Abstract:
There is provided a system for detecting distant seaborne objects by an airborne vehicle, including a seeker head having an axis in the direction of flight, a sensor mounted on the seeker&#39;s head, the sensor being operative to transmit towards the sea surface a laser radiation beam of selected wavelength and to receive from the sea water surface radiation reflected from the sea water surface and from a seaborne object, and a computing unit for differentiating between the reflection received from the sea water surface and from the seaborne object. A method for detecting distant seaborne objects by an airborne vehicle is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a PCT National Phase of International Patent Application No. PCT/IL2009/000881 filed Sep. 10, 2009, which claims priority from Israel Application No. 194029 filed in the Israel Patent Office on Sep. 11, 2008, the entire disclosures of which are hereby incorporated by reference in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates to a system and method for detecting distant objects, and more particularly to a system for remote identification and determination of the location of a seaborne vehicle, e.g., a boat. More specifically, the present invention is also concerned with an airborne projectile or missile (hereinafter a “seeker”) head bearing such a system, which may advantageously be equipped with proximity sensors for timely triggering an explosive. 
     BACKGROUND OF THE INVENTION 
     Various lasers are used today as proximity detectors for target seeker heads. Most of the employed systems are fast range finders determining the distance from the seeker head to the target, and when this distance is below a predetermined value, the proximity detectors activate the explosive material. 
     The existing proximity detection systems operate effectively in an airborne atmosphere, where no adjacent bodies generate reflections of the laser range finder. In ground applications, greater care must be taken not to be triggered by dust or clatter, and hence targets are identified by their size and shape or by other means such as temperature and magnetism. 
     The location of small boats on the sea water surface is a challenge due to the small dimensional features of the boats which are of the same magnitude as the sea waves, and thus, in such cases, a novel approach is required. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a system for detecting distant seaborne objects by an airborne vehicle, comprising a seeker head having an axis in the direction of flight, at least one sensor mounted on said seeker&#39;s head, said sensor being operative to transmit towards the sea surface a laser radiation beam of selected wavelength and to receive from the sea water surface radiation reflected from the sea water surface and from a seaborne object, and computing means for differentiating between the reflection received from the sea water surface and from the seaborne object. 
     The invention further provides a method for detecting distant seaborne objects by an airborne vehicle, comprising providing a system according to claim  1 , irradiating said sea water surface with at least one laser beam of a predetermined selectable wavelength, receiving by radiation detectors radiation reflected from the sea water surface and possibly from a seaborne object and converting received radiation into corresponding computable signals, and computing the received signals to differentiate between the reflection received from the sea water and from a seaborne object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood. 
       With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purpose of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
       In the drawings: 
         FIG. 1  is a schematic representation of a seeker head emitting signals impinging on a sea surface, in accordance with the present invention; 
         FIG. 2  is a schematic representation of a seeker head emitting signals, impinging on a seaborne object, according to the embodiment of  FIG. 1 ; 
         FIG. 3  is a front view of the two sets of proximity sensors shown in  FIGS. 1 and 2 , in accordance with the present invention, and 
         FIGS. 4A to 4D  illustrate intensity versus time curves of reflected pulses as received by the seeker&#39;s head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the Figures in detail illustrating a preferred embodiment of a laser object seeker system, and first referring to  FIGS. 1 and 3 , there are shown schematic representations of a seeker head  2 , e.g., a cylindrical, tapered or conical head, moving forward in direction  4 , also constituting the seeker&#39;s head axis. On the seeker head  2  there are located a plurality of laser proximity sensors (PS&#39;s)  6  to  6 ″″ and  8  to  8 ″″ (see  FIGS. 1 and 3 ) pointing sideways into the sea water  10 . One or more of the PS&#39;s  6  to  6 ″ emit a laser beam  12  in the direction of the sea water  10 . The laser wavelength of the beams is selected so as not to penetrate the sea water  10  and to be reflected from the sea surface  14 , back in direction  16  into at least one of the dedicated detectors of the PS&#39;s  6  to  6 ″. Further seen are a plurality of laser PS&#39;s  8  to  8 ″″ pointing sideways and emitting laser beams  18  in the direction of the sea water  10 . The laser wavelength of the beams of these PS&#39;s is selected to penetrate the sea water  10 , to be partially reflected from the sea water surface  14  and partially reflected by the sea bed  20 , back in directions  22  and  24  respectively, into dedicated detectors of PS&#39;s  8  to  8 ″″. As the reflected amplitude and time of the beams are known, they are used to determine the various reflection distances, as per-se known. In this embodiment, the PS  6 ′ serves as a reference measuring distance to the surface  14  and PS  8 ′ measures the depth of the sea bed  20 . While in the shown embodiment, the PS&#39;s point sideways, in other embodiments the PS&#39;s may be arranged on the seeker&#39;s head  2  to point at a downwardly angle forwards in the direction of flight. Also, the seeker&#39;s head  2  may be rotatable about its axis. 
