WO1992001236A1

WO1992001236A1 - A radio direction finding system

Info

Publication number: WO1992001236A1
Application number: PCT/GB1991/001128
Authority: WO
Inventors: David Murdoch Mackenzie
Original assignee: Gec Ferranti Defence Systems Limited
Priority date: 1990-07-10
Filing date: 1991-07-09
Publication date: 1992-01-23
Also published as: PT98265A; ZA915077B; CN1059967A; GB2246041B; GB9015219D0; MX9100152A; GB2246041A; AU8200391A; IE912391A1

Abstract

The wavefront orientations of transmissions from a plurality of OMEGA navigation transmitters are determined to establish absolute geographical bearing. Respective signals induced in the individual antennae (A, B, C, D) of a fixed antenna array are sampled and stored. A microprocessor (69) synthesises the induced waveforms utilizing stored polar response data for the antenna and compares the relative signal amplitudes of the synthesised waveforms to determine the wavefront orientation. The wavefront orientations of transmissions from different transmitters may be determined simultaneously.

Description

A Radio Direction Finding System

This invention relates to a radio direction finding system and particularly, though not exclusively to an OMEGA compass navigation system in which direction is established with reference to VLF radio transmissions broadcast by OMEGA transmitter stations.

In low and middle latitudes the magnetic compass provides a satisfactory means of establishing the bearing of a vehicle such as an aircraft. However, the magnetic compass becomes increasingly unreliable in higher latitudes and becomes unusable in the vicinity of north and south magnetic poles.

The OMEGA VLF radio navigation system provides a worldwide pattern of VLF signals by means of which absolute position can be determined using a suitable receiver. Because it has not been found practicable to construct a goniometer which will work at VLF, the commonly used radio direction finding array comprising a pair of fixed antennae coupled to a receiver by a goniometer cannot be used. However, it has been demonstrated that by using a rotatable antenna array, OMEGA transmissions can indeed be used to

SUBSTITUTE SHEET establish bearings. One known method uses a single antenna which is rotated to null out the signal of interest. Another known method uses an array comprising a pair of identical antennae disposed at a predetermined angle to each other, the array being rotated and the signal strength in each antenna measured. The bearing is given by the position at which both strengths are equal. In both these cases the absolute position of the observer is determined from the OMEGA transmissions and the absolute bearing is obtained from a knowledge of the direction of the wavefront of OMEGA transmission in the vicinity of the observer. These prior techniques are described in "A feasibility study of VLF radio compass for arctic navigation" (Petrov et al) in Journal of the Institute of Navigation Volume 31 No. 4 Winter 1984-5 pages 338-358, and "The OMEGA Compass" (J.F. Kemp) Proceedings 13th Annual Meeting of the International OMEGA Association - October, 1988, pages 8-1 to 8-4, and while they are capable of yielding accurate bearings, the requirement of taking a large number of readings while the antenna is being rotated means that it is impracticable to utilize the technique in an aircraft or other vehicle whose direction and position are changing rapidly.

A known microwave radio direction finding technique involves the use of a plurality of directional antennae arranged such that their beam patterns overlap. An incident wave will induce signals in at least two of the antennae. In general, the two adjacent antenna most closely aligned with the incident wave will have the strongest signals induced in them. By measuring the relative amplitudes of these signals and from a knowledge of the polar response patterns of the antennae, the direction of the incident wavefront may be determined. However, this technique is not suitable for use at VLF as it is not practicable to construct the suitable directional antennae required. The present invention has arisen in an attempt to provide an improved VLF radio compass.

According to the invention there is provided apparatus for determining the wavefront orientation of at least one radio wave comprising a fixed antenna array consisting of a plurality of antennae arranged at pre-determined orientations to each other, sample and hold means for sampling and storing signals representing the magnitudes of the voltage induced in each respective antenna by the at least one radio wave, and signal processing means utilising said stored signals and the antenna response data for each antenna to determine the wavefront orientation of the at least one radio wave.

