Weapon training systems
Ashford et al X mcw&#39;f v I WEAPON TRAINING SYSTEMS inventors: David William Ashfurd; Sydney Stuart Hartley, both of Farnborough; Bruce John Pike. Church Crookham. all of England Assignee: The Solartron Electronic Group Limited, Farnborough, England Filed: Aug. 8, 1973 Appl. No.: 386,659  
 Foreign Application Priority Data Aug. I8, 1972 United Kingdom 38760/72 US. Cl 35/25; 273/l0l.l Int. Cl. G09b 9/00 Field of Search 35/2S;273/l0i.1, 101.2;  
  References Cited UNITED STATES PATENTS 6/l97l Ormiston 35/25 X a 1 3,877,157 I [451 Apr. 15, 1975 3,70l,206 l0/l972 Ormiston ..35/25 Primary ExaminerJohn H. Wolff Attorney, Agent, or Firm-Roylance, Abrams, Berdo &amp; Kaul lated impact point of a projectile.  
 One or two beams may be employed and measurement of the transit time of the energy allows range corrections and the simulation of a laser ranget&#39;mder.  
 16 Claims, 7 Drawing Figures RFL/NK OR IN 35/25 P&#39;ATENTEDAPR 1 Ems 3.877. 157  
 sum 1 or 3 7 RFL/NK (253 LASfR BEAMS I ATTACA&#39;ER 3 man 5 FIG 2 F163 7x THRESHOLD a PRF, we?? 15 0mm A mcx srsrzm ETC WEAPON TRAINING SYSTEMS This invention relates to weapon training systems and is particularly but not exclusively concerned with systems for simulating the use of rangefinders and the effects of firing guns at targets.  
  Devices which project a narrow beam of light or of infrared energy in the direction of the axis of a gun are well known. Further. our British Pat. No. l.228.l43 has disclosed means of indicating near misses. as well as hits. by scanning a zone in the direction of aim of a weapon at a target with a beam ofenergy and detecting the incidence of the beam on a target.  
  With the need in mind to provide an improved means of simulating the use of a weapon system. the present invention provides improvements in the scanning of a zone in direction of aim with a beam of energy in relation to an aiming direction or to the axis of the bore of a weapon.  
  According to the invention there is provided equipment for use in a weapon simulation system comprising a source associated with the weapon to be simulated and adapted in operation to produce at least one beam of electromagnetic radiation. steering means for varying the direction in which said beam or beams is directed in relation to the direction in which said weapon is directed at a target or to means of aiming the weapon at the target. scan controlling means for causing said steering means to scan with said beam with reference to a datum direction and detecting means for detecting when a beam is incident on the target. characterised in that there may be more than one beam and the scan controlling means co-operating with the steering means scans with the beam or beams separately in two directions substantially orthogonal to one another and discriminator means are provided responsive to signals representative of the direction in which a beam is directed when the detecting means detects that a beam is incident on the target for providing in operation information concerning the direction of the target in relation to the datum direction.  
  Various embodiments of the invention will now be described by way of non-limitative example only by reference to the accompanying drawings in which:  
  FIG. I depicts an attacker and a target tank equipped according to the invention.  
  FIG. 2 shows the target and two beams used for scanning.  
  FIG. 3 is a simplified block diagram of the equipment carried by the target tank.  
  FIG. 4 is a simplified block diagram of the equipment carried by the attacker tank.  
  FIG. 5 shows diagrammatically a source of two beams and means of steering these beams.  
  FIG. 6 shows diagrammatically a source of one beam and means of steering this beam.  
  FIG. 7 is a simplified block diagram of means of producing signals to simulate the use ofa laser rangefinder.  
  Referring now to FIG. I, there is shown an attacking tank I. with a projector 2. mounted on a main gun 3. Simulated firing of the main gun 3, causes a pulsed beam or beams of radiation from a source (not shown in FIG. I in the projector to scan in relation to the axis of the main gun. When a beam impinges on a detector. 4, mounted on target tank. 5. a signal is transmitted by a radio frequency transmitter (not shown in FIG. I) on the target to a receiver (not shown in FIG. I) on the attacking tank by means of an omnidirectional radio frequency link.  
  FIG. 2 shows the zones illuminated by each of two laser beams directed from the projector 2, toward the target 5. A first beam 6, is relatively narrow in elevation and wide in azimuth and is used for scanning in elevation. A second beam 7, is relatively narrow in azimuth but wide in elevation and is used for scanning in azimuth. Both beams are employed in determining the range of the target from the attacker and for transmission of a hit signal from attacker to target. Although a wide variety of modes of scanning are possible with two such beams. in the system described the beams move together but may be pulsed independently.  
