GB2195851A

GB2195851A - Direction finding

Info

Publication number: GB2195851A
Application number: GB08418456A
Authority: GB
Inventors: William David Waddoup; Andrew Peter Norris; David Evan Naunton Davies
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1984-07-20
Filing date: 1984-07-20
Publication date: 1988-04-13
Also published as: GB2195851B

Abstract

A linearly polarised far-field signal incident on a cusp reflector (5) gives rise to a linearly polarised signal at its focus whose plane of polarisation depends on the azimuthal angle on arrival ( phi ) of the far-field signal. Placing a polarimeter (7,8,9) at the cusp reflector focus enables the plane of polarisation to be determined. This, combined with a coarse amplitude comparison measurement to indicate the quadrant in which the azimuthal angle of arrival lies, enables an accurate azimuthal angle of arrival to be determined. Placing a linearly polarising screen (6) around the cusp reflector ensures that the otherwise arbitrary polarisation of most signals becomes linear before encountering the reflector. <IMAGE>

Description

SPECIFICATION Direction finding This invention relates to direction finding.

According to one aspect of the present invention there is provided an azimuthal direction finder comprising a cusp reflector, as hereinafter defined, and a polarimeter disposed at the focus of the cusp reflector, the cusp reflector being such that a linearly polarised far-field signal incident thereon produces a linearly polarised signal at the focus of the cusp reflector whose plane of polarisation depends on the azimuthal angle of arrival of the far-field signal, the polarimeter serving in use of the direction finder to determine the plane of polarisation of the signal at the focus of the cusp reflector.

According to another aspect of the present invention there is provided a method of azimuthal direction finding comprising causing a linearly-polarised far-field signal, whose azimuthal angle of arrival is to be determined, to be incident on a cusp reflector, as hereinafter defined, whereby to produce a linearly polarised signal at the focus of the cusp reflector whose plane of polarisation depends on the azimuthal angle of arrival and employing a polarimeter to determine the plane of polarisation of the signal at the focus of the cusp reflector.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figs. 1 and 2 illustrate two configurations of cusp reflectors; Fig. 3 illustrates a cusp reflector with a linearly polarised feed horn and showing the farfield plane of polarisation rotating with azimuth (0); Fig. 4 shows, partially cut away, a basic direction finding system employing the polarisation properties of the cusp reflector;; Figs. 5 and 6 show, respectively, in a side view (ray approximation) and a plan view (practical occurence) a feed arrangement relative to a cusp reflector (omitted for clarity from Fig. 6) for determining the direction of incident radiation both coarsely and accurately by an amplitude comparison method, and Fig. 7 shows a direction finding system (linearly polarising screen omitted) similar to that of Fig. 4 but employing an amplitude comparison method to resolve the z ambiguity.

In our copending Application No. 8405291 (Serial No. ) (W.D.Waddoup-A.P.Norris 4-1) there is disclosed an antenna system, particularly, but not exclusively, for- mm wavelengths, that has an omnidirectional azimuth pattern and an elevation pattern shaped to suit the particular systems appiication. Such shaping can permit maximised gain at elevation angles of interest together with high rates of cut-off, for instance at the horizon, to prevent the formation of multipath grating lobes. The distinguishing feature of that antenna system is a rotationally symmetric main reflector fed from the zenith and shaped to produce the desired elevation pattern. The main reflector may be referred to as a cusp reflector" which expression is thus correspondingly defined.Such a cusp reflector may be formed by rotation of a parabolic section, an example of which is described hereinafter with reference to Fig. 5.

Two alternative configurations and associated ray patterns are shown in Figs. 1 and 2. The configuration of Fig. 1 uses a conventional horn feed 1 to illuminate the cusp reflector 2, whilst the second employs a parabolic subreflector 3 which is itself fed by a horn feed 4 disposed at the apex of cusp reflector 2'. The dual-reflector configuration of Fig. 2 produces a more compact design and may be configured to substantially eliminate adverse effects due to spillover.

