GB1566386A

GB1566386A - Apparatus for transmitting bearing information

Info

Publication number: GB1566386A
Application number: GB1455578A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-04-13
Filing date: 1978-04-13
Publication date: 1980-04-30

Description

(54) APPARATUS FOR TRANSMITTING BEARING INFORMATION (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2. England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to apparatus for transmitting bearing information, and is particularly applicable for marine navigation in coastal waters.

There are many small fishing and other harbours where small boats have to make an approach or departure along well but narrowly defined paths to avoid dangerous obstructions. Very often the safe channel can be determined by reference to no more than a bearing upon some fixed point on the shore. However, there are a large number of small boats which use such harbours which are not equipped with specialised navigational aids such as radar, or even direction finding radio. Nevertheless most boats carry, or can be equipped with, low cost communications receivers, for example even a portable domestic transistor radio, capable of receiving VHF transmissions.Such boats, lacking specialised navigation equipment, are unable to navigate in the approaches to many small harbours after dark, unless there is a lighthouse, especially in hostile weather conditions, or even with the aid of a lighthouse in heavy fog.

According to the present invention there is provided apparatus for transmitting bearing information including two fixed omnidirectional aerials spaced a predetermined distance apart, a VHF carrier frequency signal source, an audio frequency signal source, means for deriving from the VHF signal source a VHF carrier frequency signal periodically phase modulated by the audio frequency signal, means for transmitting from one aerial a combination of the modulated VHF signal and an unmodulated signal from the VHF signal source with a controlled phase relationship means for transmitting from the other aerial a combination of the modulated VHF signal and an unmodulated from the VHF signal source in a different controlled phase relationship, means for cyclically varying the phase of the signals transmitted from one aerial relative to the signals transmitted from the other aerial, and means for distinctively altering the audio frequency modulation of the carrier frequency signal at the commencement of each cyclic variation in phase of the transmitted signals.

In one embodiment of the invention there is further included means for introducing a 1800 phase shift in the modulated VHF signals relative to the unmodulated VHF signal in the combined signals transmitted from each aerial, said 1800 phase shift being effected at substantially the middle of each cyclic variation in phase of the transmitted signals.

The above and other features of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates in schematic form an apparatus for transmitting bearing information, Figure 2 illustrates vector diagrams for the transmitted signals at various times in each cyclic variation of the phase of the transmitted signals, Figure 3 illustrates phase relationships between signals at the aerials and in space for different times in each phase variation cycle, Figure 4 illustrates further phase relationships between signals at the aerials and in space, Figure 5 illustrates a vector diagram for a situation in space, and Figure 6 illustrates a receiver frequency discriminator output plot.

In the apparatus illustrated in Figure 1 a crystal controlled oscillator 10 operating at a VHF frequency, say 162.125 MHz, has two outputs. One output is fed via an amplifier 11 to one input port of a hybrid network 12. The other output of oscillator 10 is fed via a phase modulator 13 and amplifier 14 to the other input port of hybrid network 12. The two output ports of hybrid network 12 are connected to the two aerials 15, 16 respectively via adjustable phase shift networks 17,18. The hybrid network 12 is designed so that at both outputs the unmodulated and modulated signals are combined in quadrature, at one output the modulated signal leads and at the other output it lags the unmodulated signal.An audio frequency oscillator 19 is connected to the phase modulator 13 via a switch 20 which is connected to the phase modulator 13 via a switch 20 which is controlled by a timing circuit 21. The remaining elements of the arrangement will be referred to later.

The operation of the apparatus is briefly as follows. The timing circuit 21 operates the switch 20 to cause the modulator 13 to phase modulate the VHF output of oscillator 10 periodically, say for periods of 600 milliseconds separated by intervals of 150 milliseconds during which no modulation is effected. Thus each aerial will transmit a combination of unmodulated VHF signal and periodically modulated VHF signal. The phase shifters 17 and 18 each comprise 4 different lengths of transmission line which can be selectively switched in different combinations into the aerial feeds to provide a maximum of 256 possible different degrees of relative phase shift between the signals applied to the two aerials. There are four increments of phase shift during each audio burst of 100 milliseconds, i.e. one during each 150 milliseconds of audio modulation.This results in a total of 64 audio bursts for a complete phase shift cycle at a rate of one every 750 milliseconds. Depending on the changing relative phase shift of the signals transmitted from the two aerials there will be cancellation of the audio frequency modulated VHF signals along a line pivoted at and rotating about the centre of the baseline joining the two aerials.