  FIG. 3 shows a simplified block diagram of equipment fitted to the target tank comprising detecting means If), incorporating detectors 4 and arranged to give 360 coverage in azimuth and preferably at least 40 coverage in elevation round the target. These detectors are connected via a threshold circuit 11, to a radio frequency transmitter 12. and also to a pulse repetition frequency (PRF) discriminator 13. The PRF discriminator 13, provides outputs to two indicator lamps I4 under attack&#34; and 15 hit.&#34; and the latter also provides an electrical output at point 16.  
  Referring now to FIG. 4, which shows the equipment on the attacking tank. the projector 2, includes a source comprising lasers 20 and 21 connected to pulse generators 22 and 23 respectively. these lasers being placed near the focal planes of collimating lenses 24 and 25 respectively.  
  A first beam of electromagnetic energy is produced by discharge of the pulse generator 22 through laser 20, radiation from this laser being substantially collimated by the lens 24. Similarly a second beam is produced by pulse generator 23 discharging through laser ZI. the radiation therefrom being substantially collimated by the lens 25.  
  The direction of the beams may be varied with respect to the boresight of the main armament 3, by means of azimuth steering means 26, and elevation steering means 27. The elevation steering means 27, is responsive to signals from elevation correction means 29, and also from scan controller 28. Azimuth steering means 26, is responsive to signals from azimuth correction means 30, and from scan controller 28.  
 A receiver 34, is adapted to receive signals transmit ted from the target tank. by transmitter 12, and provides outputs via a mono-stable 35, and a differentiator 36, to scan controller 28. Q  
  The receiver 34. also provides an output to gates 37 and 57, which form part ofa timing and range measurement circuit which also provides means of correlation between returns from receiver 34, associated with pulses of radiation transmitted from lasers 20 and 21 respectively.  
  Pulses for pulse generators 22 and 23 are derived from PRF generator 38. via delay 42, and AND gates 40 and 41. PRF generator 38. is also connected to an input of AND gate 39. which controls flip flop 43. the outputs of which are connected to the other inputs of gates 40 and 41. The output of gates 40 and 41 are also connected to the inputs of OR gates 44. the output of which is connected to the input of delay 45. which in turn is connected at its output to the set terminal of bi-stable 46. The set output of bi-stable 46. provides one input to AND gate 47. which is also connected at its other inputs to a clock 48 and to a control signal. at point 49, from a sequence controller 50. Sequence controller 50 also provides control signals for gate 39. for the set and reset terminals of flip flop 43, and (not shown in the interests of clarity) for other elements of the system and receives signals from these elements.  
  The output of gate 47 is connected to a counter 54 and to the shift line input ofa shift register 55. The output from shift register 55. is connected back to its input through OR gate 56. the other input terminal of which is connected to the output of AND gate 57. lnputs of AND gates 57 and 37 are also connected respectively to the set and reset outputs of flip flop 43. AND gate 37 is also connected at another of its inputs to the output of shift register 55.  
  The output. indicating the accumulated count, of counter 54, is connected through gates (not shown) to range display 58. to a hit signal circuit 59, and to elevation correction circuit 29. A store and averaging circuit 60 is connected to scan controller 28, and receives signals from the scan controller and. via the scan controller, from mono-stable 35, and differentiator 36, to provide means of interpolating between signals representative ofdirections of the beam. The averaging and store circuit 60 also provides an output for a fall of shot display 61 and for hit signal circuit 59 and to scan controller 28. An output of the hit circuit. 59, is connected to mono-stable 68, the output of which is connected to PRF generator 38. The scan controller 28, and averaging and store circuit 60, together constitute a discriminator circuit for providing information concerning the accuracy of aim.  
  A two positon switch 62, allows a member of the tank crew to select the type of ammunition to be fired.&#34; for example armour piercing discarding sabot (APDS) or high explosive squash head (HESH a push button 63, is operated to simulate the act of loading a round. Switches 62 and 63, are connected to ammunition counter 64, which provides a presettable counter for each type of ammunition, each counter being decremented by one each time the appropriate round is selected and loaded.  
  Three push-buttons connected to sequence controller 50, are provided for response to the prepare for action&#34; command, 65, for operating the simulated range finder 66 and for simulated firing of the main armament 67. I  
  Referring now to H0. 5 there is shown diagrammab ically further details of the source of the first: and second beams and means of steering these beams in azimuth and in elevation.  
  A first beam. narrow in elevation. is formed by the GaAs laser diode 20, mounted with its junction lying in the horizontal plane. and the collimating lens 24. A second beam, narrow in azimuth. is formed by the GaAs laser diode 21, mounted with its junction lying in the vertical plane. and the collimating lens 25.  