For both of the configurations illustrated in Figs. 1 and 2, a linearly polarised feed 1' will produce a linearly polarised far-field, although the tilt angle of the far-field polarisation becomes a function of azimuth as illustrated in Fig. 3, that is the plane of polarisation rotates with changing azimuth. This is normally undesirable and special feeds are required in dependence on the desired far-field polarisation.

For instance, vertical polarisation in the farfield will be produced with a TM01 circular waveguide fed, which exhibits a radial electric field at its aperture, whilst horizontal polarisation will be produced with a TE01 feed, which has circular electric field lines at the aperture.

Far-field circular polarisation will be produced by tha antenna system using the appropriate circularly polarised feed.

The simply-realised 2n azimuth coverage of these antennas and the availability of gain enhancement by elevation shaping, make them attractive for use as direction finding antennas.

In order to effect direction finding, however, it is necessary to provide multi-ports to the antenna system that exhibit different amplitude and/or phase characteristics in different directions, thus enabling comparison techniques to yield ange-of-arrival information. Normally such systems can be categorised as amplitude or phase comparison systems.

Amplitude comparison systems are typically based on configurations where the effective phase centres of radiation associated with each part are coincident, or nearly so, but the port amplitude patterns exhibit marked angular differences. phase comparison systems rely on separated phase centres and, in order to determine the angle-of-arrival, effectively measure the time difference between the same signal arriving at different ports.

If the cusp reflector system is fed by a multifeed network then it is capable of direction finding by, for example, applying an amplitude comparison method.

As previously mentioned, when the cusp reflector is illuminated by a linearly polarised primary feed, the polarisation vector in the farfield undergoes a complete rotation when viewed over a 2n azimuth range. If one considers the direction of the polarised field at the focus of the cusp reflector when receiving from a vertically polarised far-field source, it will be seen that the source will now be aligned along the direction of the received polarisation vector. Hence to ascertain the direction of the source, it is only necessary to determine the polarisation angle of the received field, and to resolve the 7r ambiguity.

If the source is rotated to give horizontal polarisations, a similar action takes place, although the source direction now lies orthogonal to the polarisation direction. For other tilted linear polarisations, the angular offset between the polarisation direction and the source direction is equal to the tilt angle (from vertical) of the source direction. Thus for a source with a known angle of linear polarisation it is possible to determine the angle-ofarrival information from measurements of the received polarisations at the focus of the reflector.

For most direction finding applications it is unreasonable to assume that the polarisation state of the source will be known to any accuracy. However, by incorporating a linearly polarising screen around the direction finding antenna, it is possible to ensure that the otherwise arbitrary polarisation of most signals becomes linear before encountering the reflector, so that pure linear polarisation at the required tilt angle is incident on the cusp reflector, enabling direction finding to be performed by a polarimeter. Components of incident energy orthogonal to the polarising screen will be reflected at the screen and hence a certain loss of sensitivity will result, dependent on the source polarisation. This action, of course, is identical to the properties of any non-matched direction finding antenna.

Sources orthogonally polarised to the cho sen polarising screen will, clearly, be "invisible" to the antenna. However, if this is considered to be a likely occurrence for a particular application then it may be overcome by employing two antenna systems with mutually orthogonal polarising screens.

A basic direction finding system employing the polarisation properties of the cusp reflector is illustrated schematically in Fig. 4. It comprises a cusp reflector 5, a polarising screen 6, which is shown partially cut away, and a polarimeter comprising a polarisation sensitive mirror 7, two receivers 8 and 9, which may be comprised by conventional feed arrangements as illustrated, arranged to be orthogonal. Other methods of polarisation measurement may alternatively be employed.

