IL161331A

IL161331A - Target acquisition device, aircraft, associated trajectory estimation system and defense system

Info

Publication number: IL161331A
Application number: IL161331A
Authority: IL
Original assignee: Thales Sa
Priority date: 2001-10-23
Filing date: 2004-04-08
Publication date: 2010-05-17
Also published as: WO2003036323A1; ATE409317T1; EP1440329A1; ES2314126T3; FR2831273B1; IL161331A0; DE60229065D1; FR2831273A1; EP1440329B1

Abstract

The incoming target sensor for an aircraft has an optronic sensor path with a detector (6) and circuits to correlate the line of sight of the optronic sensor. The target acquisition sensor also has a star position sensor (8) path and correlate the optronic sensor line of sight with that of the star sensor.

Description

may n)¾n ro-iyai m»¾£> ro-iyn JID-WO ,V>O >ΪΟ ,moa nw n nn TARGET ACQUISITION DEVICE, AIRCRAFT, ASSOCIATED TRAJECTORY ESTIMATION SYSTEM AND DEFENSE SYSTEM THALES C: 51303 TARGET ACQUISITION DEVICE, ASSOCIATED AIRCRAFT, ASSOCIATED TRAJECTORY ESTIMATION SYSTEM AND ASSOCIATED DEFENSE SYSTEM The invention relates to the field of target, in particular . flying target, acquisition devices, as well as to surveillance aircraft integrating these target acquisition devices, the associated systems for target trajectory estimation, and the anti-target defense systems using trajectory, estimation systems.

The target acquisition device is used by a trajectory estimation system which locates the target so as to allow for example a defense system to neutralize this target. One of the problems that arises in effectively neutralizing the target is the locating accuracy. Several systems of the prior art and consequently several types of target acquisition devices have been successively developed, thus allowing target locating accuracy to be gradually improved. The target acquisition device according to the invention allows the trajectory estimation system that uses it to further and substantially improve this locating accuracy.

According to a first prior art, the use is known of a target acquisition device comprising a simple optronic sensor channel . The aircraft carrying such a target acquisition device comprises an inertial unit to which the optronic sensor channel is reset. This first prior art is inaccurate on account of the problems in resetting the optronic sensor channel to the inertial unit of the aircraft and on account, in particular in the event of a long-duration mission, of the problems of drifting of the inertial unit of the aircraft.

According to a second prior art, the use is known of a target acquisition device comprising a simple optronic sensor channel . The aircraft carrying such a target acquisition device also comprises an inertial unit but the optronic sensor channel is reset to the stars in the sky that are observed by way of the optronic sensor channel. However, during the motion of the boresight of the optronic sensor channel when the latter passes from the stars to the target, the additional accuracy that had been obtained during resetting to the stars is partially lost. This second prior art is also relatively inaccurate, although better than the first prior art since it is autonomous with respect to inertial unit. Moreover, the inertial unit may be reset to the stars, by way of the optronic sensor channel.

The object of the invention is to propose a target acquisition device that allows very accurate locating of said target, in contradistinction to the devices of the prior art. The target acquisition device according to the invention is intended to be mounted on an aircraft or an aerostat, that is to say on an aerial platform moving around or stationary in the air, for example of the airplane, drone or balloon type, with the exclusion of satellite type platforms moving around in space and which are in no way subjected to the same mechanical stresses, such as for example load factor or vibrations, as aircraft. An aircraft is subjected to much bigger mechanical stresses than a satellite insofar as the conditions of deployment or of vibratory environment of an aircraft are much more severe than those of a satellite. The target acquisition device according to the invention, in order to improve the accuracy of locating of potential targets, comprises a star sensor channel to which the optronic sensor channel may be very accurately reset. In the target acquisition device according to the invention, in order that the high accuracy acquired by resetting the optronic sensor channel to the star sensor channel cannot be lost through various relative movements or degrees of freedom of one of the channels with respect to the other, said channels are tied to one another.

Thus, by virtue of the presence of the star sensor channel and by virtue of the tying of the star sensor channel and optronic sensor channel, the system for globally stabilizing the target acquisition device according to the invention achieves a better compromise between the accuracy of stabilization obtained by said system and the complexity of said system than the target acquisition devices according to the prior art.

According to the invention, there is provided a target acquisition device, being intended to be mounted on an aircraft, comprising an optronic sensor channel comprising a detector and means for slaving the boresight of the optronic sensor channel, characterized in that the target acquisition device also comprises a star sensor channel comprising a detector, means for resetting the boresight of the optronic sensor channel to the boresight of the star sensor channel, and in that the boresight of the star sensor channel is mechanically tied to the boresight of the optronic sensor channel .

