A Missile for Use in a Laser Beam Riding Missile Guidance System

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(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) Int Cl.: 16.08.2017 Bulletin 2017/33 F41G 7/26 (2006.01) F41G 7/22 (2006.01) F41G 7/00 (2006.01) F41G 7/36 (2006.01) (21) Application number: 17154263.2

(22) Date of filing: 01.02.2017

(84) Designated Contracting States: (72) Inventors: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB • Mallon, Paddy GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO Nr Weybridge, Surrey KT152NX (GB) PL PT RO RS SE SI SK SM TR • McConnell, George Designated Extension States: Nr Weybridge, Surrey KT152NX (GB) BA ME Designated Validation States: (74) Representative: Grant, David Michael MA MD Marks & Clerk LLP 90 Long Acre (30) Priority: 15.02.2016 GB 201602648 London WC2E 9RA (GB)

(71) Applicant: Thales Holdings UK Plc Addlestone, Surrey KT15 2NX (GB)

(54) A MISSILE FOR USE IN A LASER BEAM RIDING MISSILE GUIDANCE SYSTEM

(57) A method for aligning a missile with a target in current direction of travel and the direction in which the a laser beam riding missile guidance system, the system target lies from the missile; determining, based on said including a laser transmitter for generating and projecting distance and angular displacement, a new point in the a laser information field towards the target and an optical laser information field with which the missile should be sight for aiming the laser beam towards the target, the aligned to reach the target; and controlling missile guid- method comprising: determining a point in the laser in- ance systems on board the missile to bring the missile formation field with which the missile is currently aligned; into alignment with the new point in the laser information determining a distance of the target from the missile; de- field. termining an angular displacement between the missile’s EP 3 205 973 A1

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Description determining, based on light received from the laser beam at the missile, a point in the laser information FIELD field with which the missile is currently aligned; determining a distance of the target from the missile; [0001] Embodiments described herein relate to a mis- 5 determining an angular displacement between the sile for use in a laser beam riding missile guidance system missile’s current direction of travel and the direction and a method for aligning a missile with a target in a laser in which the target lies from the missile; beam riding missile guidance system. determining, based on said distance and angular displacement, a new point in the laser information field BACKGROUND 10 with which the missile should be aligned to reach the target; and [0002] Beam riding is a known technique for guiding a controlling missile guidance systems on board the missile to its target. In this technique, a laser beam coded missile to bring the missile into alignment with the in azimuth and elevation is projected towards the target, new point in the laser information field. and the missile is provided with light sensors for detecting 15 the beam. Once launched, the missile uses the sensors [0008] In some embodiments, the laser information to correct its position to a specific location within the field is generated by scanning a pulsed laser beam beam, allowing it to travel along the path of the beam across a region of space, the intervals between succes- towards the target. sive laser pulses being varied as the laser scans across [0003] Figure 1 shows an example of a missile 101 20 the region of space. The point in the laser information being guided towards a target 103 by a laser beam riding field with which the missile is currently aligned may be missile guidance system. In this example, the target 103 determined based on the time interval between receiving is an aircraft, but could also be a ground based target successive laser pulses. such as a tank or a sea-based vessel. [0009] In some embodiments, determining the new [0004] The system comprises a laser operable to gen- 25 point in the laser information field comprises identifying erate an intermittently projected laser beam 105. An op- the inter-pulse interval that corresponds to the new point erator uses an optical sight to align the beam with the in the laser information field. target. The laser is scanned in lateral and vertical direc- [0010] In some embodiments, determining the new tions with respect to the direction in which the beam is point in the laser information field comprises determining propagating, so as to form a laser information field 107. 30 the spatial resolution of the laser information field at the The laser information field comprises an array of points missile’s present distance from the laser source, the spa- or grid, in which the light signal at each point is modulated tial resolution defining the lateral distance between points with information that can be used to identify that point’s in the field having different inter-pulse intervals. position within the array. [0011] Insome embodiments,the distanceof the target [0005] The missile 101 is provided with aft mounted 35 from the missile is determined by comparing the distance sensors that can detect the signal encoded in the laser of the target from the optical sight with the distance of beam and so determine the missile’s position with re- the missile from the laser transmitter. spect to the centre of the laser information field. Then, [0012] In some embodiments, the distance of the mis- by use of appropriate guidance mechanisms (e.g. fins), sile from the laser transmitter is determined by use of an the missile can adjust its position so as to remain aligned 40 inertial navigation system onboard the missile. with the centre of the beam. [0013] In some embodiments, data conveying the dis- [0006] As shown in Figure 1, a problem that may arise tance of the target from the optical sight is received by in the beam riding missile guidance system is that an the missile via the laser beam. optical alignment error 109 exists between the centre of [0014] In some embodiments, the laser transmitter is the laser information field and the target aimpoint centre 45 co-located with the missile launcher from which the mis- 111 in the optical sight. The misalignment can lead to a sile is launched. guidance error which can result in the missile missing [0015] According to a second embodiment, there is the target. provided a missile for use in a laser beam riding missile guidance system, the missile comprising: SUMMARY 50 a light sensor for detecting light in a laser information [0007] According to a first embodiment, there is pro- field generated by a laser transmitter and projected vided a method for aligning a missile with a target in a by the laser transmitter towards a target; laser beam riding missile guidance system, the system a target sensor for sensing an angular displacement including a laser transmitter for generating and projecting 55 between the missile’s current direction of travel and a laser information field towards the target and an optical the direction in which the target lies from the missile; sight for aiming the laser beam towards the target, the a guidance processor unit for determining a distance method comprising: of the target from the missile, the guidance processor

