circuit 153 to the appropriate one of 12 output leads 154. The phase-lock discriminator 106 provides an analogue output which changes in sequence as the various transmissions are received, each transmission being compared with the appropriate phase store. These analogue outputs are fed back by the switching circuit 153 to appropriate leads to control the oscillators of the phase stores so that, during the various transmissions, the appropriate oscillators 111 to 123 are phase-locked to the received transmissions. Thus signals locked in phase to the twelve received signals are now continuously available at a frequency of 100 kHz on the leads 160 to 171 from the various frequency dividers. 
     The stored phase memories include the necessary frequency shift to allow for the doppler effect due to relative movement between the receiver and transmitting stations. Thus, so long as there is no change in speed or direction of movement relative to the transmitting stations, the phase memories store information relating to instantaneous present position of the receiver, not merely past information at the various times at which the different phase memory oscillators were last compared in phase with received signals. 
     Synchronizing of the timing is effected by a synchronising discriminator unit 172 which is similar to the discriminator unit 80 in the secondary transmitters and which senses the modulation on the synchronising transmission from the prime station to provide a reset signal for initiating the cycle of the timing control unit 151. This timing control unit provides the necessary signals at the appropriate times on leads 174, 175 for operating the gates of the units 150 and 153. A manually operable switch 173, when operated, disconnects the feed from the synnchronising discriminator unit 172 but is effective only after the reception of the next trigger pulse. Thus, if the switch is operated, the timing control will receive the next trigger pulse from the synchronising discriminator unit 172 but, thereafter, the timing control depends on the phase locked φ1.1 oscillator 111 via the synthesiser 107. Since the timing cntrol unit 151 operates from a 4 kHz signal from synthesiser 107, which is derived from oscillator 111 locked to the prime station, where the timing is derived in the same way, once the timing unit has been synchronised it will continue to operate correctly and does not require synchronising during each timing cycle. Thus the synchronising discriminator need only be functional when the equipment is first switched on to receive signals from a chain of transmitting stations. Thereafter, the synchronising transmissions are not required and the switch 173 may be operated. The timing is now dependent on the narrow band filtering to which the signals are subject in the φ1.1 control loop. In effect therefore the bandwidth of the receiver is reduced so far as timing control is concerned, with increase in the signal-to-noise ratio. 
     The outputs on the leads 160 to 171 are applied as appropriate to form the inputs of three identical digitiser and phase measurement units 180, 181 and 182. Each of these units includes a phase measuring circuit 183 and counter 184 for providing both fine and coarse pattern information in binary-coded decimal form together with manually operable selector switches for providing the appropriate inputs to the digital phase comparison means selected from the signals on leads 160 to 171. The phase measurement is conveniently effected using a double-D type bistable which gives the two phase time intervals, i.e. a first time interval using signals locked to f1 transmissions and a second time interval using signals locked to f2 transmissions from the same pair of stations. These intervals are measured by a pulse count of 10 MHz signals obtained from the oscillator 111 on a lead 187. Since the comparison signals are 100 kHz signals, the count is inherently in one hundredths of a lane. To provide for the coarse pattern display, the two pairs of the 12 possible inputs are selected to give measurements employing the same base line at the two frequencies f1 and f2. The f1 pattern phase measurement is fed via the comparator into a phase angle fraction display store. The comparator updates the store by the minimum number of bits that are required and provides steering and carry data for the display whole lane counter. The f2 pattern phase measurement also is stored. The complement of the f2 store is added to the f1 pattern store and this mathematically generates the difference in phase between the two signals which is fed to the coarse pattern display. The same two stations must be used for the f1 and f2 measurements in one digitiser so as to obtain the appropriate coarse pattern. The counts for the fine and coarse patterns are displayed on display units 186. 
     In this particular receiver, provision is made for setting in manually the lane reading counts for the three displays. Once these counts have been set in, the comparator will provide carry data for altering the display whole lane counter. For this operation there is provided a set push button 185 together with digit select thumbwheels 188 for the three displays. A digit set thumbwheel 189 selects the particular digit in the display which is to be set. 
     It will be noted that the receiver uses only a single phase-lock discriminator 106 to effect all the required phase-locking of the various memory units. No phase shifters are provided between the memory units and the phase-lock discriminator 106 and thus the phase locking is effected with the same circuits for all the various memories so eliminating differential phase shift problems in the receiver. 
     No referencing of the receiver is required since all the signals are datumed with one discriminator. 
     The switching means on the displays enable any pair of staions to be selected as a base line for the phase comparison measurement. In particular, it is possible to select a pair of secondary stations as well as to compare signals from each secondary station with the prime station.