GB2459913A - System for guiding projectiles by a directing beam coded in Cartesian coordinates - Google Patents

Abstract

In this system the fire-control station) 1 comprises : - means (40 to 45 Fig 4) for tracking a target and defining a line of sight LV ; - means (33 to 39 Fig 4) for scanning with a laser beam (3) a guidance cone 5 whose axis of symmetry AS is locked to said line of sight LV , said cone 5, being made up of elemental cells (e.g. 9, 9' Fig 2) successively illuminated by said beam 3, the beam being deflected by two acousto-optic deflectors (37, 37' Fig 4) in two orthogonal planes; - means for modulating said beam 3 according to a binary sequence (Fig 3) representing said Cartesian coordinates of the cell which is illuminated; and the projectile includes : - means for receiving said laser beam 3 , for demodulating it and for decoding the coordinates; - means for determining a trajectory correction such that said projectile comes closer to the axis of symmetry AS of said guidance cone 5. The beam may carry a message. The effective beam size is controllable in dependence upon range (Fig 5).

Description

System for guiding projectiles by a directing beam coded in Cartesian coordinates The present invention relates to a system for guiding pro-jectiles by a directing beam coded in Cartesian coordinates.

The projectiles may be missiles or shells fitted with a tra-jectory correcting device.

Guidance by a directing beam (commonly referred to as beam-rider guidance) is a method which allows any number of projectiles to be guided very accurately to a target7g to determine the position of each projectile, simply by deter- mining a line of sight passing through the position to be rea-ched which is either the current position of the target or a future position which is determined from the characteristics of its trajectory. Target tracking means determine this line of sight.

The fire-control station transmits an infrared laser beam which illuminates a portion of space called a guidance cone.

The position of the axis of symmetry of the guidance cone is locked to the line of sight. The guidance cone is made up of elemental cells which are individually identified by a coding method. The projectile includes means for receiving the direc- ting beam and determining in which cell the projectile is lo- cated. Computing means determine a correction of the trajec-tory of the projectile such that it comes closer to the axis of symmetry of the guidance cone. This correction is performed by means of control surfaces or of gas jets.

A beam-rider guidance system has a high guidance accuracy and a very good immunity to countermeasures since the means for receiving the directing beam, which are in general a lens and an opticaldetector, are directed in the direction of the fire-control station and not in the direction of the target.

There are known two main types of beam-rider guidance sys-tems. A firs.t type, the so-called space-coding type, includes means for transmitting a laser directing beam illuminating simultaneously all the elemental cells of' the guidance cone, each cell being identified by modulating locally the amplitude of the Laser beam by binary codes different for each cell.

The projectile determines in which cell it is located by re- cognizing the binary code modulating the portion or the direc-ting beam which is received by the projectile. Such a system is described in "Optical Command and Beamrider Missile Guidan-ce", R.L. Sitton, SPIE, vol. 317, pp. 358-364.

The disadvantage of' this type of guidance system is to require the transmission of a beam of relatively high power to be able to illuminate simultaneously all the cells of the guidance cone while achieving a given range. The smoke present in the battlefield, as well as the flame and the smoke from the projectile when it is a missile, attenuate signilicantly the transmission of the laser beam between the fire-control station and the means for receiving this beam. To achieve a given range, it is therefore necessary to transmit a beam ha-ving a spatial power density sufficiently high to compensate for this attenuation.

There is known a second type of beam-rider guidance sys- tem which is referred to as the time-coding system or the scan- ning system. The firing station transmits a laser beam scan-ning a guidance cone by illuminating successively each of the elemental cells. The paper "An improved acousto-optic laser scanner guidance system", by Michael Higgins, Gerald Titmuss, John Evans, Richard Martyn, British Aerospace, UK, describes a system in which the fire-control station includes a conti-nuous-wave (CW) laser and means For scanning the guidance cone along a set of horizontal lines, then along a set of vertical lines, each line being scanned twice prior to scan the next line. For example, each horizontal line is scanned from the left to the right, then from the right to the left, which al-lows the projectile to receive twice the laser beam during the period of horizontal scanning of the whole guidance cone.

The time interval between the two receptions allows to deter-mine the position of the projectile along a horizontal axis.

Similarly, the projectile receives twice the laser beam during the period of vertical scanning of the whole guidance cone. A measurement of the time interval between these two receptions allows the position of the projectile along a C erit ered vertical axis to be determined, Each elemental cell is/at the intersection of a line of vertical scanning and a line of ho-rizontal scanning. The width and the length of each cell are such that the diameter of the laser beam completely covers a cell and slightly overlaps adjacent cells.