The invention will be better understood and other features and advantages will become apparent with the aid of the following description and of the appended drawings, given by way of examples, where: figure 1 diagrammatically represents an exemplary aircraft integrating a target acquisition device according to the invention; figure 2 diagrammatically represents an exemplary target acquisition device according to the invention; figure 3 diagrammatically represents a preferred exemplary target acquisition device according to the invention; 4 161,331/2 figures 4 and 5 diagrammatically represent schematics explaining the correspondence between the pixels of the detector of the optronic sensor channel and pixels of the detector of the star sensor channel in the case of an embodiment in accordance with that of figure 3.

STATEMENT IN ACCORDANCE WITH THE COMMISSIONER'S CIRCULAR NO. 23(P) Inasmuch as the invention is defined in the appended claims, it will be apparent that the portions of the present specification, which fall outside the scope of the claims, do not relate directly to the claimed invention. This Notice is not meant to disclaim any legitimate rights to which the Patentee is legally entitled, especially any rights in accordance with Section 49 of the Israel Patent Law.

DETAILED DESCRIPTION Figure 1 diagrammatically represents an exemplary aircraft integrating a target acquisition device according to the invention. This aircraft is preferably a surveillance aircraft. The surveillance aircraft is for example a surveillance drone, only a portion of the outside surface of which is represented in figure 1 for the sake of clarity. The target acquisition device 10 according to the invention is a tracking ball into which are integrated an optronic sensor channel and a star sensor channel. The tracking ball 10 is situated on the outside surface 20 of the drone. The drone also comprises on its outside surface 20, but placed on the other side of the drone, a watching device 22, also in the form of a ball, which is a watching ball. Communication means 24, situated inside the drone, link the target acquisition device 10 to the watching device 22. Target trajectory reconstruction means 21 are also linked to the communication means 24 and are also situated in the drone. The reconstruction means 21 could 4A 161,331/1 also be situated in a ground station. A mechanically rigid structure 23 ensures that the relative position of the tracking ball 10 and of the watching ball 22 is maintained .

The target acquisition device according to the invention is intended to be mounted on an aircraft. This aircraft is for example a surveillance drone as in figure 1. This aircraft may also be a fighter plane. Specifically, the load factor on a fighter plane, that is usually high on account of rapid deployments of the fighter, would degrade the locating accuracy even more - 5 -considerably than on a surveillance drone. However, with the target acquisition device according to the invention, the mechanical tie existing between the optronic sensor channel and the star sensor channel renders the sought-after locating accuracy compatible with the use of a fighter with a high load factor. Specifically, the star sensor and optronic sensor channels are mechanically linked together in a sufficiently rigid manner for the boresight of the observation channel and the boresight of the star tracker channel to be immovable with respect to one another. The use of a fighter plane also has the advantage of not requiring an additional aircraft in order to achieve interception of the flying target. The target acquisition device according to the invention can be mounted on a fighter plane on account of its great compactness, in particular when it consists of a tracking ball into which the optronic sensor channel and the star sensor channel are integrated.

The altitude of deployment of the aircraft on which the target acquisition device according to the invention is mounted is preferably an altitude on the one hand for which the signal-to-noise ratio of the various channels, namely the optronic sensor channel and the star sensor channel, is big enough to allow the target acquisition device to lock on to a target and in particular a flying target, to allow the star sensor channel to acquire the stars, and on the other hand at which several standard types of aircraft can deploy. An altitude of around 40 000 feet therefore represents a good compromise.

Preferably, the aircraft comprises a watching device as well as means of communication between the target acquisition device and the watching device, said means of communication allowing the target acquisition device to lock on, successively over time, to several targets and to regularly return to lock on to each of said - 6 -targets. Each time the target acquisition device returns to lock on to a target, the images that it captures allow fine estimation of several positions of said target, said positions then being used by trajectory estimation means to reconstruct the trajectory of said target. This manner of operation with point-like returns of the aircraft to the target, allows the same aircraft to share its target acquisition time between several targets deploying simultaneously within the field.

The target acquisition device has to search for the target and acquire it on the basis of an initial designation of objective provided as absolute coordinates or as elevation and bearing. To do this, the target acquisition device is locked on to a designation of objective giving the estimated position in elevation and bearing of the target onto which the target acquisition device must then actually lock. To do this, the designation of objective may also sometimes provide the estimated angular speed of the target. The objective designation may be provided either by a watching device integrated into the aircraft or by a ground station. In the case where the objective designation is provided by a watching device integrated into the aircraft, the position estimate or even the angular speed estimate is coarse and will indeed be refined by way of the images captured by the channels of the target acquisition device. In the case where the objective designation is provided by a ground station, the position estimate or even the angular speed estimate has been performed by reconstructing the trajectory of the target obtained by extrapolating to the present instant positions or even speeds that are themselves obtained from the information provided previously by the devices for target acquisition and for watching of one or more surveillance aircraft (possibly including the one to which the objective designation is supplied) .

