GB2612033A

GB2612033A - Target tracking using background subtraction

Info

Publication number: GB2612033A
Application number: GB2114909.1A
Authority: GB
Inventors: William Miller Alan
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2021-10-19
Filing date: 2021-10-19
Publication date: 2023-04-26
Anticipated expiration: 2041-10-19
Also published as: GB202114909D0; GB2612033B

Abstract

A system 100 configured to process images for tracking a target 102, comprises an illumination radiation source 110 configured to illuminate the target with illumination radiation, a radiation source 104 configured to direct radiation to disrupt the target 102, a camera 116 and an image processor 118. The camera 116 is configured to capture an illumination image whilst the target 102 is being illuminated with illumination radiation from the illumination radiation source 110, and capture a disruption image whilst the target 102 is not being illuminated with illumination radiation from the illumination radiation source 110, the disruption image showing effects of the radiation disrupting the target 102. The image processor 118 is configured to process the illumination image using the disruption image to reduce or remove the effects of the radiation from the disruption image.

Description

Processing Images for Target Tracking

FIELD

[0001] The present invention relates to an apparatus and method for processing images for tracking a target. More particularly, but not exclusively, the present invention relates to processing images for tracking a target that is being disrupted by a high power laser.

BACKGROUND

[0002] Power that can be delivered by lasers has increased to the point where they could be used to take down aircraft or destroy targets from a distance. A benefit of a laser system over standard projectile weaponry is that there are essentially no consumables; bulky shells do not need to be carried. Instead, as long as the laser has a power supply, it can continue to fire. This means that the "cost per shot" is lower than conventional weaponry. The laser system may be considered to be a directed energy laser.

[0003] The laser system directed at a target can disrupt a target, e.g. damage or destroy the target. For this to be successful, it is important to track the targets so that the laser can be directed at the target and maintained on the target. However, the use of a high power laser can affect the tracking of the target as a hot spot may be created on the target which may alter the image being used for tracking. The hot spot may be considered to be an example of a laser directed energy effect. The laser directed energy effects may include smoke or flames etc. (which may have different spatial and spectral characteristics than the hot spot on the target). It is desirable to provide a laser system that overcomes or mitigates one or more of these problems.

SUMMARY

[0004] In accordance with a first aspect of the invention, there is provided a system configured to process images for tracking a target, the system comprising: an illumination radiation source configured to illuminate the target with illumination radiation; a radiation source configured to direct radiation to disrupt the target; a camera; and an image processor, wherein the camera is configured to: capture an illumination image whilst the target is being illuminated with illumination radiation from the illumination radiation source, and capture a disruption image whilst the target is not being illuminated with illumination radiation from the illumination radiation source, the disruption image showing effects of the radiation disrupting the target; wherein the image processor is configured to process the illumination image using the disruption image to reduce or remove the effects of the radiation from the disruption image.

[0005] This may have an advantage of providing improved tracking of a target.

Some, or (substantially) all, of the laser directed energy effects on the target may be removed, thereby enabling improved tracking and reduced laser power. The system may be relatively compact, simple and cheap to manufacture.

[0006] It will be appreciated that the effects of the radiation disrupting the target may be considered to be some or all the effects of the radiation disrupting the target, i.e. they do not need to be all the effects of the radiation disrupting the target.

[0007] The radiation from the radiation source to disrupt the target may be incident on the target when the target is being illuminated by the illumination radiation source and/or when the target is not being illuminated with the illumination radiation source. The radiation from the radiation source to disrupt the target may be incident on the target when the illumination image is being captured and/or when the disruption image is being captured.

[0008] The image processor may be configured to produce a processed image of the illumination image and the disruption image. An improved image for target tracking may be generated.

[0009] The image processor may be configured to use background subtraction to produce the processed image.

[00010] The image processor may be configured to use image subtraction to subtract effects of the radiation in the disruption image from the illumination image to produce the processed image.

[00011] The system may be configured to track the target using the processed image.

[00012] The illumination radiation source may be a pulsed illumination radiation source configured to produce pulsed radiation.