SUBSTITUTE SHEET The elimination of the need for rotating parts allows results to be established much faster and updated more frequently, and the simpler mechanical construction can lead to increased reliability of the arrangement.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which;

Figures la and lb respectively show the beam pattern of a single element antenna in polar co-ordinates and rectangular co-ordinates;

Figures 2a and 2b respectively show the superimposed beam patterns of a pair of crossed-single element antenna in polar co-ordinates and rectangular co-ordinates;

Figure 2c shows the ratio of the responses of the two elements in Figure 2a and 2b;

Figure 3 shows the superimposed beam plot of a three element antenna in polar co-ordinates.

Figures 4a and 4b show the superimposed responses of a four element antenna array.

SUBSTITUTE SHEET Figure 5 shows the polar plot of a two element array in greater detail for explaining the operation of the invention;

Figure 6 shows the block diagram of a first embodiment of the invention;

Figure 7 shows the block diagram of a digital signal processor for use with the invention;

Figure 8 shows the block diagram of a second embodiment of the invention.

Referring now to Figures 1 to 5, Figure la shows a polar plot of the response of a simple antenna element 1, which for a VLF receiver may be a ferrite-loaded coil antenna element such as is used in existing OMEGA receivers. The magnitude of signal induced in the antenna varies with the angle 0 which the antenna subtends to the source of radio waves. Figure lb shows that this response plotted in rectangular co-ordinates, the curve 11 representing the variation in magnitude with angle 0 .

SUBSTITUTE SHEET Arrays comprising a plurality of such elements disposed at predetermined angles to one another can be used for direction finding by determining the cross-over points at which the signals induced in the antenna are of equal magnitude. For example, two elements set at 90° to each other will provide four beams which cross-over at the 45°, 135°, 225° and 315° directions. Greater numbers of elements may be used for greater precision. Figure 3 shows the beam pattern obtained for three antennae set at 60° which provide crossings at 30°, 90°, 150°, 210°, 270° and 330°, while Figure 4 shows an arrangement for four antennae set at 45° providing eight crossing points disposed at 22.5°, 67.5°, and in 45° steps to 337.5°.

In Figure 5 antenna 1 is aligned on a first axis 4 and antenna 2 on a second axis 3, an angle γ being subtended between the two axes. Signals are received from an OMEGA station on a bearing 6 aligned at an angle to the axis 3 which, as shown, is aligned due north. One of the cross-over points of the beam responses 111, 121 of the antenna elements 1, 2 is shown at 5. By measuring the signals from consecutive pairs of antennae, in this case from the two antennae, ratios of the form COS Θ/COS ( θ - φ ) can be derived. The strongest signals will be obtained from the pair of beams straddling the direction of the incoming

^SUB^STITUTE SHEET signals most closely, and the ratio will contain the required directional information, which can be extracted provided the angular articulation of the beams is adequate and their shape is sufficiently accurately known.

Figure 6 shows a first embodiment of the invention for use with four antennae. The antennae ABCD may be spaced at 45° as shown in Figure 4a and having magnitude responses as shown in Figures 4a and 4b. Each antenna element A,B,C,D is coupled to a respective signal path a,b,c,d via a multiplexer 61 which serves to couple in calibration signals 610 from a calibration source (not shown) into each signal path a,b,c,d. Each signal path is identical and comprises a superheterodyne arrangement in which the incoming RF is band pass filtered in filter 62 and mixed with a source of local oscillator signals in a mixer 63. The resulting intermediate frequency (IF) is passed through an IF filter 64 to a sample-hold arrangement 65 which samples the instantaneous magnitude of the IF signal and which is controlled by a clock source 681. Outputs from the four sample-hold (S/H) circuits are multiplexed by multiplexer 66 and sequentially applied to an A/D converter 68 which produces a corresponding^" sequence of digital words representing the respective S/H output signals. The digital words are sequentially applied to a digital signal processor

SUBSTITUTE SHEET, 69 which determines the bearing from the data supplied to it. As is known, signals transmitted by OMEGA stations are in the form of a sequence of bursts of carrier, each having a different frequency. Each burst has a different duration and the duration of the frequencies and the sequence of the frequencies uniquely identifies the transmitting station. Accordingly, to receive the selected transmission, multiplexer 67 selectively couples the appropriate one of local oscillators 671, 672 etc. to the mixer 63 at such instants of time and with such durations that only transmissions from the selected OMEGA transmitter are received.