On the basis of the objective designation the target acquisition device can lock on to the target on condition on the one hand that the target is seen in the field of the optronic sensor channel of the target acquisition device, this requiring, for searchless acquisition, said field to be greater than twice the angular accuracy of the objective designation plus the travel of the target, and on the other hand that the signal-to-noise ratios on the optronic sensor and star sensor channels of the target acquisition device are sufficient, a signal-to-noise ratio, whose value lies between two and ten for example, being standard.

The target acquisition device according to the invention is preferably intended to be used in a surveillance aircraft which is itself advantageously part of a system for target trajectory estimation.

In a preferred embodiment, the system for estimating a target's trajectory comprises a single surveillance aircraft which comprises a telemeter. The estimation system also comprises means for reconstructing the trajectory of the target from the images provided by the aircraft and from the telemetry measurements.

In another preferred embodiment, the system for estimating a target's trajectory comprises at least two surveillance aircraft. The estimation system also comprises means for reconstructing the trajectory of the target by triangulation from the images provided by the various aircraft.

These two preferred embodiments allow the system for estimating the trajectory of the targets, in particular flying targets, to derive full benefit from the accuracy afforded by the images provided by the target acquisition device according to the invention. The accuracy would be slightly degraded in the absence both of telemetry and of triangulation or else in the absence of equivalent means making it possible for distance measurements to supplement the angle measurements originating from the images provided by the channels of the target acquisition device. The means for reconstructing the trajectory of targets can be situated for example, either on the surveillance aircraft themselves, or at the level of a ground station, or partly at the level of the ground station and partly at the level of the surveillance aircraft. These means of reconstruction include in particular databases relating to the ephemerides of the stars corresponding to the portion of field where the target or targets are presumed to deploy.

The target trajectory estimation system may for example form part of a more global system for active defense integrating at least one interception aircraft to which is supplied at least a portion of the trajectory estimated by the estimation system. By virtue of this estimated trajectory portion, the interception aircraft is guided towards the target until it can engage its own terminal guidance system. Interception will generally take place after re-entry into the atmosphere for a ballistic missile. In the case of a passive defense system, that is to say one comprising no interception aircraft, the passive defense then consisting for example in putting the civil populations into shelters, the locating of the flying target by way of the trajectory estimation system dispenses with the need for the ground systems for very long range radar detection of ballistic missiles in particular. The trajectory estimation system according to the invention has moreover a much greater range than a very long range radar detection system. In the example of a ballistic missile with a range of around 1500 km, and a flight time of around 10 min, the very long range radar system will detect the missile around 2 min before impact whilst the trajectory estimation system can - 9 -detect the same missile under certain conditions up to around 8 min before impact, this greatly facilitating the task of the associated defense system, whether the latter be active or passive.

The duration of the mission of a surveillance aircraft within a defense system may be long, for example of the order of a day, missions of the order of a week or a month even being conceivable. Specifically, target location on the basis of images provided by the channels of the target acquisition device preserves good accuracy independently of the drifting over time of the inertial unit of the aircraft, which inertial unit can then even be reset to the star sensor channel during the mission. In the event of resetting of the target acquisition device to the current inertial units of aircraft, as in the prior art, only missions of a few hours at most would be conceivable.

The type of flying targets tracked by the target acquisition device according to the invention is preferably the ballistic missile. The target acquisition device and the trajectory estimation system according to the invention may also be used against other types of flying targets such as spy satellites or airplanes deploying at high altitude. The target acquisition device and the trajectory estimation system according to the invention may also be used against other types of targets such as maritime targets or terrestrial targets, terrestrial targets then advantageously being fixed or moveable targets, for example of the headquarters type or of the ballistic missile launcher type.