This in effect is a rotating vector. A listener with a conventional VHF communications receiver located to one side of the base line will hear, during one complete phase shift cycle, a series of audio frequency bursts except when the rotating vector sweeps through the receiver position, at which time the audio bursts will momentarily cease due to cancellation effects. If the listener knows the orientation of the radial on the first beat and the increment per beat and he is informed as to the time the cycle starts, he can then determine his bearing relative to the baseline centre by simply counting the number of audio bursts received until cancellation and then performing a simple computation.The transmitter is arranged to precede each phase shift cycle with a timing signal and if desired other signals which indicate the transmitter (or beacon as it is hereinafter called) identity, beacon granularity, i.e. the angular increment in degrees of the vector rotation per audio burst, and the beacon orientaton. Thus all the listener needs to know is the fundamental operating procedures of such beacons. So long as he is aware of the existence of a beacon and its transmission carrier frequency and has a receiver he can determine his bearing on the beacon. Figure 2 illustrates vector diagrams applicable to space for different time intervals during the phase shift cycle. Figure 2 shows the unmodulated carrier signal A, the modulated carrier resultant B at the extremes of the audio modulation cycle, and the resultant C of all the components at the extremes of the modulation cycle.In Figure 2(a) the vector diagram is shown for the centre line of the beacon, i.e. the perpendicular bisector of the base line joining the two aerials, at the start of the phase shift cycle. Figures 2(b) and 2(c) show the vector diagrams for the centre line at 1/4 of the cycle and of the cycle respectively.

One problem which arises with the system described above is that distortion arises when the resultant of the unmodulated carriers from the two aerials approach cancellation of the resultant of the modulated carriers. After the rotating vector has passed through the receiver position there will be a reversal of the relative phases with greatly increased distortion. If, however, the beacon incorporates a means for introducing an extra 1800 phase shift between the modulated and unmodulated carriers at the aerials at the approximate time when the rotating vector passes through the receiver position this distortion can be minimised.Since the vector, by definition, passes through different receiver positions at different times it would not be possible to switch, at the beacon, the extra 1800 in a way which would coincide with the vector at all bearings. however, a compromise is possible that is beneficial for all bearings.

The 1800 phase shift switching is effected by means of a switch 22 (Figure 1), where either a short length of line or a long length of line can be switched into the modulated carrier feed to the hybrid network 12. The difference in length between the two lengths of line is sufficient to introduce the 1800 phase shift at the hybrid network 12. This shift 22 is operated to effect the changeover at the mid-point of the incremental phase shift cycle of phase shifters 17 and 18. Switch 22 is in the modulated path since this introduces a minimum of spectrum widening. Since the phase difference between the modulated and unmodulated carrier frequency signals is reversed at the hybrid network it will also be reversed at the aerials.

The effect of the extra 1800 phase reversal results in the polar co-ordinates of the signals fed to the aerials having the following relationships: (a) with the phase shifters 17 and 18 giving zero phase difference between the aerial feeds (e.g. the rotating vector being normal to the baseline joining the aerials) immediately before the extra 180 phase shift effect of switch 22 aerial modulated carrier unmodulated carrier is 1 + 90 1 0 16 1 -90 1 0 (b) with phase shifters 17 and 18 in the same condition but switch 22 operated to impose the extra 1800 phase shift aerial modulated carrier unmodulated carrier 15 1 900 1 0 16 1 +900 1 00 At the commencement of the variable phase shift cycle of shifter 17 and 18 there is a 90" phase difference between the two aerials such that the two unmodulated carriers commence the cycle in RF quadrature. At the completion of the cycle they are again in quadrature, one of them having meanwhile moved through 1800. This is illustrated in Figure 3.

Figure 3(a) shows for a particular point in time the phase relationships between the various signals at the aerials and Figure 3(b) shows at the same time the phase relationships in space at a constant azimuth angle of -10". The references U1 and U2 denote the unmodulated carriers transmitted from aerials 15 and 16 respectively, M1 and M2 denote the modulated carriers. Figure 5 shows the sequence at 4 different times of the cycle corresponding to the time when the phase shifts 17 and 18 have the electrical phase difference shown as 4). These vector diagrams show the combined effect of the aerial phase shifters 17 and 18 and the extra phase shift imposed by switch 22 at the mid-point of the cycle, the resultants in space being denoted by Ur and Mr respectively.Each of the four diagrams in Figure 5 is orientated to show U1 in the same position, so that U2 effectively rotates due to the phase shifter action. M2 similarly rotates but M1 does not, it only suffers the 1800 phase reversal at the mid-point of the cycle.