This type of guidance system permits all the power of the beam to be concentrated in an elemental cell at a"me.

This concentration of the full power of the laser beam in a single cell provides a significant improvement of the signal-to-noise ratio, which allows the power of the laser for a given range to be reduced.

A purpose of the present invention is to propose a guidan-ce system allowing the power of the laser for a given range to be further reduced, to increase the guidance accuracy, arid to transmit messages to the missile, for example to inform it about the evolution of the trajectory of the target.

An object of the present invention is accordingly a gui- dance system including essentially two acousto-aptic deflec- tors to deflect a laser beam in two orthogonal planes inter-secting each other along the axis of symmetry of the guidance cone, but also to modulate the amplitude of this beam in order to transmit two Cartesian coordinates and a message.

According to the present invention, there is provided a system, for guiding projectiles by a directing beam coded in Cartesian coordinates, the fire-control station including -means for tracking a target and defining a line of sight; -means for scanning with the beam transmitted by a laser a portion of space referred to as a guidance cone, having an axis of symmetry whose position is locked to the line of sight, and this guidance cone being made up of elemental cells suc-cessively illuminated by the beam, said projectile including -means for receiving the laser beam, determining in which cell the projectile is located and deriving from it a correc-tion of the trajectory of the projectile such that it comes closer to the axis of symmetry of the guidance cone; -means for performing this trajectory correction; wherein said scanning means include -means for deflecting the laser beam along two directions orthogonal to each other; -means for modulating the amplitude of the laser beam trans-mitted in each elemental cell according to a separate binary sequence for each cell; and in that said means for receiving the laser beam inclu-de means for demodulating said laser beam and decoding said binary sequence.

The present invention will be better understood and other features and advantages will become apparent from the folio-wing detailed description of a preferred embodiment given as a non-limitative example with reference to the accompanying drawings, in which -Figure 1 shows schematically the fire-control station, the projectile, the target and the guidance cone; -Figure 2 shows schematically the intersection of the guidan-ce cone with a plane perpendicular to its axis of symmetry; -Figure 3 is a timing diagram illustrating the coding of the laser beam for a pair of cells of the guidance beam; -Figure 4 is a block diagram of an embodiment of the system according to the present invention; and -Figures 5 and 6 illustrate the operation of this embodiment.

Referring to Figure 1, a fire-control station 1 generates a guidance cone 5 for guiding a missile 4 in the direction of a target 2. The guidance cone 5 has an axis of symmetry AS which is locked in position to remain coincident with a line of sight LV which extends from the fire-control station 1 to the target 2. This line of sight is defined by tracking means incorporated into the fire-control station 1. At a given time, the fire-control station 1 transmits a laser beam 3 that illuminates an elemental cell of the guidance cone 5. This elemental cell has the shape of a cylinder or of a cone with a small aperture angle.

To identify each elemental cell of the guidance cone, let us consider the intersection of the guidance cone with a plane xoy perpendicular to the axis of symmetry AS and which is gone through by the missile 4 at the time of interest. The inter-section of the guidance cone with the plane xoy is a surface 7 approximately circular, and the elemental cell which is il-luminated at the time of interest by the laser beam 3 has an intersection with the plane xoy, which is theoretically a square surface 6. The position of the surface 6 is identified with respect to coordinate axes ox and oy which are orthonor-med and which intersect at the point o located on the axis of symmetry AS. The axis ox is horizontal, while the axis oy is contained in the vertical plane. The cell whose intersec-tionis the surface 6 is identified by means of the Cartesian c.ooi'dinates x,y in this coordinate system xoy.

In practice, the plane xoy and its coordinate system move while accompanying the missile 4. For the given cell to retain the same coordinate values x,y, it is necessary to put, as unit distance with respect to the axis of symmetry AS, a unit varying as the missile 4 flies away from the fire-control sta- tion 1. For example, the unit is put equal to 1/16 of the ra-dius of the intersection surface 7 of the guidance cone with the plane xoy.

Figure 2 shows the intersection surface 7 and illustrates the operation of the system by showing the position of the elemental cells which are successively illuminated in the gui-dance cone 5. The cells are represented by their intersections with the plane xoy. Scanning is performed successively along 16 columns, for example in the order of increasing y by steps equal to 1, and in the order of increasing x by steps equal to 1.

The cells have theoretically an intersection surface which is square. In fact, the illumination has a circular cross sec-tion with a Gaussian distribution. Consequently, the cells have no distinct frontiers but, on the contrary, they partial-ly overlap each other. The cells being regularly distributed along the directions ox and oy, they overlap in the same man-ner whichever their position. The energy transmitted into the guidance cone is thus uniformly distributed, and the guidance accuracy is uniform in the whole guidance cone.