Figure 2 diagrammatically represents an exemplary target acquisition device according to the invention. The target acquisition device according to the invention, represented in figure 2, is preferably intended for acquiring terrestrial or naval targets : - 10 -specifically, the star sensor channel looks towards the sky whilst the optronic sensor channel looks towards the earth or towards the bottom layers of the atmosphere. The target acquisition device according to the invention takes the form of a tracking ball 10. Once mounted on a surveillance aircraft, the ball possesses at least two degrees of freedom in rotation with respect to the structure of the surveillance aircraft, so as to be able to scan a substantial portion of space such as for example a half-space. The optronic sensor channel whose field 12 of view encompasses the target 13 comprises an optronic sensor 11 for example of observation camera type. The field (15) of the detector (8) of the star sensor channel is preferably large enough to always see at least two stars, this making it possible to determine the roll of the star sensor channel and this being particularly beneficial given the fact that the boresights of the star sensor and optronic sensor channels are not colinearly harmonized in the example represented in figure 2. The star sensor channel whose field 15 of view encompasses several stars 16 comprises a star sensor 14. The relative position of the fields 12 and 15 in space is preserved by virtue of the rigidity of the mechanical link between the optronic sensor and star sensor channels. In figure 2, this mechanical rigidity originates from the ball 10 which is itself very rigid and is moreover engineered so as to be unaffected by heat, to which ball 10 are also rigidly mechanically fixed the various elements of said channels. Openings 17 and 18 are made in the ball 10 so that the fields 12 and 15 of view respectively of the optronic sensor and star sensor channels are not obstructed by the walls of the ball 10. In figure 2, the position of the various channels is such that the directions in which said channels look are not aligned, that is to say the boresights of the various channels are not aligned. Preferably, the boresights of the star sensor and optronic sensor channels are disposed - 11 -substantially at right angles to one another, so as to allow grazing sightings of the optronic sensor channel while limiting the locating errors related to diffraction in the layers of the atmosphere.

The target acquisition device according to the invention comprises a star sensor channel having a matrix detector with an associated boresight and an associated field as well as an optronic sensor channel having a matrix detector with an associated boresight and an associated field. Preferably, to every pixel of a matrix detector of one of the channels there corresponds in a bijective manner a pixel of the field of said channel, said correspondence being effected by the optic of said channel situated upstream of said detector, said field pixel corresponding both to an angular portion of the space situated opposite the sensitive surface of said detector and to a pixel of the image of the space, obtained by said detector. The optronic sensor channel and the star sensor channel are preferably mechanically bound together in a sufficiently rigid manner for the boresight of the optronic sensor channel and the boresight of the star sensor channel to be immovable with respect to one another so that the relative position of the pixels of the field of the optronic sensor channel and of the pixels of the field of the star sensor channel is preserved. The relative position of the pixels is preserved if the amplitude of the tiny relative movement that may exist between the boresights of the various channels does not exceed, at the level of each detector field of one of said channels, the detector field of the other channel then being taken as reference, the size of half a pixel.

In the last -described preferential case of figure 3, the preferential preservation of the relative position of the pixels of the field of the optronic sensor channel and of the pixels of the field of the star - 12 -sensor channel is manifested by the maintaining of an existing correspondence between the pixels of the detector of the optronic sensor channel and pixels of the detector of the star sensor channel. The existing correspondence is maintained if for example when a given pixel pi of the detector of the optronic sensor channel corresponds to a given pixel pj of the detector of the star sensor channel, that is to say when the pixels pi and pj both correspond to the same angular portion of space, then pixel pi always corresponds to pixel pj regardless of the absolute movements of the aircraft, since the boresights of the various channels remain immovable with respect to one another. The existing correspondence is maintained if for example when a set of given pixels pk of the detector of the optronic sensor channel corresponds to a given pixel pj of the detector of the star sensor channel, then the set of pixels pk always corresponds to pixel pj regardless of the absolute movements of the aircraft, since the boresights of the various channels remain immovable with respect to one another, and vice versa.

This relative immovability of the boresight of one of the channels with respect to the boresight of the other channel makes it possible to mount the target acquisition device according to the invention on aircraft subject to considerably bigger disturbing torques than those with which the devices of the prior art were compatible without degrading the accuracy and without requiring a more enhanced stabilizing system than in the prior art .

Another cause of inaccuracy could be the smallness of the refresh rate of the star sensor channel, which refresh rate is generally smaller than the image rate of the optronic sensor channel. Thus, an image obtained by the star sensor channel at a given instant serves of course to reset the image obtained at the same instant by the optronic sensor channel but also several images - 13 -obtained by the optronic sensor channel at instants close to the given instant . In order to reduce or even eliminate this cause of inaccuracy, computation means carry out interpolations. In order to reduce, in a simple manner, the errors induced by the interpolations, the optronic sensor channel and the star sensor channel each preferably have an image rate, one of which is a multiple of the other. In order to eliminate, in a simple manner, the errors induced by the interpolations, the optronic sensor channel and the star sensor channel have the same image rate and are temporally synchronized. In this latter advantageous case, the similarity of the image rates between the various channels and their temporal synchronization makes it possible, without requiring interpolation computations, to preserve the accuracy obtained by associating with each image of the optronic sensor channel a temporally simultaneous image of the star sensor channel thus -allowing very accurate resetting of the image of the optronic sensor channel to the image of the star sensor channel, which image of the star sensor channel generally comprises several reference stars whose position in space is very accurately known by virtue of databases containing the ephemerides of these reference stars.