Resolution of the lr ambiguity may be obtained by a secondary amplitude comparison technique yielding coarse directional information. This may be achieved by replacing the single feed receivers 8 and 9 with dual feeds which straddle the focal points and are aligned along the respective polarisation vectors. This arrangement will now be described with reference to Fig. 5. The cusp reflector 11 is considered, for the purpose of this description, to be formed by rotation of a parabolic section about the point P. The focus of the reflector is the rotation of the focal point F of the parabola, that is a circle 12 (Fig.6). Energy arriving at the same azimuth but at different elevation angles will be focussed to different points, as shown in Fig. 5.Fig. 6 is a plan indicating the arrangement of four receivers u1, u2, v1 and v2 disposed on the circle 12 comprising the focus of the reflector in the plane x-x, which plane in practice corresponds to a polarimeter input port. By suitable choice in the geometry it can be ensured that energy within the focal spots from all elevation angles of interest will be preferentially directed to a focal sector lying "towards" the source, i.e.

the shaded region marked energy distribution in plane x-x. Amplitude comparison between the signals in receivers u, and u2 and separately v, and v2 will then readily give the coarse angle of arrival of the received signal.

Fine angle of arrival can then be obtained by obtaining u = biggest (u1, u2) and v = biggest (v1, v2) and then operating on u and v to determine the direction of polarisation which corresponds to 0 the angle of arrival and is given by tan-1 (v/u). 0 is as indicated in the inset diagram of Fig. 3 Fig. 7 illustrates a direction finder with means for overcoming the 7r ambiguity present with the basic arrangement of Fig. 4. A signal is incident from the zenith upon a polarisation sensitive mirror 13 which is inclied at or/4 to the zenith. One polarisation component is reflected by the mirror 13 through it/2 and is shown focussed on a feed of receiver 14.The other component passes through the mirror and is shown focussed on a feed of receiver 15. Depending on the azimuthal angle of arrival the components will be focussed on one or other feed or a position between then. Hence, r.f. signals corresponding to the two orthogonal components of signal are obtained from the feeds of the receivers 14 and 15, which signals can be processed to determine 0(0 = tan-1v/u) and to resolve the K ambiguity by comparing the signal amplitudes in the different feeds of the receivers 14 and 15. In practice, this is more likely to be performed after conversion of the signals, from r.f. to i.f. which would be carried out in the usual way using conventional mixers and coherent local oscillator signals.

As mentioned above, in order that the total direction finding system is not "blind" to a particular polarisation vector direction two antenna systems with mutually orthogonal polarising screens, but otherwise identical, may be employed. Which antenna systems would be stacked one on top of the other with a common axis for their individual axes of symmetry.

Claims

1. An azimuthal direction finder comprising a cusp reflector, as hereinbefore defined, and a polarimeter disposed at the focus of the cusp reflector, the cusp reflector being such that a linearly polarised far-field signal incident thereon produces a linearly polarised signal at the focus of the cusp reflector whose plane of polarisation depends on the azimuthal angle of arrival of the far-field signal, the polarimeter serving in use of the direction finder to determine the plane of polarisation of the signal at the focus of the cusp reflector.

2. A direction finder as claimed in claim 1 including means for determining in which quadrant the azimuthal angle of arrival lies whereby to overcome ambiguity in the determined angle of arrival.

3. A direction finder as claimed in claim 2, wherein said quadrant determining means comprises an amplitude comparison measurement system.

4. A direction finder as claimed in any of the preceding claims, including a linearly polarising screen disposed around the cusp reflector for linearising the polarisation of an arbitrarily polarised incident far-field signal, prior to incidence on the cusp reflector, whereby to produce said linearly polarised far-field signal.

5. A direction finder as claimed in any one of the preceding claims, wherein the polarimeter comprises a polarisation sensitive mirror disposed between the cusp reflector and its focus at 7r/4 to the zenith for splitting the signal as reflected from the cusp reflector into two orthogonally polarised components, one being reflected by the mirror and the other being transmitted by the mirror, respective means for receiving each said orthogonally polarised component and producing an output signal corresponding thereto, which receiving means output is an r.f. signal or an i.f. signal, and means for determining the angle of arrival 0 from the receiving means outputs from the expression 0 = tan~ .(vtu) where V corresponds to the amplitude of the receiving means output associated with the mirror-reflected component and u corresponds to the amplitude of the receiving means output associated with the mirror-transmitted component.