  Whilst the radiating slits of lasers and 21, may be located in the focal planes of lenses 24 and respectively, we have found it advantageous to arrange for a small displacement of the lasers from the focal planes of these lenses so that each beam is given a small but specific angular divergence in the plane of minimum divergence thereof.  
  Lasers 20 and 21 and lenses 24 and 25 are mounted on a common frame 70 which is pivotable about an axis 74 in relation to a subframe 75. A screw 77, is screw- 4 threadedly engaged in frame 70, and is free to rotate in, but not to move axially with respect to, subframe 75. The frame 70, may be tilted about axis 74 with respect to sub-frame 75, by operation of a geared electric motor 76, which drives the screw 77, through a worm gear 78.  
  The sub-frame 75 may also be rotated about a bearing 79, with respect to a base 80. by means of a screw 81, engaged in a screwed hole of sub-frame 75. and driven by a geared electric motor 82. The base 80. is in operation positively located with respect to the boresight of the main armament 3, on the attacking tank. The geared electric motors 82 and 76, may cor-.veniently be stepping motors and these, with associated control circuits. which may, for example, be described in British Pat. No. l.298.332, comprise the azimuth steering means 26, and elevation steering means 27, respectively.  
  The operation of the system as a whole will now be described by reference to the preceding FIGS. 1 to 5.  
  When the equipment as described in FIGS. 1 to 5 is fitted to an attacking vehicle the first step is to adjust the base 80, with respect to the main gun 3, so that at a specified distance and at a reference setting of beam steering means 26 and 27, the overlap area between the two beams, 6 and 7 shown in FIG. 2, is aligned with the axis of the main gun 3.  
  Prior to a training exercise, ammunition counters in ammunition counter means 64, are set to represent the numbers of each type of round available in the tank for the exercise.  
  At the start of a simulated engagement the loader presses the action&#34; push button 65. Signals from sequence controller 50, to azimuth steering 26, and elevation steering 27, then return both mechanisms to the reference setting where both beams are aligned with the boresight of the main armament. This allows the use of the equipment in the range-finding mode. Signals from the sequence controller 50, also reset to zero counter 54, and shift register 55, and reset other counters, registers and gates to a reference state.  
  The main armament is directed towards the target and when range-finder switch 66 is operated a signal from the sequence controller 50, to gate 39, allows pulses from PRF generator 38, operating at 280 pulses per second to pass to flip flop 43. causing it to set and reset alternately, Signals from flip flop 43 open and close gates 40 and 41 alternately, allowing pulses from PRF generator 38, delayed by delay 42, to pass alternately to pulse generators 22 and 23 which discharge into lasers 20 and 21 respectively which each operate at I40 pulses per second;  
  Gates 40 and 41 are also connected at their outputs to the two inputs of OR gate 44 the output of which is connected to delay 45. which is provided to compensate for fixed delays in the detector 10, threshold 11, and transmitter 12, on the target tank 5 and for delays in the receiver 34 and pulse generators 22 and 23 on the attacking tank 1. The delayed pulse from delay 45 is applied to the set terminal of bi-stable 46. A signal from sequence controller 50. at point 49, is also applied as an input to AND gate 47, allowing pulses from the 6MHz clock 48. to pass to counter 54 and to shift register 55. I  
  The shift register 55, contains 128 bits. The counter 54. makes the transition from full house&#34; to zero on a count of 128 bits and provides an output to reset bistable 46 at this point.  
  Flip flop 43 is initially set so that the first pulse transmitted will be laser 20. Any target vehicle carrying the equipment shown in FIG. 3 and lying within the zone illuminated 6, will transmit a pulse from its transmitter 12. This pulse is received by receiver 34, and passed through AND gate 57 and OR gate 56 into the shift register 55, which is being clocked by clock pulses from gate 47. Thus any returned pulse received by receiver 34 is stored in shift registers 55. in a position corresponding to its range. After counter 54 has accumulated 128 pulses from clock 48, an output signal from said counter resets flip flop 46, thereby closing gate 47. it will be appreciated that the total time elapsed since the pulse transmitted from laser 20, and including the delay introduced at delay 45, is typically less than 30 microseconds. whereas the time interval between successive pulses from the PRF generator 38, is approximately 3.5 milliseconds fora PRF of 280 pps.  