The digital signal processor 69 operates on the signal strength data to obtain an output indicative of the bearing of the transmitter of interest.

Figure 7 shows a block diagram of a digital signal processor which can be used to extract magnitude and phase information from the digital signals. Incoming data is fed to two signal paths each having a respective multiplier 71, 710, and integrator 72, 720. The multipliers are also supplied with respective SIN wt and COS wt signals from respective read-only memories 75, 76 which are addressed by a counter 77 fed by a clock. The integrated outputs are fed to a summer 73 which indicates magnitude, and to a phase sensitive circuit 74 which indicates the phase.

The arrangement of Figures 6 and 7 operates on one transmitter at a time. It can be tuned to a different transmitter by instructing the multiplexer 67 to connect the local oscillators 671, 672 in a different time and duration sequence corresponding to the desired transmitter.

A second embodiment of the invention will now be described with reference to the Figure 8. This arrangement uses direct radio frequency sampling and is capable of receiving the broadcast of a number of OMEGA transmitters simultaneously. As in Figure 6, the arrangement is for use with an antenna array comprising four antenna A,B,C,D and the signal processing path includes four substantially identical paths a,b,c,d. Only the first path will be described, the others being identical.

Each antenna comprises a ferrite rod 81 having a test signal winding 801 and an array winding 802. The main winding is connected to a buffer amplifier 803 whose output is coupled via a multiplexer 804 to a first band pass filter 805 which only passes signals lying within the 10 to 14 kHz band. Following this are three narrow band pass filters 806, 807, 808 connected in parallel, each being tuned to a respective one of the frequencies 10.2, 11.33 and 13.6 kHz on which OMEGA signals are transmitted. The sum of the outputs of these filters is applied to a track and hold amplifier 809 controlled by clock signals from a clock source 821. The outputs of the track and hold amplifier for each channel are applied to a second multiplexer 810 which connects the output of each channel in turn to the following A/D conversion circuit. This couples a first, flash ranging A/D circuit 811 which determines the exponent 815 as an eight bit word and programmable gain amplifier 812 and second A/D converter 813 which determines the mantissa portion 814 as a sixteen bit word. The exponent and mantissa are applied to the digital signal processor 816 which in the non-limiting example of the embodiment may be a dedicated signal processor such as a Texas Instruments TMS 320C3O. The digital signal processor 816 is connected to and operates under the control of a main system micro-processor 818 via data bus 817, the micro-processor having an external I/O bus 819, 820.

The digital signal processor uses a correlating filter to successively correlate the digitised output from the A/D converter with in-phase and quadrature 10.2, 11.33 and 13.6 kHz waveforms loaded from the system memory. The results

^SUBSTITUTESHEET are forwarded to the main processor which determines the bearing from the amplitude information-containing data received from the co-processor. This can be done in a number of ways. A first technique is to calculate the ratios between the amplitudes of signals induced in the consecutive antenna pairs and obtain the bearing from the ratios so calculated. A second technique involves a beam scan approach in which the amplitude responses of two orthogonal antennae elements are combined to form a single response pattern which can be steered by a system of software.

The arrangements described provide a bearing line only, and the consequential 180° directional ambiguity must be resolved independently. This is readily done using known ambiguity-resolving techniques.

For example, the absolute direction may be established by reference to a conventional magnetic compass when still in a region where the magnetic compass is reliable. High precision is not required, as the magnetic compass only needs to distinguish North from South. Once set up, the OMEGA compass will retain its setting for as long as it remains energised.

^SUB^STITUTE SHEET For a land-based vehicle, if the compass has to be de-energised after the vehicle has stopped, data representing the absolute bearing can be stored eg. in non-volatile memory, and used to recalibrate the OMEGA compass when it is re-energised prior to moving the vehicle. This can be done manually or automatically.

