GB2347572A

GB2347572A - Alignment system using reference source

Info

Publication number: GB2347572A
Application number: GB9905052A
Authority: GB
Inventors: Roger Keith Titmuss
Original assignee: Marconi Electronic Systems Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1999-03-05
Filing date: 1999-03-05
Publication date: 2000-09-06
Also published as: GB9905052D0

Abstract

The imaging means is sighted on the reference source (30), for example a carbon dioxide infra-red laser, whose intensity and size is modified in order to align an imaging system(11). The system preferably uses a thermal imaging camera. A filter (preferably an infra red band pass filter) is used to attenuate energy emitted by the background scene. The transmissivity of the filter may be low over the bandwidth of the energy emitted by the reference source and the scene, so as to reduce the apparent temperature to below the noise level of the system. The system may include a diverging mirror or lens (31).

Description

Improvements in or Relating to Alignment The invention relates to an alignment system and a method of aligning a receiver with a reference.

In a tracking system it is often necessary to collimate the pointing direction of at least a pair of tracking sensors. When one of the tracking sensors is a thermal imaging camera, collimation is achieved by alignment of the pointing direction of the camera with a distant target, which may be mounted on a suitable platform such as a tower. Once the pointing direction of the camera has been determined the point direction or directions of the other tracking sensor or sensors can then be collimated with the pointing direction of the camera.

The present invention is concerned with alignment of a tracking sensor with a distant target and not the collimation of the pointing direction of a plurality of tracking sensors. A problem with aligning the tracking sensor with the target is that the tower and land geometries can produce clutter since solar energy may heat the tower and land geometries and machinary within the vicinity of either the tracking sensor or the target may produce heat.

It is an object of the invention to obviate or mitigate problems associated with aligning a receiver, such as a tracking sensor, with a reference, such as a distant target.

According to a first aspect of the invention an alignment system comprises a reference arranged to emit energy, a receiver, to be aligned with the reference, which is arranged to receive energy emitted by the reference and a background scene containing the reference, and a filter arranged between the receiver and the reference to attenuate energy emitted from the background scene, wherein the energy emitted by the reference is of such intensity that the receiver can distinguish it from energy emitted from the background scene and the receiver can be sighted on the reference.

In this manner, the energy emitted by the background scene is attenuated by the filter such that the receiver perceives a reduction in energy emitted within its field of view and the energy emitted by the reference is of such intensity that the receiver can distinguish it from energy emitted by the background scene thereby allowing alignment of the receiver with the reference.

The reference may be arranged to emit energy in the infrared band and may be a carbon dioxide type laser.

Preferably, the filter may be arranged to attenuate energy in the infrared band.

Alternatively, the filter may be a band pass filter arranged to allow energy emitted by the reference to pass and to attenuate energy emitted by the background scene. In either of these embodiments, infrared energy emitted by the background scene through, for example, solar energy heating objects in the background scene or heat generated by machinary, is reduced. Effectively this results in a reduction in the environmental temperature as perceived by the receiver thereby allowing a reference producing energy at a greater intensity to that produced by the background scene to be distinguished by the receiver with a reduction in clutter from the background scene.

The energy emitted by the reference may be arranged to be adjustable in intensity to allow variation in the reference size. In this manner, the size of the reference, as perceived by the receiver, can be altered to allow the receiver to track a reference which appears to be moving either towards or away from the receiver.

Preferably, the reference may comprise a diverging optical arrangement positioned between the reference and the receiver. In this manner the energy emitted by the reference can be arranged to cover a greater area such that the receiver is more likely to detect the emitted energy. The diverging optical arrangement may be a diverging lens or the diverging optical arrangement may be a diverging mirror, for example a concave or convex mirror.

The receiver may be a thermal imaging camera.

According to a second aspect of the invention a method of aligning a receiver with a reference comprises arranging the reference to emit energy, arranging the receiver to receive energy emitted by the reference and a background scene containing the reference, and filtering the energy to be received by the receiver so as to attenuate energy emitted by the background scene and to allow energy emitted by the reference to pass. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates the present invention; Figure 2 illustrates the range of frequencies emitted by a background scene and the boundaries of the energy emitted by a reference; Figure 3 schematically illustrates a further embodiment of the present invention, and Figure 4 illustrates an alternative arrangement to that shown in Figure 3.

In Figure 1, an alignment system 10 comprises a reference 11, which may be a carbon dioxide type laser, arranged to emit energy in the form of a beam 12 which is directed towards a receiver 13, which may be a thermal imaging camera.

Disposed between the reference 11 and receiver 13 is a filter 14 which is arranged to attenuate energy emitted from the reference 11 and a background scene 15 containing the reference 11. The background scene 15 may contain a variety of land geometries and/or machinary within the vicinity of either the reference 11 or the receiver 13. The background scene 15 may also include any mounting arrangement, such as a tower, on which the reference 11 is situated. The land geometries or machinary may be heated by solar energy or may produce their own source of heat which will be detected by the receiver 13 and cause clutter of the energy emitted from the reference 11 thereby presenting a number of different energy sources to the receiver 13.