 The next pulse from PRF generator 38, will reset flip would introduce to compensate for [he finite time of flop 43, closing gates 40 and S7, and opening gates 41 and 37. After the delay provided by delay 45, bi-stable 46, will be set and pulses from clock 48. will be counted in counter 54, and will step shift register 55. The pulses appearing on the output of shift register 55, correspond in time delay after the outgoing pulse from laser 21 to the returns and received and stored from the previous pulse from laser 20. These pulses are rotated back into the shift register through gate 56. but are also presented. together with any returns from receiver 34, at AND gate 37. Simultaneous appearance of a pulse at the output of shift register 55 and from receiver 34, opens gate 37, and resets bi-stable 46, closing gate 47, and stopping further accumulation of pulses in counter 54. it will be appreciated that the action of the circuit isto open gate 37. only if pulses are received after exactly the same time interval from the two beams and this therefore indicates the presence of a target vehicle in the overlap area between the two beams 6 and 7 shown in FIG. 2.  
  Further. the choice of frequency of 6 MHz for a clock 48. corresponds to a range interval of metres so that the contents of the counter 54, when bi-stable 46 is&#39;reset. represents the range in 25 metre increments. with a maximum range of three thousand two hundred metres.  
 la order to reduce the effects of target movement, or  
 small misalignment, or atmospheric scintillation. the contents of shift register 55. are recycled through OR gate 56 even when gate 57 is closed. so that all returns from pulses transmitted from consecutive pulses of said laser 20, are stored in shift register 55. for correlation with&#39;individual returns from the beam from laser 21.  
  An output signal from gate 37, also passes to the sequence controller indicating that the range has been determined correctly and is now contained in counter 54. The sequence controller will then remove the enabling voltage from point 49, and will open gates (not shown) connecting counter 54 to the range display 58. and to elevation correction circuit 29.  
  After determination of range to target. the loader selects the required type of ammunition by means of switch 62. and presses push button 63. which decrements the appropriate counter by one and opens a gate to pass a signal representative of the ammunition selected to elevation correction circuit 29. The function of the elevation correction circuit 29 is. with elevation steering means 27. to depress both beams below the boresight of main armament 3, by an amount appropriate to the range. measured in counter 54, and the type of ammunition, signalled from counter 64. Elevation correction means 29, signals a number of pulses to elevation steering means 27, appropriate to the number of steps to be made by the stepping motor 76 (shown in FIG. 5) contained therein. This involves generation of a non-linear function of two variables which may conveniently be made by polynomial expansion means disclosed in French Pat. No. 2,099,446 alternatively the values may be stored in a read only memory which is used to set a counter which is counted down by pulses supplied to elevation steering 27.  
  it is important in training that tank gunner should follow the same procedures and practices which may be required in a real engagement and accordinly some compensation must be made for lead angle&#34; which either the gunner or an automatic fire control system flight of the ammunition and crossing speed in azimuth of a target. Accordingly, there is provided an azimuth correction circuit 30, responsive to a manual input 31, which provides output signals to azimuth steering means 26, enabling stepping motor 82, (FIG. 5) to introduce the appropriate azimuth offset.  
  The direction of the beam after these corrections have been made for elevation and azimuth is the &#34;beam datum direction&#34; and this is the direction of aim adopted as the reference direction for comparison with the actual direction of the target to allow assesment of a hit or a miss.  
  During the loading operation the main armament will have been elevated, either automatically or by the gunner. and the main gun is now ready for tiring&#34; by operation of push button 67. The sequence of operations initiated by switch 67 is. first a scan in azimuth by the beam narrow in azimuth from laser 21, to determine the angular deviation of the target in azimuth from the beam datum direction; second. a scan in elevation by the beam narrow in elevation from laser 20 to determine the angular deviation of the target in elevation with respect to the beam datum direction; third. provided certain conditions are satisfied. a display of these angular deviations on the fall of shot display 61. fourth. determination whether the shot fired &#34;hit&#34; a target of specified size at the measured range; and fifth, if a hit is determined, transmission of a hit signal to the target.  
  in the first step signals from sequence controller 50. reset flip flop 43, and open gate 39, allowing pulses from PRF generator 38, delayed by delay 42. to pass through gate 41 to pulse generater 23 and lire laser 21 at the frequency of PRF generator 38. At the same time the azimuth steering mechanism 26 will be enabled by scan controller 28 so that the direction of the beam moves away to the right from the datum direction. The scanning logic determines. within predetermined limits of scan. the directions of the beam between which the target is illuminated by the beam and returns are received by receiver 34 from transmitter 12.  
  Mono-stable 35. which is set by pulse signals from receiver 34, resets after four milliseconds. i.e. slightly longer than the time interval between the successive pulses. and so provides a continuous signal to scan controller 28. when the target is illuminated by the beam.  