A remarkable feature of the guidance cone scanning is that each column is scanned virtually simultaneously with the syrn-metrical column with respect to o. This feature results from the coding method used for identifying each cell of the gui-dance cone by modulating the laser beam. This coding method consists essentially in transmitting a separate binary sequen-ce for each cell and in transmitting a complementary binary sequence for the symmetrical cell with respect to the axis of symmetry AS.

The laser beam is modulated to transmit, in the form of a binary sequence, two coordinates x, y, identifying the posi-tion of a cell in the guidance cone.

In a a preferred embodiment, the laser is a continuous- wave (CW) laser. Each bit of the binary sequence is transmit-ted by modulating the amplitude of the laser beam.

Various conventional modulation methods can be used. A modulation consisting in an illumination or in an absence of illumination for the values 1 and 0, respectively, of the bi-nary sequence would have the disadvantage not to ensure a zero average component. A better modulation method may consist, for example, in representing each bit of the sequence by two elemental informations having a duration equal to a half bit period. For example, each bit with a value 1 is transmitted in the form of an illumination during a bit half-period, fol-lowed by an absence of illumination during a bit half-period, and conversely for each bit with a value 0. This modulation method allows in addition an error detection, for each trans-mitted bit includes a transition. A lacking bit is therefore detectable by counting the transitions.

The implementation of this on/off amplitude modulation consists in deflecting the laser bean toward a given cell 9 to transmit an elemental information, and in deflecting the laser beam toward a cell 9' symmetrical to the given cell with respect to o to transmit an opposite elemental information.

The binary sequence transmitted toward the given cell 9 is thus complementary of the bit sequence transmitted during the same time interval toward the symmetrical cell 9'.

This special manner to on/off-modulate the laser beam has a remarkable efficiency since, in fact, the laser beam is transmitted without interruption. This efficiency adds up to the effect of concentration of the laser beam in a single cell, and permits a significant reduction of the required ave-rage power of the laser as compared to that of the laser of a system of the type with spatial coding.

A partial overlap of the cells remains that has a signi-ficant advantage over the method of the spatial coding type of the prior art. As a matter of fact, the means for receiving the laser beam, located in the missile, can receive almost at the same time, the coordinates of several cells and can compute interpolated coordinates by taking into account the amplitudes respectively received for each of the received bi- nary sequences. This interpolation allows to refine the deter- mination of the position of the missile and, therefore, impro-yes the accuracy of guidance.

Figure 3 illustrates the coding method used for coding the coordinates of the cells 9 and 9' by means of a binary sequence S and of the complementary sequence. This sequence S begins with two discrimination high/low bits allowing to dis-criminate a cell belonging to the high half-space, such as the cell 9, and a symmetrical cell belonging to the low half-space, such as the cell 9' . These two high/low discrimination bits consists of a so-called reference bit and a so-called sign bit which represents the sign of the y-coordinate of the cell of interest. The sequence S then includes 3 bits repre-senting the absolute value of y. The sequence S then includes 4 bits representing the sign and the absolute value of x.

The decoding of the modulation of the laser beam received by the missile is performed by a decoder aboard the missile.

This decoder first reconstructs a binary sequence T in which a bit has a value equal to 1 when the decoder detects an illu-mination for a duration equal to a bit half-period, then an absence of illumination for a duration equal to a bit hal-period. A bit of the sequence I has a value equal to 0 when the decoder detects an absence of illumination for a duration equal to a bit half-period, then detects an illumination for a duration equal to a bit half-period.

Then, the sequence T is decoded to discriminate the high half-space and the low half-space and to reconstruct a value x and a value y. If the missile is located in the high half- space, the reception of the directing beam begins with an ii- lumination for a duration equal to a bit period. If the missi- le is located in the low half-space, the reception of the di- recting beam begins with an illumination during a bit half-period followed by an absence of illumination for a duration equal to a bit period. The value 01 or 10 of the first two bits in the sequence I selects, therefore, the high half-space or the low half-space. The value of x and the value of y re-main to be decoded.

For example, if the missile is located in the cell 9, the demodulation of the laser beam received by the missile provi-des a binary sequence T(9) :011011010.

The first two bits 01 indicate that the missile is located -10 -in the high half-space. The second, the third, the fourth and the fifth bit give directly the coded value y 1 101 + 5.