[00013] The illumination radiation source may be an illumination laser beam transmitter and/or the radiation source is a laser beam transmitter.

[00014] The effects of the radiation (i.e. the radiation to disrupt the target) may comprise resultant radiation emitted from the target that obscures all or part of the target in the disruption image.

[00015] The disruption image may show the effects of the radiation disrupting the target.

[00016] The illumination image may show the target and the effects of the radiation disrupting the target.

[00017] In accordance with a second aspect of the present invention, there is provided a method of tracking a target, the method comprising: disrupting a target with radiation directed from a radiation source; illuminating a target with illumination radiation from an illumination radiation source; capturing an illumination image whilst the target is illuminated with illumination radiation from the illumination radiation source; capturing a disruption image whilst the target is not being illuminated with illumination radiation from the illumination radiation source, the disruption image showing effects of the radiation disrupting the target; and processing the illumination image using the disruption image in an image processor to reduce or remove the effects of the radiation from the disruption image.

[00018] The method may further comprise disrupting the target with radiation directed from the radiation source when the target is being illuminated by the illumination radiation source and/or when the target is not being illuminated with the illumination radiation source. The method may further comprise disrupting the target with radiation directed from the radiation source when the illumination image is being captured and/or when the disruption image is being captured.

[00019] The method may further comprise producing a processed image of the illumination image and the disruption image.

[00020] The method may further comprise subtracting the effects of the radiation in the disruption image from the illumination image to produce the processed image. [00021] The method may further comprise tracking the target using the processed image.

[00022] In accordance with a third aspect of the present invention, there is provided a computer program comprising computer readable instructions configured to cause a processor to carry out a method as described above.

[00023] In accordance with a fourth aspect of the present invention, there is provided a computer readable medium carrying a computer program as described above.

[00024] In accordance with a fifth aspect of the present invention, there is provided a computer system for operating a system comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[00025] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 depicts a system for processing images for tracking a target in accordance with an embodiment of the invention; Figure 2 depicts a diagram showing a process of the removal of the laser directed energy effects in accordance with an embodiment of the invention; Figure 3 depicts a flow chart of steps in removal of the laser directed energy effects in accordance with an embodiment of the invention; Figure 4 depicts a diagram showing steps in parallel of the removal of the laser directed energy effects in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[00026] Figure 1 shows a system 100 for processing images for tracking a target 102 in accordance with an embodiment. Example targets may be rockets, mortars, unmanned aerial vehicles (UAVs) and helicopters. The system 100 comprises a laser beam transmitter 104, i.e. a high power laser beam transmitter which is used to project a laser beam 106 (i.e. high power (high energy) laser beam) to disrupt the target 102. The power may be high enough to cause structural damage of the target 102. The power may be dependent on range, target, defeat mechanism, environmental and pointing accuracy. The disruption of the target 102 may include damage and destruction. That is, structural damage by melting, fire, ablation or softening through heat. More generally, the high power laser beam transmitter 104 may be a radiation source (i.e. high power radiation source) which directs radiation (i.e. high power radiation) at the target 102.

[00027] The high power laser beam transmitter 104 produces a highly directed laser beam 106 such that the laser beam 106 has a relatively high power when it reaches the target 102. This means that the laser beam 106 has sufficient power to disrupt (e.g. damage) the target 102. The laser beam transmitter may include a beam shaping, pointing and stabilisation system. The laser beam 106 may create a hot spot 108 (i.e. laser directed energy effects) on the target 102 when disrupting the target 102.

This may alter an image being used for tracking the target 102. In extreme cases (e.g. a head on air-to-surface missile (ASM)) then the hot spot 108 may cover the majority of the target 102 and thus may obscure the target 102. The high power laser induced hot spot 108 may be considered to seduce the tracker away from a true aimpoint. This seduction issue may be more of an issue for a target 102 of presented dimensions much greater than the high power laser hot spot 108 width. The seduction issue may be less of a concern if the hot spot 108 encompasses all or most of the target image as presented to the system 100, e.g. for the head on missile case. This may be because in placing the high power laser beam 106 on the target 102 and keeping it to the extent that it is heated up to produce the hot spot 108 then it may be considered that an aiming beacon has been generated that may potentially provide more robust tracking.