With or without interpolation computations, by virtue of the target acquisition device according to the invention, the accuracy of the angular position of a flying target visible on several images of the optronic sensor channel will be very high for all the images concerned, thus allowing very accurate reconstruction of the trajectory of the flying target. By way of example, with detectors of the order of 320 by 240 pixels, the accuracy of the angular position of a flying target may reach 30 μ^ά.

The target acquisition device according to the invention is preferably a tracking ball and the two 14 161,331/2 channels are advantageously integrated inside the ball. Thus, the rigidity of the mechanical link between the various channels may be obtained more easily. The target acquisition device according to the invention can also be integrated into a pod or even be disposed inside the aircraft on which it is mounted in the event of no pod.

The target acquisition device according to the invention possesses means for slaving the optronic sensor channel in a specified direction. This specified direction may be chosen in several ways.

In a first embodiment, the means for slaving the optronic sensor channel slave the boresight of the optronic sensor channel to the target. An advantage of this first embodiment is the absence of any velocity streak for the target, thereby improving the signal-to-noise ratio and allowing better locating of the target in the reference frame of the optronic sensor channel. A drawback of this first embodiment is the presence of velocity streak for the stars, thereby degrading the signal-to-noise ratio and giving rise to less good locating of the stars in the reference frame of the star sensor channel.

In a second embodiment, the means for slaving the optronic sensor channel slave the boresight of the optronic sensor channel to the stars in the sky. An advantage of this second embodiment is the absence of any velocity streak for the stars, thereby improving the signal-to-noise ratio and allowing better locating of the stars in the reference frame of the star sensor channel. A drawback of this second embodiment is the presence of velocity streak for the target, thereby degrading the signal-to-noise ratio and giving rise to less good locating of the target in the reference frame of the optronic sensor channel. In the case described subsequently of a single detector for both channels so - 15 -aligned, the velocity streak for the target is not as troublesome as the velocity streak for the stars, consequently this second embodiment is then preferable.

Figure 3 diagrammatically represents an exemplary preferential target acquisition device according to the invention. The target acquisition device according to the invention, represented in figure 3, is preferably intended for the acquisition of flying targets, that is to say aerial and space targets, and in particular flying targets deploying at high altitude. In the following description of figure 3, unless mentioned to the contrary, an optic may either designate a simple optical element such as for example a lens or may designate any complex optical combination comprising several optical elements. The target acquisition device according to the invention takes the form of a tracking ball 10 only a portion of which is represented in figure 3 for the sake of clarity. Once mounted on the surveillance aircraft, the ball 10 possesses at least two degrees of freedom in rotation with respect to the structure of the surveillance aircraft, so as to be able to scan a substantial portion of space such as for example a half-space. The block 1, called the tracker block 1, symbolically represents the rigidity of the mechanical link between the optronic sensor channel on the one hand and the star sensor channel on the other hand. The optronic sensor channel comprises, in succession from outside the ball 10 to inside the ball 10, a vertical filter 2 whose manner of operation will be explained subsequently, an entrance optic 3, a beam splitter 4, a focusing optic 5, a detector 6. The star sensor channel comprises, in succession from outside the ball 10 to inside the ball 10, a vertical filter 2 whose manner of operation will be explained subsequently, an entrance optic 3, a beam splitter 4, a focusing optic 7, a detector 8. The filter 2, the entrance optic 3 and the splitter 4, are common to both channels in figure 3, this not being compulsory but - 16 -preferable by virtue of the fact that an element common to both channels contributes by construction to improving the rigidity of the mechanical link between said channels. The beam f arriving from outside enters the ball 10, passing in succession through the filter 2 and the entrance optic 3 and arriving at the splitter 4 which is for example a simple semi-reflecting plate. The splitter 4 splits the beam f into two beams, a beam fo propagating towards the detector 6 of the optronic sensor channel and a beam fve propagating towards the detector 8 of the star sensor channel . The beam fo is focused by the focusing optic 5 onto the detector 6 while the beam fve is focused by the focusing optic 7 onto the detector 8.

Whether the boresights of the star sensor and optronic sensor channels are colinearly harmonized or not, the detector 8 of the star sensor channel is preferably disposed in such a way as to obtain a defocused image of the stars so that the image of each star covers several pixels of said detector 8. In the case of stars emitting a lot of light, accurate locating of the boresight of the star sensor channel with respect to the stars observed is thus easier to obtain.