The sixth, the seventh, the eighth and the nineth bit give directly the value x 1 010 +2.

When the missile is located in the cell 9', the demodula- tion of the laser beam received by the missile provides a bi-nary sequence 1(9') :looloolol.

The first two bits 10 indicate that the missile is located in the low half-space. The second bit provides the sign of y for the cell 9'. The third, the fourth and the fifth bit provi-de the absolute value of y complemented to 1. A comple-mentation allows to find the absolute value of y ii = 101 and y -5.

The sixth bit provides the sign of x for the cell 9'. The seventh, the eighth and the ninth bit provide the absolute value of x complemented to 1. A complementation allows to find the absolute value of x lxi = 010 and x -2.

The sequence S may include in addition an additional bina- ry word constituting a message. This message can be some in-formation about the evolution of the trajectory of the target to permit the on-board computer of the missile to generate better trajectory corrections. This message can also be some information about a possible pointing error of the axis of symmetry of the guidance cone to allow the on-board computer to compute a trajectory correction such that the missile comes closer to an axis offset relative to the axis of symmetry of the guidance cone. Finally, this message can control the ac-tivation of a proximity fuze carried by the missile.

If the missile is located in the high half-space, the bi- -11 -nary sequence obtained by demodulating the laser beam provides directly the bits of the message. If the missile is located in the low half-space, the decoder complements the binary se-quence obtained by demodulating the laser beam to recover the accurate value of the message bits.

If the message is long, it can be divided into several binary words, each fitted with an identifying prefix and which are transmitted during several successive scanning periods.

To increase the rate of scanning the guidance cone, it is possible to alternate scans with transmission of coordi-.

nates only and scans with trasnmission of the message only, so as to devote time to the transmission of messages only when there is effectively a message to be transmitted. The messages are then distinguished from the coordinates by an additional bit.

Another embodiment of the coding method consists in using a pulsed laser having a rate higher than that of the bits in-stead of the CW laser, and in using a receiving device adapted to the pulses of the laser. The pulses from the laser are then a subcarrier modulated in the on/off mode by the binary se-quence in a manner similar to that described for a CW laser.

This results in an improvement of the signal-to-noise ratio for a given average power of the laser and a lower sensitivity to jamming caused by the flame and smoke from the motor of a missile.

A further embodiment consists in using a continuous-wave laser and in chopping its beam at a rate much higher than that of the bits of the sequence to be transmitted. The encoder 31 must then include logic means to provide a logic signal which is the product of the sequence to be transmitted and a chop-ping signal. This product signal replaces the previous binary sequence to control the hops of the laser beam.

-12 -A still further embodiment consists in using a pulsed la-S ser having a rate twice that of the bits and including means for synchronizing it to the bit rate. The modulation of the beam consists in transmitting two pulses for each bit whatever its value. To send a bit of value 1 to the cell 9, for exam-ple, the first pulse is deflected toward the cell 9, whereas the second pulse is deflected toward the symmetrical cell 9' Conversely, to send a bit of value 0 to the cell 9' , the first pulse is deflected toward the cell 9' and the second pulse is deflected toward the cell 9.

Figure 4 is a block diagram of an embodiment of the gui- dance system according to the present invention. This embodi-ment comprises means for scanning the guidance cone, made up of a laser 33, two deflecting mirrors, an afocal optical mat-ching device 36, two acousto-optic deflectors 37 and 37', an afocal zoom 38, and control means 39. It includes tracking means comprising a scanning device 40, an optical device 41, a cooled linear-array sensor 42, a tracking computing device 43, a pointing mechanism 44, and a dichroic mirror 45. It fur-ther includes means for automatically co-aligning the lines of sight, comprising a shutter 46, an attenuating plate 47, a cube corner reflector 48, and a device 49 controlling the co-alignment of the lines of sight.

It should be noted that the tracking means and the means for scanning the guidance cone include a common optical chan-nel formed by the pointing device 44.

The laser 33 is a CO2 laser, with continuous-wave emission and with a 10.6-microns wavelength, having a linear polariza-tion. The deflecting mirrors 34 and 35 have the only function to reduce the size of the whole device. They deflect the beam of the laser 33 at the input of the device 36 that adjusts the diameter and the divergence of the laser beam to values -13 -suitable for the subsequent optical elements. The acousto-.

optic deflectors 37 and 37' deflect the beam within a vertical plane and within a horizontal plane, respectively. Each of them consists of a Bragg cell, of conventional design, which allows to rapidly deflect a laser beam.