The laser directed energy effects may be considered to be resultant radiation emitted from the target 102 caused by the laser beam 106 that obscures part, or all, of the target 102 in an image. This may be thought of as thermal emission caused by the laser interaction with the target 102 affecting the quality of the image. Other laser directed energy effects may be smoke or flames. It may be considered that disrupting the target 102 may include a time period when the laser beam 106 is incident on the target 102 and also may include a time period after the laser beam 106 has ceased to be incident on the target 102 but the effects of the laser beam are visible (at least to a camera).

[00028] The system 100 also comprises an illumination laser beam transmitter 110 which is used to illuminate the target 102 with an illumination laser beam 112. It may not matter what the radiation exactly is, e.g. power, intensity etc. as long as there is sufficient intensity to be seen. Thus, in embodiments, different sources of illumination may be used. The illumination laser beam 112 is shown diverging from the illumination laser beam transmitter 110 to cover an area 114 at the target 102. However, it will be appreciated that the area 114 may be larger or smaller than as shown depending on the properties of the illumination laser beam 112 (e.g. focus) and the distance from the illumination laser beam transmitter 110 to the target 102. The area 114 is shown as a rectangle but it will be appreciated that the actual shape does not need to be rectangular. For example, it may be circular, i.e. being consistent with the normal profile of a laser beam. More generally, the illumination laser beam transmitter 110 may be an illumination radiation source which illuminates the target 102 with illumination radiation.

[00029] The illumination laser beam transmitter 110 may be pulsed, i.e. it may produce a pulsed illumination laser beam 112. The illumination laser beam transmitter can produce a laser beam with a relatively high energy suitable to illuminate the target 102. In some embodiments, the illumination laser beam may be continuous (i.e. not pulsed) with its intensity modulated when required.

[00030] The system 100 also includes a camera 116 which is configured to capture images of the target 102. The images taken by the camera 116 may be used to track the target 102, e.g. across the background which may include sky, terrain or sea surface. That is, the camera 116 may be a tracking camera. Connected to the camera 116 is an image processor 118 which is configured to process the images captured by the camera 116. The image processor 118 may be part of the camera 116 (as shown) or may be separate from the camera 116 and connected electrically. It will also be appreciated that the components of the system 100 (i.e. the high power laser beam transmitter 104, the illumination laser beam transmitter 110, and the camera 116) may be separate to each other or may be included together in one or more units.

[00031] The camera 116 is configured to capture images when the target 102 is disrupted by the high power laser beam 106 from the high power laser beam transmitter 104. That is, images of the target 102 are taken during the time period when the high power radiation is incident on the target 102 or at least when the effects of the high power radiation on the target 102 are visible (to at least the camera sensor). Disrupted by the high power laser beam 106 may be considered to be defined as the target 102 being affected to the extent that an image may show effects of the radiation disrupting the target 102 (i.e. effects of the high power laser beam 106 on the target 102). That is, disrupted by the high power laser beam 106 may be considered to be heated up sufficiently for the camera 116 to detect a difference in an image in which the high power laser beam 106 is incident on the target 102 from an image in which the high power laser beam 106 is not incident on the target 102. Disruption may be considered to be defined broadly as e.g. damage or destruction of a target 102 or only heating up of a target by the high power laser beam 106. In some cases, the target 102 may be being disrupted by the high power laser beam 106 but may not, as yet, have its behaviour or function changed. However, it may still be considered to be disrupted in the sense of being heated up by the high power laser 106.