The target acquisition device according to the invention preferably comprises at least one rectilinear polarizing filter which is disposed in such a way that, during the acquisition of a flying target, the horizontal rectilinear polarization component of the light that arrives at the detector of one of the channels, advantageously of each of the channels, is substantially more attenuated than the vertical rectilinear polarization component of the light that arrives at the detector of said channel, advantageously of each of the channels. This filter is called a vertical filter, figure 3 presents a vertical filter 2. The flying target is usually detected on an unstructured sky background, that is to say the lower - 17 -part of the field of the detector of the optronic sensor channel points above the cloud ceiling. Nevertheless, the light originating from the sun during the day and reflecting off the clouds or limbs is scattered by these clouds or limbs in the form of mainly horizontally polarized light. The function of vertical filter 2 is consequently to increase the signal-to-noise ratio by stopping, before its arrival at the detectors, the horizontally polarized light.

Preferably, at least a part of the boresight of the star sensor channel and a part of the boresight of the optronic sensor channel are aligned with one another and the preservation of the relative position of the pixels of the field of the optronic sensor channel and of the pixels of the field of the star sensor channel is manifested by the maintaining of an existing correspondence between the pixels of the matrix detector of the optronic sensor channel and pixels of the matrix detector of the star sensor channel.

The direction of observation of the detectors 6 and 8 then being the same, the optronic sensor and star sensor channels are consequently aligned, that is to say colinearly harmonized. Thus, in particular in the case of viewing angles that may be very grazing with respect to the atmosphere, by virtue of the auto-compensation for the dispersions in propagation over the path, said dispersions being related to the crossing of the various atmospheric layers, the sinuous trajectory of the light rays is no longer a source of significant locating errors. Even in figure 3, the optronic sensor and star sensor channels exhibit a common pupil, thereby further reducing the errors of harmonization between said channels. The optic of the common pupil may however be multifield and/or multispectral in the case where the detectors of said channels do not have the same field and/or do not have the same spectral domain of sensitivity. Said channels - 18 -may also have separate pupils, this being less advantageous at the level of the mechanical rigidity of the link between said channels, but this being simpler to implement in the case of detectors of different fields and/or of different spectral domains of sensitivity for the two channels insofar as this does away with a multifield and/or multispectral optic. Unless detectors having a very large number of pixels, for example several million, are used for better accuracy, the star sensor channel preferably possesses a larger field than that of the optronic sensor channel .

The preferential preservation of the relative position of the pixels of the field of the optronic sensor channel and of the pixels of the field of the star sensor channel is manifested by the maintaining of an existing correspondence between the pixels of the detector of the optronic sensor channel and pixels of the detector of the star sensor channel. Figures 4 and 5 diagrammatically represent schematics explaining this correspondence. A target 13, represented in the form of a right arrow, is contained in the field 12 of the optronic sensor channel. Several stars 16, respectively represented in the form of asterisks, are contained in the field 15 of the star sensor channel. In figure 4 and preferentially, the field 15 of the star sensor channel encompasses the field 12 of the optronic sensor channel. Figure 5 shows, with the aid of a curved arrow, the correspondence existing between one or more given pixels of the sensitive surface 120 of the detector 6 of the optronic sensor channel, represented in figure 5 by the zone 30 and representing an angular sector SA, in space, belonging to the field 12, and one or more given pixels of the sensitive surface 150 of the detector 8 of the star sensor channel, represented in figure 5 by the zone 31 and representing the same angular sector SA, in space, belonging to the field 15. The dashed lines on the sensitive surface 150 of figure 5 delimit a zone of pixels having corresponding counterparts on the sensitive surface 120. The mechanical link between the optronic sensor channel and the star sensor channel is sufficiently rigid that on the one hand, in the case where the zone 30 continues to represent the angular sector SA, then the zone 31 also continues to represent the angular sector SA, and that on the other hand, in the case where the zone 30 begins to represent another angular sector aSA, then the zone 31 begins also to represent this same angular sector aSA: accordingly, the relative offset between the field 12 and the field 15 should then not exceed either half the size of a pixel of the sensitive surface 120 of the detector 6, or half the size of a pixel of the sensitive surface 150 of the detector 8. The pixels of the detectors 6 and 8 do not necessarily have the same size, but this is preferable. In the preferable case where the field 15 is bigger than the field 12, on the one hand certain pixels of the sensitive surface 150 of the detector 8, such as the pixel 32, have no corresponding counterparts on the sensitive surface 120 of the detector 6, and on the other hand a group of several given pixels (or a function, such as linear combination for example, of the pixels of this group) of the sensitive surface 120 of the detector 6 of the optronic sensor channel corresponds to a single pixel of the surface 150 of the detector 8 of the star sensor channel. The adjusting of this correspondence may generally be done once and for all and not necessarily in the factory.

