GB2120492A

GB2120492A - Apparatus for sensing the direction of a source of radiation of unknown frequency

Info

Publication number: GB2120492A
Application number: GB08310754A
Authority: GB
Inventors: Stephen Cecil Gratze
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1982-04-22
Filing date: 1983-04-20
Publication date: 1983-11-30
Also published as: GB2120492B

Abstract

The invention uses acousto-optic techniques to find the direction of a radiation source of unknown frequency, e.g. a jammer. The radiation is incident on antenna elements 1 which transmit signals to electro-acoustic transducers 2 which launch acoustic waves, having a direction frequency and power dependent on the direction, frequency and power of the source, into a lithium niobate block 3, where they interact with a beam 5 of white light at 6 to deflect the beam 5. By measuring the amount of deflection with detector arrays 11 and 13 having different spectral responses, the direction of the source is determined. The frequency of the source is determined by comparing in circuit 14, 15 and 16 the outputs of corresponding detectors of the arrays 11 and 13. A level sensor 17 indicates the power of the source. <IMAGE>

Description

SPECIFICATION Apparatus for sensing the direction of a source of radiation of unknown frequency This invention relates to apparatus for sensing the direction of a source of radiation of unknown frequency and constitutes, it is believed, an entirely new principle for direction finding.

The invention provides apparatus for sensing the direction of an external source or reflector of radiation of unknown frequency comprising: an array of transducers constructed and arranged to launch mechanical waves into a medium in a direction dependent on the direction of the external source or reflector relative to the apparatus, the mechanical waves having a wavelength depending on the frequency of the said radiation; a local source of electromagnetic radiation arranged to direct a beam of radiation containing a plurality of frequencies into a path of the mechanical waves in the medium, the medium being such that one frequency component, depending on the wavelength of the mechanical waves of the said beam, is deflected by effects of the mechanical waves through an angle dependent on their direction; and detecting means for receiving the deflected radiation from the local source and detecting the said angle.

It is preferable that further detecting means is included for detecting the frequency of the radiation deflected through the said angle, thereby producing an indication of the frequency of the radiation from the external source or reflector.

One way in which the invention may be performed will now be described by way of example, with reference to the accompanying drawing in which: Figure 1 shows schematically apparatus constructed in accordance with the invention, and Figure 2 is a diagram illustrating the operation of part of the apparatus.

An array of antenna elements 1 receives radiation from an external R.F. source (not illustrated) so that the relative phases of received signals on lines 1A are determined by the direction of the source. The frequency of the radiation from the source is not known.

The signals on lines 1 A are passed to a receiver 1 B which reduces the frequency of each signal by equal amounts to produce a UHF frequency and amplifies it whilst maintaining a similar phase relationship on its output lines 1C as on its input lines 1 A. The lines 1 C are connected to respective electro acoustic transducers 2 of an array of transducers attached to a block 3 of lithium niobate. When a signal is received by a transducer 2 an acoustic bulk wave is launched into the block 3. The relative phases of the signals received by the elements of antenna 1 thus determine the relative phases of the acoustic waves and hence the resultant direction in which most of the acoustic energy is propagated. Also, the frequency of the signals received at the antenna elements determines the wavelength of the acoustic signals in the block 3.

A light source 4 with a broadband frequency spectrum emits a collimated beam 5 which is incident on the block 3 at a fixed angle. Because the lithium niobate has acousto-optic properties, the beam 5 interacts with the acoustic waves at a region 6 in the block 3. A specific wavelength of the beam 5 is deflected by Bragg interaction through an amount depending on the direction of the acoustic waves and on their frequency, since it undergoes Bragg diffraction. An acoustic absorber 7 is located at the edge of the block 3 opposite the transducers 2.

The deflected beam, shown at 8, emerges from the block 3 and is incident upon a beam splitter 9 which divides the beam into two parts 8A and 8B.

The first part 8A passes through a lens 10 and is focussed on to a first detector array 1 The other beam part 8B is similarly incident on a lens 12 and focussed at a second detector array 1 3. Each detector of the array 11 has a response to different wavelengths as shown by the continuous line on Figure 2 and each detector of the array 1 3 has a response as shown by the broken line. The range of wavelengths produced by the source is A, to 2 as shown in Figure 2.

The direction of the external R.F. source is indicated by the point on either of the detector arrays 11 and 13 where the deflected beam is focussed, since the angle of deflection depends on the direction of the acoustic waves in the block 3.

The direction of the source or a reflector may thus be determined, even though the frequency of the radiation they emit is unkown.

The frequency of the R.F. radiation emitted by a source or reflector in any given direction can be determined by comparing the outputs of the two detectors of arrays 11 and 1 3 which correspond to that given direction. This is because the wavelength A of the deflected beam depends on the frequency of the R.F. source or reflector. In the illustrated embodiment, the comparison of each pair of outputs is effected by a circuit formed by components 14, 15, 1 6 and 1 7 as shown in Figure 1. A number of identical circuits are included, one for each pair of detector outputs though only one such circuit is illustrated.

The circuit 14, 15, 1 6 and 1 7 performs the comparison by adding the detector outputs at 14, calculating their difference at 1 5 and calculating at 1 6 the ratio of the sum and difference signals.

The result of the comparison is a signal at 18 whose magnitude represents the frequency of radiation received from a particular direction.

Similar outputs are obtained from the other identical circuits previously referred to for other directions.

A level sensor 1 7 indicates whether the signal received at 1 is sufficiently strong for it to be assumed to emanate from a target and if so, give an indication of the power of the signal.

In alternative embodiments, surface acoustic waves or surface skimming bulk waves may be used instead of bulk waves.

Apparatus employing the invention may be used to detect the direction and frequency, whici could be varying, of radiation from a jammer. In such an application, the resultant information could then be used to reduce interference of a radar, for example, by altering its directional sensitivity and/or by using the information to control a filter, cutting out a frequency at which the jammer is operating.

Claims

1. Apparatus for sensing the direction of an external source or reflector of radiation of unknown frequency comprising: an array of transducers constructed and arranged to launch mechanical waves into a medium in a direction dependent on the direction of the external source or reflector relative to the apparatus, the mechanical waves having a wavelength depending on the frequency of the said radiation; a local source of electromagnetic radiation arranged to direct a beam of radiation containing a plurality of frequencies into a path of the mechanical waves in the medium, the medium being such that one frequency component, depending on the wavelength of the mechanical waves, of the said beam, is deflected by effects of the mechanical waves through an angle dependent on their direction; and detecting means for receiving the deflected radiation from the local source and detecting the said angle.

2. Apparatus as claimed in claim 1 and including further detecting means for detecting the frequency of the radiation deflected through the said angle thereby producing an indication of the frequency of the radiation from the external source or reflector.

3. Apparatus according to claim 1 or 2 and including a level sensor for sensing the output level of the detecting means.

4. Apparatus substantially as illustrated in and described with reference to the accompanying drawing.