For a direction cup = -10" the path length difference 0O from the two aerials in 360 d sin , where d is the distance between the aerials. If d = 0.8761 then Oo = 54.77 . It is assumed that a negative angle is closer to aerial 16 so that a signal from aerial 16 leads that from aerial 15.The vector diagram in the upper right hand corner of Figure 5 shows M2 - the modulated carrier on aerial 16 to have arrived at the receiver leading its transmitted position by 6o and M1 - the modulated carrier on aerial 15 to have arrived at the receiver lagging the transmitted position by Ȯ,) - as would be the situation if the distance between receiver and the phase centre of the aerial were an integral number of wavelengths. U2 is similarly given Ȯo lead and U1 Ȯo lag.

The vector sum of the modulated carriers and the vector sum of the unmodulated carriers will be 1.906 and 0.605 respectively.

When < D = 55" U2 and M2 have both moved 90" - 55" and the resultants have moved as shown. A similar process shows the result at (t) = -55" and 4) = -90".

These vector diagrams all illustrate the situation at that part of the audio period when the modulated carrier passes through its mean position. Figure 5 shows at (a) the vector relationship at the aerials when the phase shifter has = 90 ; and correspondingly in space for o = -10" U2 leads M1 by 85" and M2 leads U1 by 85 . In Figure 5(b) is shown the corresponding situation for 4) = 55" and Q = -10 . For the situation of Figure 5(b) the resultant in space of the modulated components is 0.996 and the resultant of the unmodulated components is 1.734 with the two resultants being in phase as the modulated carrier passes through its mean position.

Figure 4 shows for that value of the phase shifter how the situation varies within the audio modulation cycle, the vector alignments being drawn out for the condition when t = 3T/48.

Figure 4(b) shows the position for the same asimuth angle but with (t) = -5 which is just after the extra 1800 have been switched in such that the vector sum now remains on the 'left' side of the head of the unmodulated carrier.

By comparing Figure 4(a) with Figure 4(b) one may observe the effects of this switching a) During the first quarter of the audio modulation cycle the 'in phase' situation has the vector sum rotating counter clockwise whereas the 'antiphase' situation has the vector sum rotating clockwise. The two demodulated signals will therefore in the main be in antiphase with each other - a transient being introduced at the time of switching.

b) In Figure 4(b) the most rapid phase change, which occurs when n - 0 will greatly exceed that for Figure 4(a). (The angle moved by the modulated carrier is identical in the two cases but the length of the resultant is greatest for Figure 4)a)). Thus the peak demodulated carrier in the receiver will be greatest for the antiphase case.

c) For 0 < n < 12 the resultant moves only counter-clockwise in the 'in phase' case, but in the 'antiphase' case it moves first clockwise then slightly anticlockwise. Thus the distortion will be greatly increased.

Figure 6 shows a plot of the output waveform from the frequency discriminator for azimuth angle -10" for both 4 > = +5 and Q, = -5 . These waveforms support the above 3 observations.

The arrangement of Figure 1 also incorporates switches 23 and 24 which are used to ensure that while signalling to the receiver the beacon alignment, granularity and start of notation, only one transmitter is operating and only one antenna is radiating. Thus the tone level will for this part of the cycle be independent of bearing and have a good level. The arrangement also has adjustments at 25, 26 and 27 so that the various signal levels can be adjusted as required. Additional fixed phase adjustments may be inserted at points X and/or Y to obtain the correct basic phase relationship of the signals at the aerials.

WHAT WE CLAIM IS: 1. Apparatus for transmitting bearing information including two fixed omnidirectional aerials spaced a predetermined distance apart, a VHF carrier frequency signal source, an audio frequency signal source, means for deriving from the VHF signal source a VHF carrier frequency signal periodically phase modulated by the audio frequency signal, means for tramsitting from one aerial a combination of the modulated VHF signal and an umodulated signal from the VHF signal source in a predetermined phase relationship, means for transmitting from the other aerial a combination of the modulated VHF signal and an unmodulated from the VHF signal source in a different phase relationship, means for cyclically varying the phase of the signals transmitted from one aerial relative to the signals transmitted from the other aerial, means for distinctively altering the audio frequency modulation of the carrier frequency signal at the commencement of each cyclic variation in phase of the transmitted signals.

2. Apparatus according to claim 1 further including means for introducing a 1800 phase shift in the modulated VHF signals relative to the unmodulated VHF signal in the combined signals transmitted from each aerial, said 1800 phase shift being effected at substantially the middle of each cyclic variation in phase of the transmitted signals.