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being configured to determine, based on the detect- BRIEF DESCRIPTION OF DRAWINGS ed light, a point in the laser information field with which the missile is currently aligned and to deter- [0024] Embodiments of the invention will now be de- mine, based on said distance and angular displace- scribed by way of example with reference to the accom- ment, a new point in the laser information field with 5 panying drawings in which: which the missile should be aligned to reach the target; Figure 1 shows an example of a missile being guided the missile further comprising a guidance control for towards a target by a laser beam rising missile guid- controlling the flight of the missile, the guidance con- ance system; trol being configured to bring the missile into align- 10 ment with the new point in the laser information field. Figure 2 shows a schematic of a missile in a laser beam riding missile guidance system according to [0016] In some embodiments, the laser information an embodiment; field is generated by scanning a pulsed laser beam across a region of space, the intervals between succes- 15 Figure 3 shows components of the missile shown in sive laser pulses being varied as the laser scans across Figure 2. the region of space. The guidance processor unit may be configured to determine the point in the laser informa- Figure 4 shows an example of a guidance processor tion field with which the missile is currently aligned based unit according to an embodiment; on the time interval that occurs between detecting suc- 20 cessive laser pulses at the light sensor. Figure 5 shows an example of a missile guidance [0017] In some embodiments, when determining the system according to an embodiment; and newpoint in thelaser informationfield, the guidanceproc- essor unit is configured to identify the inter-pulse interval Figure 6 shows an example of steps carried out in that corresponds to the new point in the laser information 25 maintaining the alignment of a missile with a target field. according to an embodiment. [0018] In some embodiments, in determining the new point in the laser information field, the guidance proces- DETAILED DESCRIPTION sor unit is configured to determine the spatial resolution of the laser information field at the missile’s present dis- 30 [0025] Embodiments described herein can help to re- tance from the laser transmitter, the spatial resolution duce or remove a primary source of guidance error in a defining the lateral distance between points in the field Laser beam riding Line of sight (LBR LOS) missile sys- having different inter-pulse intervals. tem, namely the alignment error associated with the cen- [0019] In some embodiments, the detected light en- tre of the laser beam pattern and the target aimpoint in codes data conveying the distance of the target from an 35 the optical sight. By doing so, the system can be used to optical sight of the laser transmitter. The guidance proc- engage smaller targets such as unmanned aerial vehi- essor unit may comprise a range calculator that is con- cles (UAVs) and rockets, artillery and mortars (RAMs). figured to determine the distance of the target from the [0026] An embodiment will now be described with ref- missile by comparing the distance of the target from the erence to Figure 2, which shows a missile 201 being optical sight of the laser transmitter with the distance of 40 launched towards a target 203 which is viewed in the the missile from the laser transmitter. optical sight 205. In this case, the target 203 is an aircraft. [0020] In some embodiments, the missile comprises The missile 201 may be used to carry an explosive device an inertial navigation system for determining the distance for detonation at the target 203. The missile flies along of the missile from the laser transmitter. the centre of the Laser Information Field 207, projected [0021] In some embodiments, the laser transmitter is 45 from the laser transmitter 209. The laser transmitter and co-located with the missile launcher from which the mis- optical sight both form part of the same device that is sile is launched. used to launch the missile 201; that is, the laser trans- [0022] In some embodiments, the missile comprises mitter and optical sight are co-located with the missile one or more guidance fins and the guidance control is launcher 211. configured to control the flight of the missile by adjusting 50 [0027] The laser information field 207, shown in cross the fin(s). section in Figure 2, encodes spatial information that can [0023] According to a third embodiment, there is pro- be used by the missile to determine its position within the vided a laser transmitter for generating a laser informa- laser beam (it will be understood that, although the laser tion field and projecting the laser information field towards information field is shown at a single position along the a target; and a missile according to the second embod- 55 beam length in Figure 2, in practice, the laser information iment. field will be present along the entire length of the beam). In the present embodiment, the laser information field 207 is generated by operating the laser transmitter 209