Each cell comprises a germanium crystal travelled by me-chanical waves generated by a piezoelectric transducer excited by a RF generator. The implementation of this generator is conventional. The deflector 37 includes an input for receiving a set-point voltage provided by a first output of the control means 39 and that defines a deflection angle 4x. This angle corresponds to a coordinate x through an approximately linear relationship, for the deflection angles are smaller than 100.

The deflector 37' includes an input for receiving a set-point voltage provided by a second output of the control means 39 and that defines a deflection angle 4y corresponding to a co-ordinate y. Then, the output beam from the deflector 37' goes through the afocal zoom 38 that allows to adjust the diver-gence of the laser beam and the amplitude of its deflection as a function of the distance between the fire-control station and the missile. The utilization of this zoom will be descri- bed hereinafter. The zoom 38 includes a conventional servo-system controlled by a set-point voltage provided by a third output of the control means 39. This voltage represents a mag- nification value C which is a function of the distance tra-velled by the missile from its launch.

The dichroic mirror 45 is transparent for the wavelengths longer than 10 microns, and reflecting for the wavelengths shorter than 10 microns. The laser beam from the zoom 38 goes through the dichroic mirror 45 with a low attenuation due to the reflection of a small fraction of its power. Then it is deflected by a pointing device 44 which is a conventional op- -14 - tomechanical device controlled by the tracking computing de- vice 43 so that the line of sight of the device 44 is cons-tantly directed to the target or to a future position which is predicted by the device 43. Thus, an output of the device 44 transmits a laser beam 3 having as an average direction the line of sight defined by the computing device 43, and mo-ving about this average position to scan the guidance cone according tO the deflections caused by the deflectors 37 and 37' . Tracking is performed by detecting the infrared radiation which belongs to the target. This radiation is received in a beam50 that goes through the device 44, then is reflected alm'ost entirely by the dichroic mirror 45 to the optical de-vice 41 that forms an image on the sensor 42. The scanning device 40 is interposed between the linear-array sensor 42 and the device 41 to scroll the image on the sensor 42. It can produce a circular scanning, for example, and its imple-rnentation is conventional. The sensor 41 consists of a row of cells which are photosensitive in the infrared range. It is cooled by cryogenic means. It has an output connected to an input of the tracking computing device 43. The circular scanning performed by the device 40 scrolls the image on the linear-array sensor 42. The output or the sensor 42 thus pro-vides a video signal representing a radially scanned image.

The device 43 determines a line of sight from the position of the target and its maneuvers in the sequence of images.

The algorithm implemented in the device 43 is conventional.

The devices 40 and 44 are optomechanical devices having a wide optical bandwidth which allows to obtain an image of the target in a wide spectral region.

Any guidance system requires a co-alignment of the line of sight of the tracking means and of the line of sight of -15 -the means for scanning the guidance cone. These lines of sight are not exactly colinear because the dichroic mirror 45 is not tilted exactly by 45° to the optical axis of the zoom 38 and of the pointing device 44, and to the optical axis of the optical device 41, and because the optical axes of all the remaining optical elements are not perfectly stable when there are vibrations or temperature changes.

Prior to each launch, the device 49 controls an automatic co-alignment of the lines of sight. To this end, it controls the opening of a shutter 46 which is interposed between the dichroic mirror 45 and the cube corner reflector 48, in pro-longation of the optical axis of the optical device 41. The shutter 46 passes then a fraction of the laser beam which is reflected by the dichroic mirror 45. This fraction of the la-ser beam is reflected back by the cube corner reflector 48 parallel to itself, whatever the orientation error of the cube corner reflector 48. Then, it goes back through the shutter 46, then goes through the dichroic mirror 45, and then through the optical device 41, and then arrives on a cell of the sen-sor 42. When the position of the dichroic mirror 45 is not perfect, the fraction of the laser beam that arrives on the sensor 42 does not falls at the point forming the center of the image. To measure the co-alignment error along two ortho-gonal directions, the device 49 controls the deflector 37 to perform a scan with a predetermined amplitude in the vertical plane, which results in a scan of the laser beam arriving on the sensor 42. An output of the sensor 42 is connected to an input of the control device 49 to provide it with the video signal resulting from the scan of the laser beam on the sensor 42. The control device 49 determines two correction signals that it provides respectively to a first correction input of the deflector 37, and to a first correction input of the de--16 flector 37'. These correction signals add up algebraically to the set-point values provided by the control means. When the device 49 has completed this measurement, it closes the shutter 46 again to prevent the fraction of the laser beam to disturb the reception of an image in the sensor 42.