The target acquisition device according to the invention is generally rendered unaffected by heat so that the differences in temperature do not disturb the correspondence between the pixels of the detectors of the various channels. The material employed for this rendering unaffected by heat is for example INVAR. The target acquisition device according to the invention preferably takes the form of a ball integrating the - 20 -optronic sensor and star sensor channels. The ball, being relatively compact, may more easily be rendered unaffected by heat. The ball is for example around 400 mm in diameter.

Preferably, the respective detectors of the various channels have a spectral domain of sensitivity which if not identical is at least very similar. Thus, the correlation between the images provided by the various channels is easier to achieve at the level of the trajectory estimation system encompassing the target acquisition device according to the invention.

In order for the target acquisition device according to the invention to be able to preserve this accuracy of target locating equally well in the daytime mode of operation and in the nighttime mode of operation, the detector of the star sensor channel is chosen in such a way that its spectral domain of sensitivity is at least partially in the infrared, even if the infrared is situated in the tail of the emission spectrum of the stars, that is to say in a relatively weak part of this emission spectrum, the availability of detectors that are very sensitive in the infrared making it possible to substantially reduce this drawback; thus, the target acquisition device according to the invention circumvents the background noise constituted by solar scattering .

In order to remain more accurate and to more completely circumvent both solar scattering and the thermal flux of the optical elements of the star sensor channel, the target acquisition device according to the invention comprises a detector of the star sensor channel whose spectral domain of sensitivity comprises at least a part of the II band of the infrared.

Specifically, bands I and II of the infrared are better for the star sensor channel whereas bands II and III of - 21 -the infrared are generally better for the optronic sensor channel. In the case where the targets are ballistic missiles, for the optronic sensor channel, band II may be better at the start of the ballistic phase of the missile when the latter is still hot whereas band III may be better at the end of the ballistic phase of the missile when the latter is already cold. At least a part of band II of the infrared is preferably included within the domain of sensitivity of the detector of each of the channels. Thus, the target acquisition device according to the invention makes it possible to at least partly solve the paradox of being able "to see stars in full daylight while being close to the sun" . Moreover, in band II of the infrared, the thermal flux of the optical elements of the channels is small.

Preferably, the domain of sensitivity of the detectors of the various channels has a lower limit substantially equal to 1 μπι and an upper limit lying substantially between 2.5 μπι and 4 μιτι. The lower limit of substantially 1 μπι represents an optimal insofar as the background noise increases with solar scattering below this lower limit. Beyond an upper limit of substantially 4 μτη, the thermal flux of the optical elements of the channels as well as of the atmosphere increases substantially. Preferably, the detectors are detectors of high sensitivity such as MCT, InSb or MPQ. These detectors advantageously possess a minimum of 640 x 480 pixels.

A first option consists in using for the two channels a single detector instead of the two detectors described previously and shared out one per channel . In this case, the detector of the star sensor channel and the detector of the optronic sensor channel are merged into one and the same detector. To obtain very good accuracy of locating of the target, this single detector preferably possesses several million pixels.

A second option consists in using for each of the channels, two detectors instead of one detector. In this case, the star sensor channel possesses two detectors, preferably one sensitive in the visible and the other in at least a part of the infrared, and the optronic sensor channel possesses two detectors, preferably one sensitive in band II of the infrared and the other sensitive in band III of the infrared. The entrance optic of the optronic sensor channel at least or else, as appropriate, the common optic of both channels, is then multispectral. 23 161, 331/4

Claims

1. A target (13) acquisition device, intended to be mounted on an aircraft or on an aerostat, comprising an optronic sensor channel for the acquisition of a target comprising a detector (6) and means for slaving the boresight of the optronic sensor channel, the optronic sensor channel being reset to the stars of the sky so as to ensure the locating of the target, characterized in that the target (13) acquisition device comprises for said resetting a star sensor channel comprising a detector (8), and a rigid mechanical link between the boresights of the star sensor and optronic sensor channels.

2. The target acquisition device as claimed in claim 1, characterized in that the detectors (6, 8) of the star sensor and optronic sensor channels are matrix__detejrtp. s,,__ in that at least a part of the boresight of the star sensor channel and a part of the boresight of the optronic sensor channel are aligned with one another, and in that the mechanical link between the boresights of the star sensor and optronic sensor channels is sufficiently rigid to maintain the correspondence existing between the pixels (30) of the detector (6) of the optronic sensor channel and pixels (31) of the detector (8) of the star sensor channel.

3. The target acquisition device as claimed in claim 2, characterized in that the target acquisition device comprises a single entrance optic which is common to the star sensor and optronic sensor channels.

