ARRANGEMENT FOR THE DETECTION OF TARGETS

Abstract: The invention relates to an arrangement for detecting approaching high-velocity targets using a rotatable optical detector (2, 3) of the line array type. With a view to the suppression of false alarms, the arrangement moreover comprises a hollow rotation axis (4) containing a mirror (14) that can be orientated such that a detector of the staring array type, a laser range-finder or a CW Doppler laser detector, all likewise contained in the hollow rotation axis, can be directed at a potential target.

Description: Arrangement for the detection of targets The present invention relates to an arrangement for the detection of targets, comprising a detector unit rotatable about an at least vertical search axis, provided with focusing means and a detector of the line array type for cyclically generating a two-dimensional image representing the surroundings of the arrangement, and a target extractor connected to the detector for generating directions of detected potential targets. An arrangement of this type is known from EP-B 0.205.794. The aim of the target extractor is to reduce the number of false alarms generated by a detector unit. Without the use of a target extractor, a detector unit will usually generate thousands of false alarms per revolution. If the target extractor possesses all features known in the art, this number can be reduced to only a few false alarms per revolution. By incorporating several detectors in the detector unit and by subsequently combinating the images generated by these detectors, the number of false alarms can still be further reduced. The present invention is likewise aimed at reducing the number of false alarms, namely by subjecting the individual detected potential targets to a closer examination. It is thereto characterized in that the arrangement furthermore comprises optical verification means for verifying the detected potential targets, which means are controlled on the basis of data generated by the target extractor. In a favourable embodiment of the invention, the verification means form an integral part of the detector unit, but are incapable of rotation. They are characterized in that an optical axis of the optical verification means at least substantially coincides with the search axis. In an advantageous realisation, the detector unit is positioned such that it is arranged for rotating around a hollow search axis, the optical verification means being contained in the hollow search axis. A further advantageous embodiment of the invention is characterized in that the optical verification means comprise an optical window provided in a wall section of the hollow search axis that projects beyond the detector unit, and in that the optical verification means comprise a mirror, contained in the hollow search axis, an azimuth direction and an elevation direction of which can, on the basis of data generated by the target extractor, be adjusted in such a way that radiation emanating from a selected potential target is passed on at least substantially parallel to the search axis. A still further favourable embodiment of the invention has the advantage that the optical window is flat and is characterized in that the optical window can, on the basis of data generated by the target extractor, be adjusted in azimuth direction in such a way that its azimuth direction at least substantially corresponds with the mirror's azimuth direction. It is now possible to provide the optical verification means with various optical means which can be accommodated in the hollow search axis. In a first favourable embodiment, the optical verification means comprise an optical detector of the staring array type. Such a detector is highly suitable for verifying a potential target, since it generally allows longer integration periods and has a higher angular resolution than a rotary line array selected for volume search. In a second favourable embodiment, the optical verification means comprise a laser range-finder which, besides a target verification, also yields a target range. In a third favourable embodiment, the optical verification means comprise a CW Doppler laser detector which, besides a target verification, moreover yields a target velocity and at least an indication of the range. An exceptionally favourable embodiment of the apparatus is characterized in that means are provided for optionally placing, in operating conditions, a detector of the staring array type, a laser range-finder or a CW Doppler laser detector in the hollow search axis. This renders it possible to select and position the optimal verification means depending on the operational conditions. By giving the hollow search axis sufficiently wide dimensions, several optical verification means can be used simultaneously. The invention will now be further explained in greater detail with reference to Fig. 1 which is a schematic representation in cross-section of an embodiment of the arrangement. Fig. 1 shows an embodiment of the arrangement in cross- section, in which a detector unit 1 comprising two detectors 2, 3 rotates about a hollow axis 4 which is rigidly attached to a base 5, for instance a deck. Rotation is made possible by known provisions such as drive means with integrated bearing 6, additional bearing 7, a rotary joint 8 for the connection of the supply voltages and the supply of the command signals to detector unit 1 and for the output of signals generated by the detectors 2, 3. An essential condition is that within the hollow search axis 4, a cavity 9 remains available to incorporate the optical means to be described below, which means form part of the optical verification means the optical axis of which at least substantially coincides with the longitudinal axis 10 of the hollow search axis 4. The upper part 11 of hollow search axis 4 may rotate with respect to search axis 4 by means of additional bearing and drive means 12. The upper part 11 furthermore comprises a flat window 13 and a mirror 14 which is tiltable about an axis of tilt 15, the tilting movement being controlled by means of a linear drive 16. If the rotating detectors 2, 3 observe a potential target, which observation is essentially achieved by a target extractor connected to the detectors through rotary joint 8, the extractor generates a direction of the potential target from which control signals for the drive means 12 and linear drive 16 are derived such that the potential target can be observed along the longitudinal axis 10 of hollow search axis 4 via flat window 13 and mirror 14. During target observation, flat window 13 and mirror 14 are at least substantially stationary. Alternative solutions are to control flat window 13 and mirror 14 by means of a track computer connected to the extractor so that a predicted or measured target trajectory is followed, or to control flat window 13 and mirror 14 on the basis of available roll and pitch signals in such a way that the own motion of the platform or vessel on which the arrangement is mounted is compensated for in a way known in the art. It is now possible to provide the optical verification means with various optical means to be located in cavity 9. Irrespective of the rotation of detector unit 1 and the vehicle's own motion, this renders it possible to distinguish between a real target and a false alarm. A camera provided with a detector of the staring array type is highly suitable for this purpose. Preference should be given to an infrared-sensitive array since it matches the spectral sensitivity of detectors 2, 3 to a greater extent and can moreover be used at night or in fog conditions. An advantageous selection of the known systems of lenses disposed in front of the array results in a considerably higher angular resolution than that of detectors 2, 3. In order to increase the sensitivity, the integration period of the array can additionally be selected such that it is considerably longer than the integration period of detectors 2, 3. Particularly a combination of both factors enables target verification at all times without the reaction time of the arrangement being significantly affected. Apart from unambiguously determining whether the detection yields a real target or a false alarm, it is usually possible to observe details that are usually imperceptible for detectors 2, 3 such as the type of aircraft or vessel, the number of persons in a rubber dinghy and the like. The drawback to this is that the image, depending on the position of mirror 14 together with upper part 11, may rotate in bearing. This drawback can be overcome in a manner known in the art by incorporating a Dove prism in cavity 9 which, by gear means also fitted in cavity 9, is continuously rotated through half the bearing angle. A further, exceptionally suitable verification means is obtained by installing a laser range-finder, as such known in the art, in cavity 9. The strength of an echo reflected by a potential target generally indicates whether or not a real target is involved; moreover, the target range can be directly derived from the delay time. A likewise exceptionally suitable verification means is obtained by installing a CW Doppler laser detector in cavity 9. A monochromatic laser beam is generated by for instance a C02 laser and is directed to the potential target via mirror 14 and flat window 13. A part of the beam of light is reflected and directed, together with a reference beam derived from the laser beam, to a detector positioned close to the laser via flat window 13 and mirror 14. The Doppler signal generated by the detector can then be used to determine the dimension and range of the potential target. By linearly modulating the laser in frequency, it is moreover possible to generate range information in a manner known in the art. The disposition of the above-mentioned and possible other optical means in a uniform housing allows the selection of an optical verification means depending on the operational conditions. This can be effected both manually and automatically, for instance by accommodating the various optical means in an exchange unit. An added advantage of the arrangement is that cavity 9 is capable of accommodating other suitable optical instruments, such as a TV camera to be used as an auxiliary navigation tool when entering a port. Also in this case, a Dove prism incorporated in cavity 9 will prevent the image from rotating in bearing when mirror 14 rotates together with upper part 11. A TV camera thus disposed takes advantage of the fact that the arrangement is usually located on the most favourable place of the ship.