Although the fraction of the laser beam reflected by the dichroic mirror 45 is very small as compared to the remaining fraction of the laser beam which is transmitted by this di-chroic mirror, it is necessary to provide an attenuating plate in the path of the fraction which is reflected by the cube corner reflector 48 so as not to saturate the sensor 42.

The measurement of the co-alignment error is performed for several values of the magnification of the zoom 38. The various corresponding values of the correction signals are stored in the device 49, then are provided in real time to the devices 37 and 37' as the magnification value increases during the flight of the missile.

The control means 39 comprise a sequencer 30, an encoder 31, and a magnification computing device 32. The sequencer 30 has three outputs: a first and a second output providing the coordinates x and y to two inputs of the encoder 31, and a third output providing a missile launch signal to an input of the magnification computing device 32. The device 32 deter-mines the distance traveled by the missile from the time of its launch and derives from it the the magnification value C. An output of the device 32 forms the third output of the means 39.

The encoder 31 has in addition an input connected to an output of the tracking computing device 43 which provides it with a binary word constituting a message to be transmitted to the missile. The encoder 31 generates a binary sequence formed by the value of x, the value of y, and the message word.

-17 -From the values of x and of y, the encoder 31 determines two values of angular deflection 4x, y corresponding to the posi-tion of a cell to be illuminated at the time of interest, and corresponding to the symmetrical cell with respect to the axis of symmetry of the guidance cone. To illuminate both of these cells by modulating the laser beam according to the binary sequence, the encoder 31 provides the deflector 37 with a set- point signal whose value represents now ++x, now -4x. Similar- ly, it provides the deflector 37' with a set-point signal who-se value represents now �4y, now The deflectors 37 and 37' have second correction inputs respectively connected to two outputs of the tracking compu-ting device 43. These inputs receive at each time correction signals 4xo and 4yo which add up algebraically to the set-point signals 4x and 4y, respectively, inside the deflectors 37 and 37'. These correction signals allow to correct the poin-ting of the axis AS of the guidance cone to align it with the line of sight LV computed by the device 43 in the event the latter detects a pointing error due to the inertia of the poin-ting device 44 during sudden maneuvers of the target. Thus, the rapidity of the deflectors 37 and 37' permits to compen- sate for the pointing errors due to the inertia of the poin-ting device 44.

Figures 5 and 6 illustrate the operation of this embodi- ment of the guidance system according to the present inven-tion. Figure 5 shows the evolution of the guidance cone during the flight of the missile 4. At the time of interest, the mis-sile 4 is guided by a guidance cone 5 which has a surface of intersection 55 with a plane perpendicular to the axis of sym- metry AS of the guidance cone. This surface 55 is approxima-tely circular and has a diameter that varies as a function of the distance traveled by the missile 4. The evolution of -18 -the diameter of this surface 55 is shown in dashed lines.

When the missile is launched from a ramp 58, the directing beam is transmitted by a guidance system 57 which is located in the vicinity of the ramp 58. The line of sight LV of the guidance system 57 is coincident with the axis of symmetry of the guidance cone. The line of fire of the ramp 58 is pa-rallel to this line of sight but is not coincident with it.

At the start of its flight, the missile is therefore outside the guidanáe cone. To avoid that the missile escapes the gui- dance cone and becomes lost, the aperture angle of the gui-dance cone is increased during a so-called homing phase which lasts-until the missile has traveled a distance dl.

During the homing phase, the guidance cone 51 has an aper- ture angle, predetermined and relatively wide, which is obtai-ned by increasing the amplitude of the deflection effected by the acousto-optic deflectors 37 and 37'. To this end, the encoder 31 provides the deflectors 37 with set-point values 4x, 4y which are higher than those corresponding to the flight phase after the homing phase. During this homing phase, the zoom 38 receives a magnification value C which is the minimal value. The number of elemental cells making up the guidance cone is increased by four times, for example, and the scanning period is increased by the same amount.

The scanning period cannot be increased indefinitely. If the number of cells is not sufficient to cover a guidance cone 51 having a given aperture angle, a solution consists in dis-tributing these cells only at the periphery of the guidance cone 51. There is then an inactive cone 56 in which there is consequently no guidance. The missile 4 enters the guidance cone 51 by necessarily flying through the periphery; there-fore, the absence of cells near the axis of the guidance cone is not important. 19 -

Figure 6 shows the intersection of the guidance cells with a plane perpendicular to the axis of symmetry AS at a time of the homing phase. The inactive cone 56 is represented by hachures.