3 5 EP 3 205 973 A1 6 in a pulsed mode and scanning the beam in the horizontal range from the target range. and vertical directions, perpendicular to its direction of [0033] Figure 3 shows the components of the missile propagation. As the beam is scanned, the intervals be- 201 in more detail. The missile includes one or more laser tween successive laser pulses are varied, such that the optical power receivers 303, for detecting light in the laser intervals detected by the missile vary across the height 5 information field. The optical power receivers are, for ex- and width of the Information field. ample, aft mounted, so as to face towards the laser trans- [0028] As shown in Figure 2, the axes of the optical mitter 209. By detecting the pulse-to-pulse intervals, the sight205 and the lasertransmitter 209 may have an offset laser optical power receiver(s) 303 are able to determine due to mechanical tolerancing and thermal effects. As a the position of the missile with respect to the centre of result, the missile 201 is misaligned with the target. Spe- 10 the laser information field 207. In addition, the laser op- cifically, the target 203 is displaced by an angle θ from tical power receiver(s) may decode information that is the missile velocity vector 213. In this context, the term contained in the laser beam and which specifies the cur- "missile velocity vector" refers to the axis of the direction rent target range. of travel of the missile 201. [0034] At the head of the missile, there is provided a [0029] As will be discussed in more detail below, in 15 sensor 305, which is used to sense the position of the order to realign itself with the target, the missile 201 will target relative to the missile. The sensor may, for exam- perform calculations based on a number of parameters, ple, comprise a visual sensor, an infra-red sensor or a including the "target range", "missile range" and "closing radar sensor. The sensor 305 is used to determine the range". The target range defines the true distance along angular displacement θ between the missile axis and the the line of sight of the target 203 from the optical sight 20 target. For example, the sensor may determine the tar- 205, whilst the missile range defines the distance of the get’s portion in the laser information field by detecting a missile 201 along the line of sight from the laser trans- portion of the field reflected by the target in both the ver- mitter 209. tical and horizontal directions. The sensor will have a [0030] The target range may be measured on one of defined field of view (FOV) and the angular offset θ can several ways known in the art. For example, the target 25 be computed based on the location of the target in that range may be determined using a (separate) laser range field of view. finder provided in the same unit as the missile launcher [0035] The missile 201 also includes an Inertial Navi- / optical sight; alternatively, the target range may be de- gation System (INS) 307, used to determine the missile’s termined by use of a radar based system, again associ- position in space relative to its point of origin (i.e. the ated with the same unit as the missile launcher / optical 30 missile launcher, and correspondingly, the laser trans- sight. Other conventional means for determining the dis- mitter). The INS 307 may, for example, comprise one or tance from the optical sight to the target may also be more accelerometers and / or gyrometers for detecting employed. The target range may be communicated to changes in acceleration which can in turn be used to the missile using the laser transmitter 209; that is, in ad- monitor the change in its position with respect to the origin dition to the spatial information encoded in the inter-pulse 35 over time. The INS 307 is used to determine the missile separation, the laser beam emitted from the laser trans- range and may also determine the missile’s velocity vec- mitter209 may also beused to transmit datato the missile tor. 201 indicating the target range. [0036] The missile range and missile velocity vector, [0031] As discussed above, the optical sight 205 and as determined by the INS 307, are input to a guidance laser transmitter 209 are co-located with one another and 40 processor unit 309. The target range and angular dis- the missile launcher 211; this means that the target range placement, as determined by the optical power receiv- andmissile rangeare bothmeasured from thesame point er(s) 303 and the sensor 305, respectively, are also input of origin (in practice, the nature of these devices means to the guidance processor unit 309. The guidance proc- thatthe optical sight and the outputof thelaser transmitter essor unit 309 is used to calibrate for the misalignment may be offset from one another slightly; however, since 45 between the centre of the laser information field and the the target range will typically be of the order of one or aimpoint on the target from the optical sight. In the more kilometres, the assumption that the missile range present embodiment, the guidance processor unit 309 and target range originate from the same point is valid sends commands to the fin control 311 to control the for the purpose of correcting the missile’s trajectory). position of the missile by suitable adjustment of the mis- [0032] The closing range defines the distance of the 50 sile fins 313. target 203 from the missile 201, as measured along the [0037] The function of the guidance processor unit will current direction of travel of the missile 201. It will be now be explained in more detail with reference to Figure understood that Figure 2 is provided by way of illustration 4. The guidance processor unit includes a clock or timer only and in practice, the misalignment between the target 401 and a look-up table that describes the width of the 203 and the missile 201 (i.e. the angleθ ) will be much 55 beam (more specifically, the area of the laser information smaller than that shown - typically of the order of 1 mRad field) as well as the expected inter-pulse separation at or below.As a result,the closing range can be determined the missile location across the laser information field, as to sufficient accuracy by simply subtracting the missile a functionof time.In Figure 4, the look-up tableis depicted