For an aircraft, ambiguity can be resolved during flight by measuring the phase shift of a given OMEGA transmission, the direction of phase shift indicating whether the aircraft is moving towards or away from the transmitter.

Having obtained the bearing information it has to be displayed. Considering the already crowded control panel of an aircraft, it is convenient to utilise the existing fluxgate compass display, or an existing radio compass display. Accordingly, the bearing information may be converted into drive signals suitable for driving either of these compass displays by means not shown.

The above embodiments are given by way of example only and a number of modifications are possible within the scope of the invention. For example, although a single A/D converter is shown in Figures 6 and 8, each channel could

SUBSTITUTE SHEET well have its own gated A/D converter.

Claims

1. Apparatus for determining the wavefront orientation of at least one radio wave comprising a fixed antenna array consisting of a plurality of antennae arranged at pre-determined orientations to each other, sample and hold means for sampling and storing signals representing the magnitudes of the voltage induced in each respective antenna by the at least one radio wave, and signal processing means utilising said stored signals and the antenna response data for each antenna to determine the said wavefront orientation of the at least one radio wave.

2. Apparatus as claimed in claim 1, in which the at least one radio wave lies in the very low frequency band.

3. Apparatus as claimed in claim 2 , in which the at least one radio wave is transmitted by an OMEGA radio navigation transmitter.

4. Apparatus as claimed in any preceding claim in which the antenna array comprises four antennae.

^SUBSTITUTESHEET

5. Apparatus as claimed in claim 4, in which each antenna is oriented substantially at 45° to its neighbouring antenna.

6. Apparatus as claimed in any preceding claim including a respective sample and hold arrangement for each antenna, a signal path coupling each antenna to its respective sample and hold arrangement.

7. Apparatus as claimed in claim 6, further comprising analogue to digital conversion means to provide respective digital representations of the signals, and digital signal processing means responsive to the respective digital representations to determine said wavefront orientation and to provide output data representative thereof.

8. Apparatus as claimed in claim 7, comprising a multiplexer to selectively sequentially couple the output of each sample and hold means to the analogue to digital converter means.

9. Apparatus as claimed in any of claims 3 and 6 to 8 in which each signal path comprises a mixer to convert the voltage induced in the respective antenna to an intermediate frequency signal, the respective intermediate frequency signals being applied to the respective sample and hold means, a local oscillator frequency being sequentially switched between selective ones of a plurality of local oscillator frequency sources so as to tune the apparatus to a selected OMEGA transmitter, the signal processing means determining the wavefront orientation of the OMEGA transmissions said transmitter.

10. Apparatus as claimed in claim 9 in which the same local oscillator frequency signal is applied to each respective mixer simultaneously.

11. Apparatus as claimed in any of claims 3 and 6 to 10 in which each signal path comprises a respective band pass filter means so as to pass only frequencies transmitted by OMEGA transmitters.

12. Apparatus as claimed in claim 11, in which each filter means comprises a plurality of pass bands, each pass band corresponding to one of the frequencies transmitted by OMEGA transmitters.

13. Apparatus as claimed in claim 12, in which the pass bands are such as to pass all frequencies transmitted by OMEGA transmitters.

^SUBSTITUTESHEET

14. Apparatus as claimed in any claims 11 to 13, in which the signal processor determines the wavefront orientation of each of a plurality of wavefronts from a plurality of OMEGA transmitters substantially simultaneously.

15. Apparatus as claimed in any preceding claim comprising means to convert output data representing the wavefront orientation into further data representing the geographical bearing of the apparatus.

16. Apparatus as claimed in claim 14 comprising means for converting said further data into signals suitable for driving the indicator of a fluxgate compass thereby displaying the geographical bearing of the apparatus.

17. Apparatus as claimed in any preceding claim including means for resolving the directional ambiguity of the radio wave.

18. Apparatus substantially as described with reference to Figures 6, 6 and 7 or 8 of the accompanying drawings.