The homing phase is followed by three further guidance phases. While the missile is at a distance included between dl and d2, the diameter of the intersection surface of the guidance cone is maintained constant by reducing the amplitude of the deflection effected by the deflectors 37 and 37' , as a function of the distance traveled by the missile. The sur- face 55 moves then while generating a cylinder 52. As the dis-tance of the missile increases, the intersection surface of each elemental cell increases. The number of elemental cells being constant and the diameter of the guidance cone being constant, the cells progressively occupy a more and more sig-nificant fraction of the surface 55.

When the missile 4 is located at a distance included bet- ween a distance d2 and a distance d3, the diameter of the in-tersection surface 55 is maintained constant by increasing the set-point value of the magnification C which is provided to the zoom 38. Thus, the surface 55 moves with the missile 4 in a cylinder 53.

When the missile exceeds the distance d3, the zoom 38 has reached its maximum magnification and it is no longer possible to maintain the diameter of the surface 55 constant. This sur-face moves then while increasing linearly as a function of the traveled distance and generates a cone having a small aper-ture angle.

The magnification computing device 32 receives a start signal provided by the sequencer 30. From that time onwards, it determines the magnification values C as a function of a predetermined time law. The sequencer 30 determines a sequence -20 -of x-values and a sequence of y-values which always produce a periodic scanning but which evolve as a function of the time elapsed from the launch of the missile according to a prede-termined time law. The determination of both of these laws from the laws of optics and from the characteristics of the zoom 38 and the deflectors 37 and 37' is possible for those skilled in the art.

The present invention is not limited to the embodiments described above as many variants thereof are possible for tho-se skilled in the art. In particular, it is possible to use a dichroic mirror reflecting the wavelength of the laser while passing the wavelengths intended to form an image of the tar- get. In this case, the optical channel in the means for scan-ning the guidance cone is exchanged with the optical channel made up by the shutter, the attenuating plate and the cube corner reflector.

It is also possible to use a laser having a wavelength different from 10.6 microns. It is also possible to use a two-dimensional image sensor by eliminating the scanning device 40, or to use an infrared camera.

Claims (11)