Claims: Claims : 1. Arrangement for the detection of targets, comprising a detector unit rotatable about an at least vertical search axis, provided with focusing means and a detector of the line array type for cyclically generating a two-dimensional image representing the surroundings of the arrangement, and a target extractor connected to the detector for generating directions of detected potential targets, characterized in that the arrangement furthermore comprises optical verification means for verifying the detected potential targets, which means are controlled on the basis of the data generated by the target extractor. 2. Arrangement as claimed in claim 1, characterized in that an optical axis of the optical verification means at least substantially coincides with the search axis. 3. Arrangement as claimed in claim 2, characterized in that the detector unit is arranged for rotating around a hollow search axis and in that the optical verification means are at least substantially accommodated in the hollow search axis. 4. Arrangement as claimed in claim 3, characterized in that the optical verification means comprise an optical window provided in a wall section of the hollow search axis that projects beyond the detector unit. 5. Arrangement as claimed in claim 4, characterized in that the optical verification means comprise a mirror, contained in the hollow search axis, an azimuth direction and an elevation direction of which can, on the basis of a direction generated by the target extractor, be adjusted in such a way that radiation emanating from a selected potential target is passed on at least substantially parallel to the search axis. 6. Arrangement as claimed in claim 5, characterized in that the optical window can, on the basis of a direction generated by the target extractor, be adjusted in such a way that its azimuth direction at least substantially corresponds with the mirror's azimuth direction. 7. Arrangement as claimed in claim 5, characterized in that the optical verification means comprise an optical detector of the staring array type, an optical axis of which at least substantially coincides with the search axis. 8. Arrangement as claimed in claim 5, characterized in that the optical verification means comprise a laser range- finder, an optical axis of which at least substantially coincides with the search axis. 9. Arrangement as claimed in claim 5, characterized in that the optical verification means comprise a CW Doppler laser detector, an optical axis of which at least substantially coincides with the search axis. 10. Arrangement as claimed in claim 5, characterized in that means are provided for optionally placing, in operating conditions, a detector of the staring array type, a laser range-finder or a CW Doppler laser detector in the hollow search axis.

Inventors: OLBERTZ ANTONIUS HENDRICUS MAR, OLBERTZ, ANTONIUS, HENDRICUS, MARIA
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, OLBERTZ ANTONIUS HENDRICUS MAR, HOLLANDSE SIGNAALAPPARATEN B.V., OLBERTZ, ANTONIUS, HENDRICUS, MARIA
Application year: 1996
Main classification: G01S3
IPC codes: G01S3, G01S17