 Differentiator 36 provides a signal to scan controller 28. when the moo-stable resets. indicating that the target is then no longer illuminated by the beam. Signals representative of the directions of the beam when monostable 35 changes state are passed by scan controller 28, to store and averaging means 60. which provides output signals to hit&#34; discriminator 59 and to fall of shot display 61. representative of the mean value of these directions in azimuth and in elevation.  
  If the target is illuminated at the start of the scan. when the beam is in datum direction. the scan will stop as soon as the beam moves beyond the target. the direction of scan will then be reversed so as to determine the left hand edge of the target. If the target is not illuminated by the beam in its datum direction. the beam scans to the right until the target has been illuminated and continues until the beam has passed beyond the target and returns are again lost. if the scan controller reaches the limit ofscan to the right. which may for example be 32 milliradians from the datum position. without illuminating the target. the direction of movement of the scan will be reversed and a similar logic will apply to the processing of signals received when the beam is to the left of the datum position. If however the target is still in the beam when the beam reaches its limit of scan a second signal representative of the scan limit plus a correction will be passed by scan controller to store and averaging unit 60. lf no returns are received from the transmitter on the target during scan between right and left azimuth limits. the result is a &#34;bad miss&#34; and the firing sequence is terminated at this point.  
  If however a measurement has been made of the direction of the target in azimuth with reference to the datum direction the beam is next returned in azimuth to the direction so measured. by signals from sequence controller 50 to scan controller 28 which causes scan controller 28 to enable azimuth steering means 26 to direct the beam in the direction of the target in accordance with signals representative of the mean direction of the target in azimuth received by scan controller from store and averaging means 60. A scan in elevation is then initiated by a signal from sequence controller 50 setting flip flop 43 so as to steer pulses from PRF generator 38 through gate to pulse generator 22 and laser 20. Thereupon scan controller 28 enables elevation steering means 27 which moves the direction of the beam from laser 20 upwards away from the beam datum direction and the scanning and averaging sequence follows a similar logic for determination of direction in elevation as for azimuth.  
  The hit signal circuit 59 receives from store and averaging circuit 60 signals representative of the angular deviations of the target from the beam datum direction and from counter 54 signal representative of the range. Hit detector 59 provides multiplying means and a com; parator (not shown) for multiplying these signals representative of range and angle. to determine whether the product thereof is less than a predetermined value which is representative of the dimensions of the target. Such multiplying means are well known and may be effected by either analogue or digital circuitry and in the latter case may conveniently be incorporated in a function generator of the type disclosed in French Pat. No.  
 2.099.446 which may also be used for other functions in the system. if a hit is indicated. signals from sequence controller to scan controller 28 cause scan controller 28 to enable elevation steering means 27 to direct the beam in the direction of the target in accordance with signals representative of said mean direction in elevation received by scan controller 28 from store and averaging means 60. A signal from sequence controller 50 then opens gate 39 allowing pulses from PRF generator 39 to set and reset flip flop 43, as in the range finding mode. A further signal from said sequence controller enables hit discriminator 59 to pass a hit signal to mono stable 68. which causes PRF generator 38 to operate at a higher frequency than in the range finding and scanning modes at 360 pps for the duration of the delay provided by this monostable. The alternate setting and resetting of flip flop 43 therefore results in pulses being transmitted alternately from lasers 20 and 21. each operating at a PRF of pps. A target receiving pulses from both beams. as shown in FIG. 2. will therefore receive 360 pps at PRF discriminator 13 which contains frequency sensitive circuits distinguishing the hit&#34; frequency of 360 pps from the frequency of 280 pps used in range finding and scanning. This has the effect of illuminating a hit indicator 15 in the target tank and providing a signal at point 16 to disable the attack system of said target tank and if required to ignite pyrotechnics etc. Any other vehicle similarly equipped lying either in the beam 6 or in the beam 7. but not in the area in which both beams overlap. will receive only 180 pps to which its PRF discriminator 13 is not responsive so that only a vehicle receiving both signals will be disabled.  
  lt will be appreciated that the invention lends itself to numerous different embodiments.  
  For exampie. means may be provided to allow beams 6 and 7 to scan simultaneously and independently. rather than sequentially. as&#39;described. In such case each beam may conveniently be modulated at a different PRF and frequency sensitive circuits. connected to receiver 34. responsive separately to the different PRF&#39;s to determine which beam is incident upon the detector at any one moment.  
  Alternatively gating circuits responsive during a predetermined interval following the transmission of a pulse of each respective beam may be used to distinguish returns from one beam from returns from the other beam and so determine which beam or both is incident on the target.  
  Further. the elevation correction circuit. 29, may be omitted altogether. for example. where an automatic fire control system is provided capable of sending the appropriate elevation correction signals to the elevation steering means. 27.  