1. -21.--Claims 1. A system for guiding projectiles by a directing beam coded in Cartesian coordinates, the fire-control station comprising: -means for tracking a target and defining a line of sight; -means for scanning, with the beam transmitted by a laser, a portion of space called "guidance cone", having an axis of symmetry whose position is locked to said line of sight, said guidance cone being made up of elemental cells successively illuminated by said laser beam; said projectile including -means for receiving said laser beam, for determining in which cell the projectile is located, and for deriving from it a correction of the trajectory of the projectile such that it comes closer to the axis of symmetry of said guidance cone; -means for performing said correction of trajectory, wherein said means for scanning comprise -means for deflecting said laser beam in two directions or-thogonal to each other; -means for modulating the amplitude of said laser beam trans-mitted in each elemental cell, according to a separate binary sequence for each cell; and -said means for receiving the laser beam include means for demodulating said laser beam and for decoding said binary se-quence.
2. 2. A system according to claim 1, wherein said means for mo- dulating the laser beam include an encoder for providing bi- -22 -Snary sequences such that each sequence corresponding to a par- ticular cell includes a first and a second binary word res-pectively representing two Cartesian coordinates identifying the position of the cell of interest in a Cartesian reference system havingan origin located on the axis of symmetry of said guidance cone.
3. 3. A system according to claim 1, wherein said means for mo-dulating the amplitude of the laser beam illuminating a given cell include means for deflecting said laser beam toward a cell syrnmetrica1 of said given cell with respect to said axis of symmetry for each elemental information having a predeter-mined value, and for deflecting said laser beam toward said given cell for each elemental information having an opposite value.
4. 4. A system according to claim 2, wherein said means for mo-dulating include an encoder for encoding in addition a message to be transmitted in said binary sequence, and wherein said means for receiving the laser beam include a decoder to decode in addition said message.
5. 5. A system according to claim 1, wherein said means for tracking the target and said means for scanning the guidace cone include a common optical channel formed by a pointing device orienting the line of sight of said means for tracking and the line of sight of said means for scanning, said poin-ting device being traveled in a direction by said laser beam, and in the opposite direction by a light beam received along said line of sight.
6. 6. A system according to claim 5, in which said means for tracking the target include an optical sensor, wherein for co-aligning the line of sight of said means for tracking the target and the line of sight of said means for scanning the guidance cone, it includes a semitransparent plate and a cata-dioptric device, disposed so as to project a fraction of said laser beam onto a predetermined point of said optical sensor when said lines of sight are co-aligned.
7. 7. A system according to claim 1, wherein said laser is a continuous-wave laser, and wherein said means for modulating include in addition means for chopping said laser beam at a rate much higher than the bit rate of said binary sequence to be transmitted.
8. 8. A system according to claim 1, wherein said laser is a pulsed laser having a rate higher than that of the bits in the binary sequence to be transmitted.
9. 9. A system according to claim 1, wherein said laser is a pulsed laser having a rate twice the bit rate of said binary sequence to be transmitted, and wherein said means for modu-lating include in addition means for synchronizing said laser to the rate of the bits in said binary sequence.
10. 10. A system according to claim 1, wherein said means for scanning include means for varying the aperture angle of said guidance cone as a function of the distance traveled by the projectile, said aperture angle being wider during a so-called homing phase during which said projectile enters said guidance cone, and for distributing said elemental cells at the pen-phery of said guidance cone during said homing phase while leaving a central zone without cells.
11. 11. A system for guiding projectiles by a directing beam coded in Cartesian coordinates and substantially as hereinbefore described and as shown in Figures 4, and 6 of the accompanying drawings.Amendments to the claims have been filed as follows 1. A system for guiding projectiles by a directing beam coded * including a in Cartesian coordinates, / fire-control station comprising: -means for tracking a target and defining a line of sight;and -means for scanning, with the beam transmitted by a laser, a portion of space called "guidance cone", having an axis of lo symmetry whose position is locked to said line of sight, said guidance cone being made up of elemental cells successively illuminated by said laser beam; to be guided a Projectile/including -means for receiving said laser beam, for determining in which cell the projectile is located, and for deriving from it a correction of the trajectory of the projectile such thatd it comes closer to the axis of symmetry of said guidance cone;, -means for performing said correction of trajectory, said means for scanning comprising: -means for deflecting said laser beam in two directions or-thogonal to each other; -means for modulating the amplitude of said laser beam trans-mitted in each elemental cell, according to a separate binary sequence for edch cell; and -said means for receiving the laser beam include means for demodulating said laser beam and for decoding said binary se-quence wherein said means for modulating the amplitude of the laser beam illuminating a given cell include means for deflecting said laser beam toward -a ceJJ symmetrical of said given cell with respect to said axis of symmetry for each elemental information having a pre-determined value, and for deflecting said laser beam toward said given cell for each elemental information having an opposite value.2. A system according to claim 1, wherein said means for modulating the laser beam include an encoder for providing bi- nary sequences such that each sequence corresponding to a par- ticular cell includes a first and a second binary word res-pectively representing two Cartesian coordinates identifying the position of the cell of interest in a Cartesian reference system having an origin located on the axis of symmetry of said guidance cone.3. A system according to claim 2, wherein said means for mo-dulating include an encoder for encoding in addition a message to be transmitted in said binary sequence, and wherein said means for receiving the laser beam include a decoder to decode in addition said message.1j A system according to claim 1, wherein said means for tracking the target and said means for scanning the guidace cone include a common optical channel formed by a pointing device orienting the line of sight of said means for tracking and the line of sight of said means for scanning, said poin-ting device being traveled in a direction by said laser beam, and in the opposite direction by a light beam received along said line of sight.. A system according to claim I, in which said means for tracking the target include an optical sensor, wherein for co-aligning the line of sight of said means for tracking the target and the line of sight of said means for scanning the guidance cone, it includes a semitransparent plate and a cata-dioptrjc device, disposed so as to project a fraction of said laser beam onto a predetermined point of said optical sensor when said lines of sight are co-aligned.6. A system according to claim 1, wherein said laser is a ccntjnuous_wave laser, and wherein said means for modulating include in addition means for chopping said laser beam at a rate much higher than the bit rate of said binary sequence to be transmitted.7. A system according to claim 1, wherein said laser is a pulsed laser having a rate higher than that of the bits in the binary sequence to be transmitted.8. A system according to claim 1, wherein said laser is a pulsed laser having arate twice the bit rate of said binary sequence to be transmitted, and wherein said means for modu-lating include in addition means for synchronizing said laser to the rate of the bits in said binary sequence.9. A system according to claim 1, wherein said means for scanning include means for varying the aperture angle of said guidance cone as a function of the distance traveled by the projectile, said aperture angle being wider during a so-called homing phase during which said projectile enters said guidance cone, and for distributing said elemental cells at the pen-phery of said guidance cone during said homing phase while leaving a central zone without cells.10. A system for guiding projectiles by a directing beam coded in Cartesian coordinates arid substantially as hereinbefore described and as shoi in Figures 4, and 6 of the accompanying drawings.