[00032] The camera 116 is configured to capture an image (or images) when the target 102 is illuminated by the illumination laser beam 112 from the illumination laser beam transmitter 110 and configured to capture an image (or images) when the target 102 is not illuminated by the illumination laser beam 112 from the illumination laser beam transmitter 110. That is, an image (or images) of the target 102 is taken during illumination of the target 102 with the illumination radiation and an image (or images) of the target 102 is taken during non-illumination of the target 102. The capturing of the image (or images) of the target 102 may be timed (i.e. synchronized) with the pulse rate of the pulsed illumination laser beam 112. Non-illumination of the target 102 is when the illumination laser beam 112 is not incident on the target 102. Preferably, a single camera (i.e. the camera 116) is used to take both the image (or images) of the target 102 during illumination of the target 102 with the illumination radiation and the image (or images) of the target 102 during non-illumination of the target 102.

[00033] It will be appreciated that a plurality of images may be taken during either or both of illumination with the illumination radiation from the illumination laser beam transmitter 110 and non-illumination of the target 102.

[00034] The images captured by the camera 116 may be used by the image processor 118 to reduce or remove effects of the high power laser beam 106 (i.e. the hot spot) as will be further described. The system 100 may be used to produce data that may be used to improve the tracking of the target 102. This data may be used by the tracking camera 116 or a separate tracking system (not shown) to track the target 102. This data may be in the form of an image or images.

[00035] Figure 2 depicts a diagram showing the processing of the images for an example target 102 taken by the camera 116 to remove the laser directed energy effects in accordance with an embodiment of the invention.

[00036] An image captured when the target 102 is disrupted by the laser beam 106 and illuminated by the illumination laser beam 112 may be referred to as an illumination image 120 as shown. An image captured when the target 102 is disrupted by the laser beam 106 but not illuminated may be referred to as a disruption image 122 (non-illumination image) as shown. As can be seen, the disruption image 122 includes the light from the hot spot 108 whereas the illumination image 120 also shows light 109 reflected from the target 102 over a relatively larger area due to the illumination of the target 102 by the illumination laser beam 112. In other words, the illumination image 120 contains both the effects of the high power laser beam 106 and the illumination of the target 102. The disruption image 122 only contains the effects of the high power laser beam 106 (the directed energy effects).

[00037] Preferably, the illumination image 120 and the disruption image 122 are taken at different times using the same camera 116. This may result in a reduction of optical components etc. when compared to using more than one camera to capture the images, and thus a reduction in complexity. This may mean that the system 100 is relatively compact, simple and cheap to manufacture. In addition, since the same camera 116 is used, the processing of the images 120, 122 may be simplified and relatively fast. Furthermore, since the illumination laser beam 112 may not be on at all times, (i.e. it may be pulsed), then there may be a reduction in energy usage.

[00038] In the camera 116, the captured illumination image 120 and the disruption image 122 are processed in the image processor 118. The image processor 118 may include a subtraction operator 124 which is used to subtract (e.g. electronically subtract) the background (i.e. the disruption image 120) from the foreground (i.e. the illumination image 120). Thus, the image processor 118 may be considered to be carrying out background subtraction. Following the processing (e.g. the background subtraction -there may be more steps prior to the subtraction), a processed image 126 is produced which shows the target without the effects of the laser directed energy from the high power laser beam 106. Thus, the processed image 126 does not have (substantially) any, or at least has a reduced amount, of the directed energy effects.

The processed image 126 may be considered to be a new image of the illumination image 120 which is produced using the disruption image 122.

[00039] With the correct exposure the disruption image 122 only contains the high energy laser effects on the target 102. The illumination image 120 contains both the image of the target 102 and the high energy laser effects. Image subtraction is used on an active image with a passive image to remove the laser energy effects from the high power laser beam 106. In other words, the image processor 118 may use image subtraction to subtract effects of the high power radiation in the disruption image 122 from the illumination image 120 to produce the processed image 126. More generally, the image processor 118 processes the illumination image 120 using the disruption image 122 to reduce or remove effects of the radiation from the disruption image 122.

Figure 2 only shows example single frames of the images but, it will be appreciated, that a plurality of frames may be used. The process may be repeated or continuous. It will be further appreciated that a variety of targets may be used, e.g. air based or ground based targets.