  Considerable latitude is possible within the scope of the invention in the design of the beam or beams used for scanning. The choice of a Gallium Arsenide laser diode. which typically has a radiating slit width of only a few microns allows very narrow beams to be produced. The direction of such a beam when detector is illuminated by the beam will for many purposes be a sufficiently accurate representation of.the.d.rection of the target. However. we have preferred to work with wider beams in order to minimise the effects of discontinuities both in the laser junction itself and also in the atmosphere between attacker and target; a known divergence can easily be provided by displacing the laser slightly from the focal plane of the lens either toward the lens or away from it. The use of a slightly divergent beam has the further advantage of reducing the apparent variations in the linear. rather than angular. width of the beam when measured at different ranges with a detector having a specified threshold of sensitivity. On the other hand if the beam is caused to diverge substantially there will he a loss of range when using a laser of specified power and any diffusion of the edges of the beam is likely to degrade the accuracy of measurement of the direction of the beam and therefore of the direction of the target with reference to datum direction even if interpolating means are providedd to establish the direction of the target. Although by using masks and an extended radiating source it is possible to work with round or square beams we have found that the best results are obtained when using a beam the minimum divergence of which is less than one-third of the divergence in a plane orthogonal to its plane of minimum divergence. That is to say a beam having an aspect ratio of more than 3 to l. The term angular divergence&#34; used in this context is understood to mean the same as angular beam width commonly used of electromagnetic radiation. being the angle between the directions in which the intensity of the main lobe of radiation has fallen 3dB below the peak intensity thereof.  
  Further we have found that in order to reduce such variations in the apparent size of the beam it is also desirable to provide a feed back circuit to control the power delivered to the laser; a photodiode monitors radiation from the laser during each pusle and controls the energy stored for the next pulse so as to maintain a constant radiant output on successive pulses.  
  Whilst we have preferred to use two separate lasers for scanning in azimuth and in elevation. a single laser may be used; the main requirement is then that the edges of the beam should be well defined and substantially symmetrical so that the averaging and store circuit 60 may effectively determine the direction of the centre of the beam. The beam may have any shape but conveniently will be elongate.  
  FIG. 6 shows an arrangement of a source including a single laser which is employed for both azimuth and elevation scanning. Means are provided for rotating the laser about the centre of the radiating slit so that the plane of minimum divergence may be rotated so as to be co-planar with the plane in which the direction of the beam moves during scanning.  
  Referring to FIG. 6 a laser 90 is mounted in a mount 94 which may rotate about a centre line 95 with respect to a frame 96. The laser 90 is mounted so that the centre of the radiating slit is substantially conincident with centre line 95. Mounting 94 carries a flange 97 one posite directions within an annular housing 108 which is secured to base 102. Each thin prism 106 and I07 is mounted in an annular mounting 109 and 110 respectively. the annular mountings having inclined radial teeth cut on the periphery of one face so as to engage with a bevel gear 111. Rotation of the bevel gear by means of an electric motor (not shown) causes the two thin prisms to rotate in opposite directions. The two prisms 106 and 107 are accurately matched to each other so that in one position the deviations of the beam produced by the two prisms cancel. If the prisms are each rotated through 90 from the positions where the deviations cancel. the deviations add. At intermediate settings the net deviation is the vector sum of deviations produced by each and lies in the plane in which they also give maximum deviation; this is adjusted during the setting up procedure to lie in azimuth. Such an arrangement of counter rotating thin prisms to produce a variable deviation in a specified plane is known as Risleys prisms.  
 In the arrangement of FIG. 6 a scan in elevation may conveniently be made with the plane of the laser juncquadrant of which is provided with gear teeth (not shown) which are engaged with a pinion 98 driven by a geared electric motor 99 which is affixed to the frame 96.  
  The frame 96 also carries a collimating lens 100. the laser 90 being located substantially in the focal plane of the collimating lens. The frame 96 can pivot about an axis 101 with respect to a base 102 but is constrained by a spring 103 to remain in contact with a cam 104 which can be rotated about an axis 105 by &#39;an electric motor (not shown). In operation the base 102 is fixed in relation to the boresight of main weapon 3 so that energising of said electric motor (not shown) t rotate cam 104 provides for steering a beam from laser 90 in elevation.  
 . The beam is steered in azimuth by means of two thin prisms 106 and 107 which are arranged to rotate in op tion lying in azimuth after which motor 99 is energised to rotate the mounting 94 through so that the plane of the laser junction lies in the vertical plane for azimuth scan.  