3. Apparatus according to claim 1 or 2 wherein the means for transmitting the combinations of signals from the aerials comprises a hybrid network to two opposing input ports both of which are fed the unmodulated and modulated VHF signal respectively, the two output ports being connected to the two aerials respectively, the hybrid network being so arranged that at one output port the modulated and unmodulated VHF signals are in quadrature leading and at the other output port they are in quadrature lagging.

4. Apparatus according to claim 1, 2 or 3 wherein the means for cyclically varying the phase of the signals transmitted from one aerial relative to those transmitted from the other aerial comprises adjustable phase shift networks in the aerial feeds, each such network consisting of a number of different lengths of transmission line and means for selectively switching different combinations of said lengths into each aerial feed.

5. Apparatus according to claim 2 or any one of claims 3 and 4 when dependent on claim 2 wherein the means for introducing a 1800 phase shift comprises means for switching a predetermined length of transmission line into either the unmodulated VHF carrier signal to be combined with the modulated signal for transmission from the aerials or the modulated VHF signal to be combined with the unmodilated signal for transmission from the aerials.

6. Apparatus for transmitting bearing information substantially as described with reference to the accompanying drawings.

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Claims

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rotating clockwise. The two demodulated signals will therefore in the main be in antiphase with each other - a transient being introduced at the time of switching.

b) In Figure 4(b) the most rapid phase change, which occurs when n - 0 will greatly exceed that for Figure 4(a). (The angle moved by the modulated carrier is identical in the two cases but the length of the resultant is greatest for Figure 4)a)). Thus the peak demodulated carrier in the receiver will be greatest for the antiphase case.

c) For 0 < n < 12 the resultant moves only counter-clockwise in the 'in phase' case, but in the 'antiphase' case it moves first clockwise then slightly anticlockwise. Thus the distortion will be greatly increased.

Figure 6 shows a plot of the output waveform from the frequency discriminator for azimuth angle -10" for both 4 > = +5 and Q, = -5 . These waveforms support the above 3 observations.

The arrangement of Figure 1 also incorporates switches 23 and 24 which are used to ensure that while signalling to the receiver the beacon alignment, granularity and start of notation, only one transmitter is operating and only one antenna is radiating. Thus the tone level will for this part of the cycle be independent of bearing and have a good level. The arrangement also has adjustments at 25, 26 and 27 so that the various signal levels can be adjusted as required. Additional fixed phase adjustments may be inserted at points X and/or Y to obtain the correct basic phase relationship of the signals at the aerials.

WHAT WE CLAIM IS: 1. Apparatus for transmitting bearing information including two fixed omnidirectional aerials spaced a predetermined distance apart, a VHF carrier frequency signal source, an audio frequency signal source, means for deriving from the VHF signal source a VHF carrier frequency signal periodically phase modulated by the audio frequency signal, means for tramsitting from one aerial a combination of the modulated VHF signal and an umodulated signal from the VHF signal source in a predetermined phase relationship, means for transmitting from the other aerial a combination of the modulated VHF signal and an unmodulated from the VHF signal source in a different phase relationship, means for cyclically varying the phase of the signals transmitted from one aerial relative to the signals transmitted from the other aerial, means for distinctively altering the audio frequency modulation of the carrier frequency signal at the commencement of each cyclic variation in phase of the transmitted signals.
2. Apparatus according to claim 1 further including means for introducing a 1800 phase shift in the modulated VHF signals relative to the unmodulated VHF signal in the combined signals transmitted from each aerial, said 1800 phase shift being effected at substantially the middle of each cyclic variation in phase of the transmitted signals.
3. Apparatus according to claim 1 or 2 wherein the means for transmitting the combinations of signals from the aerials comprises a hybrid network to two opposing input ports both of which are fed the unmodulated and modulated VHF signal respectively, the two output ports being connected to the two aerials respectively, the hybrid network being so arranged that at one output port the modulated and unmodulated VHF signals are in quadrature leading and at the other output port they are in quadrature lagging.
4. Apparatus according to claim 1, 2 or 3 wherein the means for cyclically varying the phase of the signals transmitted from one aerial relative to those transmitted from the other aerial comprises adjustable phase shift networks in the aerial feeds, each such network consisting of a number of different lengths of transmission line and means for selectively switching different combinations of said lengths into each aerial feed.
5. Apparatus according to claim 2 or any one of claims 3 and 4 when dependent on claim 2 wherein the means for introducing a 1800 phase shift comprises means for switching a predetermined length of transmission line into either the unmodulated VHF carrier signal to be combined with the modulated signal for transmission from the aerials or the modulated VHF signal to be combined with the unmodilated signal for transmission from the aerials.
6. Apparatus for transmitting bearing information substantially as described with reference to the accompanying drawings.