4 7 EP 3 205 973 A1 8 graphically as a plot 403 of the laser information field information field (and where appropriate the missile ve- pattern dimension as a function of time. Based on the locity vector) together with the new inter-pulse interval clock signal 401, and the look up table 403, the guidance (IPI) as determined by the beam offset calculator 409 in processor unit is able to determine the expected beam the guidance processor unit. The processor 503 in turn resolution i.e. the grid spacing in the laser information 5 generates appropriate commands that are sent to the field at a particular point in time. missile fins, so as to control the motion of the missile [0038] The guidance processor unit also includes a within the laser information field. range calculator 405. The range calculator 405 receives [0042] Figure 6 shows a flow chart summarising the as input the target range and missile range. By subtract- steps performed by the missile components according ing the missile range from the target range, the range 10 to an embodiment. Beginning in steps S601 and S602, calculator is able to determine the closing range (i.e. the the target range and missile range are determined. In distance currently remaining between the missile and the step S603, the sensor on board the missile determines target, as measured along the missile axis). the angular position of the target relative to the missile [0039] The closing range, as determined by the range axis. Then, in step S604, the closing range is calculated. calculator 405 is input to an offset calculator 407, together 15 In step S605, the measurements are used to determine with data indicating the missile’s current position in the the spatial offset between the missile’s current position laser information field, and the angular displacementθ with respect to the laser information field and the position between the target and the missile axis. The data indi- that the missile should adopt in order to remain on course cating the missile’s current position in the laser informa- to hit the target. In step S606, the missile’s guidance sys- tion field includes the inter-pulse separation currently be- 20 tems are used to move the missile to the new position. ing detected by the laser optical power receiver; as de- Steps S601 to S606 then continue to repeat until target scribed above, each point in the laser information field and missile lines of sight are converged. array has an associated inter-pulse separation, which [0043] As with conventional LBR Line of Sight (LOS) can be used to distinguish that point from others in the systems, the missile remains under the control of the array. The offset calculator 407 uses the inputs it receives 25 operator throughout the engagement i.e. the missile can to determine the target offset in terms of inter-pulse in- still be self-destructed by removal of the laser information tervals from the missile’s current position in the laser in- field. formation field. Here, the target offset refers to the lateral [0044] While certain embodiments have been de- / vertical distance within the laser information field that scribed, these embodiments have been presented by the missile must travel in order to remain on course to 30 way of example only and are not intended to limit the hit the target. The target offset, as measured in inter- scope of the invention. Indeed, the novel methods, de- pulse intervals may then be used to compute the distance vices and systems described herein may be embodied between the centre of the laser information field and the in a variety of forms; furthermore, various omissions, sub- location in the laser information field with which the mis- stitutions and changes in the form of the methods and sile should seek to align itself. 35 systems described herein may be made without depart- [0040] The target offset is in turn input to the beam ing from the scope of the invention. The accompanying offset calculator 409. By knowing the beam resolution at claims and their equivalents are intended to cover such the current point in time, the beam offset calculator is forms or modifications as would fall within the scope of able to determine the coordinates in the laser information the invention. field with which the missile should seek to align itself in 40 order to remain on course to hit the target. More specifically, the beam offset calculator 409 determines the in- Claims ter-pulse separation that when detected by the missile will confirm it as being correctly aligned with the target. 1. A method for aligning a missile with a target in a laser [0041] Having determined the position in the laser in- 45 beam riding missile guidance system, the system formation field with which the missile must now align itself including a laser transmitter for generating and pro- in order to stay on course for the target, the guidance jecting a laser information field towards the target processor issues instructions to the missile’s on-board and an optical sight for aiming the laser beam to- guidance systems to align the missile with the new posi- wards the target, the method comprising: tion in the laser information field. For example, the guid- 50 ance processor unit may cause the missile to adjust its determining, based on light received from the fins in such a way as to cause a lateral shift in the missile’s laser beam at the missile, a point in the laser position in space. In this way, the missile calibrates for information field with which the missile is cur- any misalignment between the centre of the laser infor- rently aligned; mation field and the target. Figure 5 shows an example 55 determining a distance of the target from the of the fin control 311 used to adjust the missile’s position missile; in space. The fin control includes a processor 503 that determining an angular displacement between receives as input the missile’s current position in the laser the missile’s current direction of travel and the