The filter 14 is formed from a transparent material, for example plastic, glass or germanium. The transmissivity of the filter 14 is selected or constructed with a suitable coating to be low over the bandwidth of the energy emitted from both the reference 11 and background scene 15. This has the effect of reducing the temperature of the reference 11 and background 15 as perceived by the receiver 13. If the perceived temperature of the background scene 15 is reduced to the below the noise level of the alignment system 10 it will not be detectable by the receiver 13 leaving the energy emitted by the reference 11 which has more intensity than that emitted by the background scene 15 as the sole energy source presented to the receiver 13.

In Figure 2, the energy emitted by a background scene 15 has a bandwidth between 8 and 12 microns, this is illustrated as a bandwidth limited by points 16 and 17 along the abscissa axis 18. The bandwidth of energy emitted by the reference 11 is centred about point 19 along the abscissa axis 18, which for a carbon dioxide type laser is typically 10.6 microns.

The intensity of the radiation emitted from the background scene 15 and the reference 11 are given along the ordinate axis 20. As it can be seen, the radiation emitted by the background scene 15 approaches the intensity of the radiation emitted by the reference 11.

The filter 14 has the effect of reducing the radiation from both the background scene 15 and reference 11, such that the radiation emitted by the background scene 15 drops below the noise level, indicated by chained line 21, of the alignment system 10. This has the effect of reducing the energy emitted by the background scene 15 to a level which is not detectable by the alignment system 10 thereby producing a relatively uncluttered background scene 15 against which the reference 11 is detectable as the sole source of emitted energy.

It is to be understood that the filter 14 can either be arranged to attenuate the energy emitted by the background scene 15 and the reference 11, as described above, or the filter 14 can be arranged as a band pass filter to allow the energy emitted by the reference 11 to pass, that is energy in the region of 10.6 microns, while attenuating energy emitted by the background scene 15.

The receiver 13 is typically situated some 200 to 700 metres from the reference 11 such that it is difficult for the receiver to be aligned with the reference 11. In this case a diverging optical arrangement can be positioned between the reference 11 and receiver 13 so as to ensure that a beam 12 can be seen by the receiver 13. Accordingly, in Figure 3, in which like references have been used to indicate integers similar as those given in Figure 1, a vehicle 30, in this case a marine vessel, carrying a receiver, not shown, can be located some 700 metres from a reference 11 and a diverging lens 31 located between the reference 11 and the receiver located on the vehicle 30. In this manner, a beam 12 emitted by the reference 11 is spread in an arc, as indicated by dotted lines 32 and 33. Therefore, the diverging lens 31 will spread the beam 12 over a diameter, for example 100 metres, in which the whole vehicle 30 is located. Alternatively, in Figure 4, in which like references have been used to indicate similar integers to those illustrated in Figures 1 and 3, diverging mirror such as a convex mirror 34 can be located between a receiver, not shown, located on a vehicle 30 and a reference 11. Such that a beam 12 emitted by the reference 11 can be spread in an arc, as indicated by dotted lines 32 and 33. This ensures that a receiver located on a vehicle 30 is capable of detecting energy emitted by the reference 11 as the vehicle 30 can be more readily located in the area illuminated by the beam 12. It will be understood that the convex mirror 34 could be replaced with a suitably constructed concave mirror.

Claims

CLAIMS: 1. An alignment system, comprising a reference arranged to emit energy, a receiver, to be aligned with the reference, which is arranged to receive energy emitted by the reference and a background scene containing the reference, and a filter arranged between the receiver and the reference to attenuate energy emitted from the background scene, wherein the energy emitted by the reference is of such intensity that the receiver can distinguish it from energy emitted from the background scene and the receiver can be sighted on the reference.
2. An alignment system, as in Claim 1, wherein the reference is arranged to emit energy in the infrared band.
3. An alignment system, as in Claims 1 or 2, wherein the reference is a carbon dioxide type laser.
4. An alignment system, as in any preceding claim, wherein the filter is arranged to attenuate energy in the infrared band.
5. An alignment system, as in Claims 1 to 3, wherein the filter is a band pass filter arranged to allow energy emitted by the reference to pass and to attenuate energy emitted by the background scene.
6. An alignment system, as in any preceding claim, wherein the energy emitted by the reference is arranged to be adjustable in intensity to allow variation in the reference size.
7. An alignment system, as in any preceding claim, wherein the reference comprises a diverging optical arrangement positioned between the reference and the receiver.
8. An alignment system, as in Claim 7, wherein the diverging optical arrangement is a diverging lens.
9. An alignment system, as in Claim 7, wherein the diverging optical arrangement is a diverging mirror.
10. An alignment system, as in any preceding claim, wherein the receiver is a thermal imaging camera.
11. An alignment system substantially as illustrated in and/or described with reference to the accompanying drawings.
12. A method of aligning a receiver with a reference, comprising arranging the reference to emit energy, arranging the receiver to receive energy emitted by the reference and a background scene containing the reference, and filtering the energy to be received by the receiver so as to attenuate energy emitted by the background scene and to allow energy emitted by the reference to pass.
13. A method of aligning a receiver with a reference as illustrated in and/or described with reference to the accompanying drawings.