[00040] The processed image 126 may then be used in a tracking system in order to track the target 102, e.g. to allow aiming (or maintaining the aim) of the illumination laser beam transmitter 110, for further disruption of the target 102 by the laser beam 106, or to check on the status of the target 102. The processed image 126 may be used in a standard image tracking system due to the removal of the high energy laser effects of the target 102. It will be appreciated that, in embodiments, only some of the high energy laser effects of the target may be removed. For example, in some cases, thermal emission may be removed but flames and smoke may not. Flames and smoke may be more dynamic than the hot spot and subtraction of temporally spaced illumination/non-illumination images may not remove them depending on the time separation between the images and the dynamics of smoke/flames. In addition, structural damage to the target which is not sufficient to destroy the target may be developing and the image processing may not remove this. This may be beneficial, if the camera has the resolution to see it.

[00041] The use of the system 100 has an advantage of providing improved tracking of a target 102 when a laser (high power laser beam transmitter 104) is being used to disrupt a target 102. That is, an improved image for target tracking is generated. Thus, some, or (substantially) all, of the laser directed energy effects on the target 102 may be removed, thereby enabling improved tracking and reduced laser power. Tracking may be improved because each type of target 102 may be assumed to have one or more preferred aimpoints which the system 100 may be aware of When the target 102 is identified by a set of criteria (e.g. including shape) the aimpoint may be selected from a database. Burning of the target 102 and smoke generation have the potential to degrade the image to the point where seeing its shape and maintaining the aimpoint become difficult or impossible. Since the laser spot (from the high power laser beam transmitter 104) may jitter about on the target, due to poor tracking, it spreads the energy more widely hence requiring more laser power to achieve the same heating. That is, tracking accuracy is improved which, in turn, enables a reduction in the high energy laser power of the high power laser beam transmitter 104 due to the high power density on the target 102.

[00042] Previously, other solutions to the problem of directed energy effects (i.e. a hot spot) affecting the tracking of a target may have involved using spatial filtering to remove the hot spot. However, this can result in tracking errors and/or the inability to track relatively small targets.

[00043] The steps discussed above are summarised in the flow chart of Figure 3. In step 5301, the target is disrupted with high power radiation directed from a high power laser beam (a high power radiation source). In step S302, the target is illuminated with illumination radiation from an illumination laser beam transmitter (an illumination radiation source). In step S303, an illumination image is captured by a camera whilst the target is illuminated with illumination radiation. In step 5304, a disruption image is captured with the camera whilst the target is not illuminated with illumination radiation.

In both steps S303 and 5304, the laser energy effects from the high power laser are detectable by at least a camera. In step S305, the illumination image and the disruption image are processed in an image processor to reduce or remove effects of the high power radiation. This may result in a processed image being created showing the target without the effects of the laser directed energy (i.e. a hot spot).

[00044] It will be appreciated that these method steps may not need to be carried out in this particular order. For example, step S304 could come before step S302. In addition, one or more of the steps may be repeated, e.g. continuously. For example, steps 5302 to 5305 may be repeated continuously. That is, this may be a continuous process during engagement. Normally, the target would be tracked first and then disrupted with the high power radiation. For example, a laser directed energy system may need to acquire a target, identify it, select an aimpoint, track both the whole target and the aimpoint then project the effector beam on to the aimpoint and hold it there with a specific point accuracy. The laser energy effects on the target are then generated.

[00045] It will be appreciated that, in some emboidments, with respect to step S301, the disrupting of the target with high power radiation directed from the high power radiation source may be occurring when one or more of steps S302-S305 are occurring. The method may comprise disrupting the target with high power radiation directed from the high power radiation source when the target is being illuminated by the illumination radiation source and/or when the target is not being illuminated with the illumination radiation source. The method may comprise disrupting the target with high power radiation directed from the high power radiation source when the illumination image is being captured and/or when the disruption image is being captured.