  The use of a single beam may cause spurious results when used for rangefindng if two targets, equipped according to FIG. 3 are simultaneously illuminated by a beam 6 or a beam 7. Similarly more than one target may receive a hit signal if a single beam illuminates a substantially larger area than that of the target hit.  
  These effects may be avoided by further providing for the laser 90 in mounting 94 to be rotated during the rangefinding mode an during transmission of a hit&#39; signal. for example by providing gear teeth round the periphery of flange 97 to allow continuous rotation when driven by motor 99 and pinion 98. Motor 99 may advantageously be a stepping motor driven by pulses derived from PRF generator 38. so that its rotational speed is locked to that of the PRF generator and motor and gear ratios are selected so that the mounting 94 moves through 90 between consecutive pulses from the PRF generator. i.e.. at l,l20 rpm during the rangefinding mode and at [.440 rpm during the transmission of a hit signal. Returns from alternate pulses are correlated by range. as described above, in the rangetinding mode.  
  A further refinement provides improved simulation of a laser rangefinder which cannot normally be used in training exercises because the powers required for reliable rangefmding entail a risk of eye damage. Such a laser rangefinder is normally equipped with a clock and counting circuits and a display; in operation the counter and clock will receive a start signal from laser corresponding to the outgoing pulse of energy and a stop signal from an optical receiver responsive to energy reflected from a target at which beam from said laser is directed. The invention provides for determination of range as described above followed by generation of start and stop signals for the laser rangefinder. these signals being separated by a time interval corresponding to the range so measured. it is not then necessary for the real rangefinder to transmit; its counting circuits are started and stopped by the start and stop signals generated as described herein so that range may be displayed on the display associated with the real rangefindcr. The equipment required to provide these start and stop signals is shown in FIG. 7 and comprises a bi-stable 120 connected at its set&#39; terminal to the sequence controller 50. and at its reset terminal to a counter 121 which. like counter-S4. provides a full house&#39; on the count of 128 pulses at its input terminal 122. The counter 121 is connected by gates (not shown) to counter 54. so that when a control signal from sequence controller 50. is applied to these gates counter 121 is set to the complement of the count held in counter 54. The set output of bi-stable 1211 provides a &#39;start&#39; signal through differentiator 123. and is also connected to gate 124, the other terminal of which is connected to bMHz clock 48. The output of counter 121 is connected through differentiator 125 to provide a stop signal and to reset terminal of bi-stable 120.  
  The range display 58 in FIG. 4 would not be provided where a laser rangefinder was already installed on the attacking tank and at the end of the rangefinding sequence as described, the range count held in counter 54 would be transferred via gates (not shown) to counter 121 instead of to range display 58. A signal from sequence controller 50. then resets bi-stable 120 providing a start pulse from differentiator 123 and opening gate 124 to admit clock pulses from clock 48 through terminal 122 to counter 12]. The counter 121 then counts from the range count complement to full house. providing an output signal to reset bi-stable 120 and. via differentiator 125 an output stop signal for the laser rangefinder circuits.  
  Equipment according to the invention also lends itself to simulation of weapons fire on a firing range. in which case it is not necessary to transmit a hit signal from the firing point to a target and some simplification of the equipment on the target is possible. Detecting means are mounted near the source, preferably associated with a telescope or other means of optical gain directed in the same direction as the beam or beams. A corner reflector is mounted on the target for returning radiation from the beam to said detecting means.  
  it will be appreciated that although the system described has referred throughout to the use of the equipment with a gun on a tank, effective and relatively inexpensive training in tactics and aiming may be given if the projector is mounted on any suitable vehicle. in such case. the equipment will be used in conjunction with a means. such as a sight. for aiming at a target and the elevating and azimuth corrections of scan will be adjusted to suit the requirements of the sight employed.  
 We claim:  
  1. Equipment for use in a system for simulating the firing of a projectile from a weapon in a trajectory at a target comprising in combination:  
 source means for producing a beam of electromagnetic radiation;  
 support means adapted to be mounted in predetermined relationship with an assumed direction &#39;of orientation of a weapon for establishing a datum direction relative to the simulated direction of initial trajectory of the projectile. and for supporting said source means:  
 steering means on said support means for varying the direction in which said beam is directed relative to said datum direction:  
 scan controlling means for causing said steering means to vary the direction of said beam separately in two substantially orthogonal planes and for pro- I 12 viding signals representative of the direction in which the beam is directed:  
 detecting means for detecting when the beam is incident on the target: and  
 discriminator means responsive to signals representative of the direction in which the beam is directed when the detecting means detects that the beam is incident on the target for providing signals representative of the direction of the target in relation to the datum direction.  