4. The target acquisition device as claimed in claim 1, characterized in that the boresights of the star sensor and optronic sensor channels are disposed substantially at right angles to one another. -24- 161,331/2

5. The target acquisition device as claimed in claim 4, characterized in that the field (15) of the detector (8) of the star sensor channel is large enough to always see at least two stars.

6. The target acquisition device as claimed in any one of the preceding claims, characterized in that the optronic sensor channel and the star sensor channel each have an image rate one of which is a multiple of the other.

7. The target acquisition device as claimed in claim 6, characterized in that the optronic sensor channel and the star sensor channel have the same image rate and are temporally synchronized.

8. The target acquisition device as claimed in any one of the preceding claims, characterized in that the detector (8) of the star senso channel is a matrix detector and in that said detector (8) is disposed in such a way as to obtain a defocused image of the stars so that the image of each star covers several pixels of said detector (8) .

9. The target acquisition device as claimed in any one of the preceding claims, characterized in that the target acquisition device is- intended to be able to operate by day and by night and in that at least a part of the II band of the infrared is included in the spectral domain of sensitivity of the detector (8) of the star sensor channel .

10. The target acquisition device as claimed in claim 9, characterized in that the spectral domain of sensitivity of the detector (8) of the star sensor channel has a lower limit substantially equal to 1 μτη 161,331/2 -25 - and an upper limit lying substantially between 2.5 μτη and 4 μπι.

11. The target acquisition device as claimed in any 5 one of the preceding claims, characterized in that the detector (8) of the star sensor channel and the detector (6) of the optronic sensor channel are merged into one and the same detector. 10

12. The target acquisition device as claimed in any one of claims 1 to 8, characterized in that the star sensor channel possesses two detectors, one sensitive in the visible and the other in at least a part of the infrared, and in that the optronic sensor channel 15 possesses two detectors, one sensitive in the II band of the infrared and the other sensitive in the III band of the infrared.

13. The target acquisition device as claimed in any 20 one of the preceding claims, characterized in that the means for slaving the optronic sensor channel slave the boresight of the optronic sensor channel to the target (13) . 25

14. The target acquisition device as claimed in any one of claims 1 to 12, characterized in that the means for slaving the optronic sensor channel slave the boresight of the optronic sensor channel to the stars (16) of the sky. 30

15. The target acquisition device as claimed in any one of the preceding claims, characterized in that the target acquisition device is a tracking ball (10) and in that the two channels are integrated inside the ball 35. (10) .

16. The target acquisition device as claimed in any one of the preceding claims, characterized in that the 26 161,331/3 target acquisition device comprises at least one rectilinear polarizing filter (2) which is disposed in such .a- way that,, duriag. the acquisition of. a flying target (13), the horizontal rectilinear polarization component of the light that arrives at the detector (6, 8) of at least one of the channels is substantially more attenuated than the vertical rectilinear polarization component of the light that arrives at the detector (6, 8) of said channel.

17. A surveillance aircraft, characterized in that the aircraft comprises a target acquisition device (10) as claimed in any one of the preceding claims.

18. The surveillance air-craft as claimed in claim 17, characterized in that the aircraft comprises a watching device (22) as well as means (24) of communication between the target acquisition device (10) and the watching device (22), said means (24) of communication allowing the target acquisition device (10) to lock on, successively over time, to several targets (13) and to regularly return to lock onto each of said targets (13) .

19. A surveillance aerostat, characterized in that the surveillance aerostat comprises a target acquisition device (10) as claimed in any one of claims 1 to 16.

20. The surveillance aerostat as claimed in claim 19, characterized in that the aerostat comprises a watching device (22) as well as means (24) of communication between the target acquisition device (10) and the watching device (22), said means (24) of communication allowing the target acquisition device (10) to lock on, successively over time, to several targets (13) and to regularly return to lock onto each of said targets (13) . 1 61 ,331/2 -27-

21. A target (13) trajectory estimation system, characterized in that the estimation system comprises a single aircraft as claimed in any one of claims 17 to 18 or a single aerostat as claimed in any one of claims 19 to 20, in that said aircraft or said aerostat comprises a telemeter and in that the estimation system comprises means (21) for reconstructing the trajectory of the target (13) from the images provided by the aircraft or by the aerostat and from the telemetry measurements.

22. A target (13) trajectory estimation system, characterized in that the estimation system comprises at least two surveillance aircraft as claimed in any one of claims 17 to 18 or two aerostats as claimed in any one of claims 19 to 20 and means (21) for reconstructing the trajectory of the target (13) by triangulation from the images provided by the various aircraft or by the various aerostats.

23. A defense system characterized in that it comprises an estimation system as claimed in any one of claims 21 to 22, and at least one interception aircraft to which is supplied at least a portion of the. trajectory estimated by the estimation system. Sanford T. Colb & Co. C: 51303