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direction in which the target lies from the missile; get; determining, based on said distance and angu- a target sensor for sensing an angular displace- lar displacement, a new point in the laser infor- ment between the missile’s current direction of mation field with which the missile should be travel and the direction in which the target lies aligned to reach the target; and 5 from the missile; controlling missile guidance systems on board a guidance processor unit for determining a dis- the missile to bring the missile into alignment tance of the target from the missile, the guidance with the new point in the laser information field. processor being configured to determine, based on the detected light, a point in the laser infor- 2. A method according to claim 1, wherein: 10 mation field with which the missile is currently aligned and to determine, based on said dis- the laser information field is generated by scan- tance and angular displacement, a new point in ning a pulsed laser beam across a region of the laser information field with which the missile space, the intervals between successive laser should be aligned to reach the target; pulses being varied as the laser scans across 15 the missile further comprising a guidance control the region of space; and wherein for controlling the flight of the missile, the guid- the point in the laser information field with which ance control being configured to bring the mis- the missile is currently aligned is determined sile into alignment with the new point in the laser based on the time interval between receiving information field. successive laser pulses. 20 9. A missile according to claim 8 wherein: 3. A method according to claim 2, wherein determining the new point in the laser information field comprises the laser information field is generated by scan- identifying the inter-pulse interval that corresponds ning a pulsed laser beam across a region of to the new point in the laser information field. 25 space, the intervals between successive laser pulses being varied as the laser scans across 4. A method according to any one of claims 1 to 3, the region of space; and wherein wherein determining the new point in the laser infor- the guidance processor unit is configured to de- mation field comprises determining the spatial res- termine the point in the laser information field olution of the laser information field at the missile’s 30 with which the missile is currently aligned based present distance from the laser source, the spatial on the time interval that occurs between detect- resolution defining the lateral distance between ing successive laser pulses at the light sensor. points in the field having different inter-pulse intervals. 10. A missile according to claim 9, wherein in determin- 35 ing the new point in the laser information field, the 5. A method according to any one of claims 1 to 4, guidance processor unit is configured to identify the wherein the distance of the target from the missile inter-pulse interval that corresponds to the new point is determined by comparing the distance of the target in the laser information field. from the optical sight with the distance of the missile from the laser transmitter. 40 11. A missile according to any one of claims 8 to 10, wherein in determining the new point in the laser 6. A method according to claim 5, wherein the distance information field, the guidance processor unit is con- of the missile from the laser transmitter is determined figured todetermine thespatial resolution of the laser by use of an inertial navigation system onboard the information field at the missile’s present distance missile; and / or wherein data conveying the distance 45 from the laser transmitter, the spatial resolution de- of the target from the optical sight is received by the fining the lateral distance between points in the field missile via the laser beam. having different inter-pulse intervals.

7. A method according to claim 6, wherein the laser 12. A missile according to any one of claims 8 to 11, transmitter is co-located with the missile launcher 50 wherein: from which the missile is launched. the detected light encodes data conveying the 8. A missile for use in a laser beam riding missile guid- distance of the target from an optical sight of the ance system, the missile comprising: laser transmitter; and 55 the guidance processor unit comprises a range a light sensor for detecting light in a laser infor- calculator that is configured to determine the dis- mation field generated by a laser transmitter and tance of the target from the missile by comparing projected by the laser transmitter towards a tar- the distance of the target from the optical sight

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of the laser transmitter with the distance of the missile from the laser transmitter.

13. A missile according to claim 12, wherein the missile comprises an inertial navigation system for deter- 5 mining the distance of the missile from the laser transmitter; and / or wherein the laser transmitter is co-located with the missile launcher from which the missile is launched. 10

14. A missile according to any one of claims 8 to 13, wherein the missile comprises one or more guidance fins and the guidance control is configured to control the flight of the missile by adjusting the fin(s). 15

15. A system comprising:

a laser transmitter for generating a laser information field and projecting the laser information 20 field towards a target; and a missile according to any one of claims 8 to 14.

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