[00046] Figure 4 illustrates an embodiment showing different steps in parallel over time. The line a) indicates whether the high power laser beam 106 is ON or OFF at that particular time. The line b) indicates whether the illumination laser beam 112 is ON or OFF at that particular time. The line c) indicates periods of time when the disruption image 122 may be captured (i.e. YES, if the disruption image 122 may be captured and, NO, if the disruption image 122 may not be captured). The line d) indicates periods of time when the illumination image 120 may be captured (i.e. YES, if the illumination image 120 may be captured and, NO, if the illumination image 120 may not be captured).

[00047] For example, at a), the high power radiation (high power laser beam 106) from the high power laser beam transmitter 104 is shown as being continuously on for the majority of the time. At b), the illumination laser beam 112 from the illumination laser beam transmitter 102 is shown as only on for some periods of time, i.e. initially off, then on, then off and then on again etc. As shown at d), it is in the periods of time when the illumination laser beam transmitter 102 is on that the illumination image 120 may be captured. As shown at c), in the periods of time when the illumination laser beam transmitter 102 is not on, the disruption image 122 may be captured. It will be appreciated that, in other embodiments, the laser beams may be OFF and ON at different times and for different durations. For example, in other embodiments, the high power laser beam transmitter 104 may not need to be on continuously (and/or incident on the target 102 continuously).

[00048] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[00049] While certain embodiments have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel methods, devices and systems described herein may be embodied in a variety of forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the scope of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope of the invention.

Claims

CLAIMS: 1 A system configured to process images for tracking a target, the system comprising: an illumination radiation source configured to illuminate the target with illumination radiation; a radiation source configured to direct radiation to disrupt the target; a camera; and an image processor, wherein the camera is configured to: capture an illumination image whilst the target is being illuminated with illumination radiation from the illumination radiation source, and capture a disruption image whilst the target is not being illuminated with illumination radiation from the illumination radiation source, the disruption image showing effects of the radiation disrupting the target; wherein the image processor is configured to process the illumination image using the disruption image to reduce or remove the effects of the radiation from the disruption image.
2. The system of claim 1, wherein the image processor is configured to produce a processed image of the illumination image using the disruption image.
3. The system of claim 2, wherein the image processor is configured to use background subtraction to produce the processed image.
4. The system of claim 3, wherein the image processor is configured to use image subtraction to subtract effects of the radiation in the disruption image from the illumination image to produce the processed image.
5. The system of any of claims 2 to 4, wherein the system is configured to track the target using the processed image.
6. The system of any preceding claim, wherein the illumination radiation source is a pulsed illumination radiation source configured to produce pulsed radiation.
7. The system of any preceding claim, wherein the illumination radiation source is an illumination laser beam transmitter and/or the radiation source is a laser beam transmitter.
8. The system of any preceding claim, wherein the effects of the radiation comprises resultant radiation emitted from the target that obscures all or part of the target in the disruption image.
9. The system of any preceding claim, wherein the disruption image only shows the effects of the radiation disrupting the target.
10. The system of any preceding claim, wherein the illumination image shows the target and the effects of the radiation disrupting the target.
11. A method of tracking a target, the method comprising: disrupting a target with radiation directed from a radiation source; illuminating a target with illumination radiation from an illumination radiation source; capturing an illumination image whilst the target is illuminated with illumination radiation from the illumination radiation source; capturing a disruption image whilst the target is not being illuminated with illumination radiation from the illumination radiation source, the disruption image showing effects of the radiation disrupting the target; and processing the illumination image using the disruption image in an image processor to reduce or remove the effects of the radiation from the disruption image.
12. The method of claim 11, further comprising producing a processed image of the illumination image using the disruption image.
13. The method of claim 12, further comprising subtracting the effects of the radiation in the disruption image from the illumination image to produce the processed image.
14. The method of claims 12 or 13, further comprising tracking the target using the processed image.
15. A computer program comprising computer readable instructions configured to cause a processor to carry out a method according to any one of claims 11 to 14.
16. A computer readable medium carrying a computer program according to claim 15.
17. A computer system for operating a system comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of claims 11 to 14.