6. A direction finder as claimed in claim 5 as appendant to claim 3, wherein the receiving means associated with the mirrortransmitted component is disposed at the focal point of the cusp reflector and the receiving means associated with the mirror-reflected component is disposed at a point corresponding to the focal point of the cusp reflector as reflected by the mirror, wherein each receiving means includes two feeds which straddle the respective focal points and are aligned along the respective polarisation vectors, the 7l ambiguity being resolved in use of the direction finder by comparison of the signal amplitudes of the different feeds of the two receivers.

7. An azimuthal direction finder substantially as herein described with reference to the accompanying drawings.

8. An azimuthal direction finder system comprising two direction finders as claimed in claim 4, or claim 5 or claim 6 as appendant to claim 4, stacked one on top of the other with the axis of symmetry of one cusp reflector in common with that of the other cusp reflector, the linearly polarising screens of the two direction finders being orthogonally polarised.

9. A method of azimuthal direction finding comprising causing a linearly-polarised far-field signal, whose azimuthal angle of arrival is to be determined, to be incident on a cusp reflector, as hereinbefore defined, whereby there results a linearly polarised signal at the focus, as hereinbefore defined, of the cusp reflector whose plane of polarisation depends on the azimuthal angle of arrival and employing a polarimeter to determine the plane of polarisation of the signal at the focus of the cusp reflector.

9. A method of azimuthal direction finding comprising causing a linearly-polarised far-field signal, whose azimuthal angle of arrival is to be determined, to be incident on a cusp reflector, as hereinbefore defined, whereby to produce a linearly polarised signal at the focus of the cusp reflector whose plane of polarisation depends on the azimuthal angle of arrival and employing a polarimeter to determine the plane of polarisation of the signal at the focus of the cusp reflector.

10. A method as claimed in claim 9, wherein in order to ensure that the incident far-field signal is linearly polarised the incident far-field signal is caused to be incident on a linearly polarising screen disposed around the cusp reflector prior to incidence on the cusp reflector.

11. A method as claimed in claim 9 or claim 10, wherein said polarimeter employing step comprises causing the signal as reflected by the cusp reflector to be incident on a polarisation sensitive mirror disposed between the cusp reflector and its focus at z/4 to the zenith whereby to split the reflected signal into two orthogonally polarised components, one being reflected by the mirror and the other being transmitted by the mirror, and mixing each component with a respective component of a reference signal whereby to produce respective i.f. signals at respective mixer outputs for said components, and calculating the angle of arrival 0 from said receiver outputs from the expression 0 = tan-1(v/u), where v corresponds to the amplitude of the mixer output associated with the mirrorreflected component and u corresponds to the amplitude of the mixer output associated with the mirror-transmitted component.

12. A method as claimed in claim 11, wherein the receiver associated with the mirror-transmitted component is disposed at the focal point of the cusp reflector and the receiver associated with mirror-reflected component is disposed at a point corresponding to the focal point of the cusp reflector as reflected by the mirror.

13. A method as claimed in claim 12, wherein each receiver includes two feeds which straddle the respective focal points and are aligned along the respective polarisation vectors, and wherein * ambiguity in the calculated angle of arrival is resolved by comparing the signal amplitudes of the different feeds of the two receivers.

14. A method of azimuthal direction finding substantially as herein described with reference to the accompanying drawings.

CLAIMS Amendments to claims filed Superseded claims 1,9 Amended claims:

1. An aximuthal direction finder comprising a cusp reflector, as hereinbefore defined, anda polarimeter, the cusp reflector being such that a linearly polarised far-field signal incident thereon results in a linearly polarised signal at the focus, as hereinbefore defined, of the cusp reflector whose plane of polarisation depends on the azimuthal angle of arrival of the far-field signal, the polarimeter serving in use of the direction finder to determine the plane of polarisation of the signal at the focus of the cusp reflector.