  2. Equipment according to claim 1 wherein the discriminator means further includes means for receiving signals representative of the angular extremes of direction between which limits the beam is incident on the target and for interpolating between said eittremes to provide signals representative of the mean direction of the target in relation to said datum direction.  
  3. Equipment according to claim 1 wherein the beam has a minimum angular divergence in a first one of said orthogonal planes and said minimum angular divergence in said first plane is less than one-third of the angular divergence of said beam in the second one of said orthogonal planes.  
  4. Equipment according to claim 3 wherein said source means comprises a diode laser and collimating means defining a focal planezand wherein said diode laser is displaced from the focal plane of said collimating means to provide a beam the minimum divergence whereof in any plane is more than one-fifth ofa milliradian.  
  5. Equipment according to claim 3 wherein means are provided for rotating the said first plane of minimum divergence of the beam through approximately to make said first plane co-planar with the plane in which the beam is moved by said steering means in either of said two substantially orthogonal planes.  
  6. Equipment according i0 claim 3 adapted to simulate the operation of a pulse rangefinder and including means for rotating said plane of minimum divergence of the beam: and means for generating third and fourth signals respectively separated in time by a time interval equal to twice the transit time of electromagnetic radiation between source and target.  
  7. Equipment according to claim 1 and further comprising a hit signal circuit for receiving information concerning the direction of the target in relation to the datum direction for providing a hit signal if the direction of the target is within defined limits of the datum direction.  
  8. Equipment according to&#39;claim 7 further compris&#39; ing means for generating a first signal related in time to the release of radiation from the source. and a second signal related in time to the incidence of radiation from the source on the detecting means and a time measuring circuit adapted to measure the time between said first and second signals and to provide therefrom a signal representative of the range of the target from the weapon wherein said defined limits are dependent on said range so as to define a hit zone the size of which is substantially constant with respect to range.  
  9. Equipment according to claim 7 wherien the minimum angular divergence of said beam lies in a first plane and is less than one-third of the angular divergence of said beam in a second plane orthogonal to the first plane and the hit signal is transmitted to the target by modulation of the beam and means are provided for rotating said first plane of minimum divergence of the beam during transmission of the hit signal to the target.  
  10. Equipment for use in a system for simulating the firing of a projectile from a weapon in a trajectory at a target comprising in combination:  
 source means for producing first and second beams of electromagnetic radiation;  
 support means adapted to be mounted in predetermined relationship with an assumed direction of orientation of a weapon for establishing a datum direction relative to the simulated direction of ini tial trajectory of the projectile. and for supporting said source means;  
 steering means on said support means for varying the directions in which said beams are directed relative to said datum direction:  
 scan Controlling means for causing said steering means to vary the directions of said beams sepa rately in two substantially orthogonal planes and for providing signals representative of beam direction;  
 detecting means for detecting when a beam is incident on the target: and  
 discriminator means responsive to signals representative of the direction in which a beam is directed when the detecting means detects that a beam is incident on the target for providing signals repre sentative ofthe direction of the target in relation to the datum direction.  
  11. Equipment according to claim further comprisng modulating means for modulating said first and second beams respectively at different frequencies; and  
 frequency sensitive circuits for receiving signals from the detecting means. said circuits being selectively responsive to said modulating frequencies for determining which beam is incident on the target.  
  12. Equipment according to claim 10 and further comprising means for correlating signals associated with the incidence of the first beam on the detecting means with signals associated with the incidence of said second beam on said detecting means according to the time interval between release of radiation in each beam and incidence of each beam on the detecting means.  
  13. Equipment according to claim 12 adapted to simulate the operation ofa pulse range-finder further comprising means for generating third and fourth signals respectively separated in time by a time interval equal to twice the transit time of electro-magnetic radiation between source and target.  
 14. Equipment according to claim 10 wherein a hit signal circuit is provided for receiving signals representative of the direction of the target in relation to the datum direction and for providing a hit&#39; signal when the direction of the target is within defined limits of the datum direction.  
 15. Equipment according to claim l4 and further comprising means for generating a first signal related in time to the release of radiation from the source and a second signal related in time to the incidence of radiation from the source on the detecting means; and  
 time measuring circuit means for measuring the time between said first and second signals and for providing therefrom a signal representative of the range of the target from the weapon; and wherein said defined limits are inversely related to said range so as to define a hit zone the size of which is substantially constant with respect to range.  
 16. Equipment according to claim 14 further comprising means for modulating said first and said second beams by pulses of the same frequency such that pulses from said respective beams are transmitted alternately and frequency sensitive circuit means for receiving signals from said detecting means, said circuit means being selectively responsive to a frequency equal to twice the modulating frequency so that the frequency sensitive circuit means responds selectively only to signals received from both beams.