     Computing means for differentiating between the reflections received from the sea water surface, possibly from the sea bed and from a seaborne object may be included in the seeker&#39;s head or in any other part of the airborne vehicle. 
       FIG. 2  illustrates a schematic representation of the seeker head  2 , moving forward in direction  4  and having a plurality of PS&#39;s  6  to  6 ′ and  8  to  8 ″″ pointing sideways. At least one of the PS&#39;s  6  to  6 ′ emits a laser beam  12  in the direction of the sea surface  14  where seaborne object  26 , e.g., a boat is present. The laser wavelength is selected not to penetrate the seawater  10  and to be reflected in direction  16  from the sea water surface  14  or from the seaborne object  26  into the dedicated detectors of PS&#39;s  6  to  6 ′. A plurality of laser PS&#39;s  8  to  8 ″″ point sideways and emit laser beams  14  towards the sea water  10 . The laser wavelength of the beams is selected to penetrate the sea water  10  and to be partially reflected in direction  22  from sea water surface  14 , and sea bed  20 , into dedicated detectors of the PS&#39;s  8  to  8 ″″. As can be seen, in this case, however, the reflection surface of the beam is the reflection from the seaborne object  26  in direction  22 . The reflected amplitude and time of the beam determine the various distances, as per-se known, with the PS  6  serving as a reference measuring distance to the water and the PS  8  measuring the sea depth. 
     In  FIG. 4A  there is illustrated a signal produced by detectors of PS&#39;s  6  to  6 ″″, while  FIG. 4B  shows the signals produced by the detectors of PS&#39;s  8  to  8 ″ for sea water. 
     In  FIG. 4C  there is illustrated signal produced by the detectors of PS&#39;s  6  to  6 ″″ while  FIG. 4D  shows a signal produced by the detectors of PS&#39;s  8  to  8 ′, which in this case, have equal timing and distance received from a seaborne object. 
     The methods of operating the systems described with reference to  FIGS. 1 to 4  will now be briefly described. 
     According to an embodiment of a method of operation of the present invention, a single laser beam is used, where the laser beam is selected to enhance or reduce seaborne object laser reflection in comparison with the sea waves. 
     According to a further embodiment of a method of operation of the present invention, there is provided a laser beam that penetrates the sea water, e.g., green laser light, wherein the impingement on the seaborne object results in a single reflection as compared with a double reflection, one from the sea water surface and one from the sea bed, when the laser radiation impinges on the sea waves. 
     A further method of operation of the present invention provides for two laser beams which are selected such that one penetrates the sea water, e.g., green laser light, where the impingement of the laser radiation on the seaborne object forms a single reflection as compared with a double reflection, one from the sea surface and one from the sea bed, when laser radiation impinges on the sea waves, and a second beam of a non-penetrating wavelength, e.g., a red laser, serving as a reference, where the impingement on the seaborne object or the sea waves results in a single reflection. 
     According to still a further method of operation of the present invention, two laser beams are selected, one that penetrates the water, e.g., green laser light, and a second, non-penetrating wavelength, e.g., a red laser, where the two laser beams operate simultaneously and share an equivalent optical path using the same single detector, wherein each of the laser beams is modulated by a different RF carrier. The detector&#39;s output RF signals are used to differentiate between the seaborne object and the sea, and the impingement on the object will result in a similar reflection pattern for both wavelengths (displayed by similar, highly correlated temporal envelopes of received RF signals), whereas the impingement from the sea will show variations in the reflection pattern between the two wavelengths, displayed by a lower correlation between the correspondingly received RF signals. 
     A further method of operation of the present invention provides for multiple laser proximity detectors placed on the circumference of the seeker&#39;s head, as shown in  FIG. 3 , enabling detection by a rotating seeker and either being redundant, or alternatively, enabling higher computation speed. 
     According to a further method of operation of the present invention, laser proximity detectors based on time-of-flight are placed on the seeker, and are continuously utilized. Similarly, laser proximity detectors based on phase detection may be placed on the seeker&#39;s head, or laser proximity detectors based on triangulation computations, may be utilized. 
     A further method of operation according to the present invention calls for laser proximity detectors data to be analyzed and processed by an on-board computing system and dedicated algorithms, and finally, according to yet a further method, multiple laser proximity detectors are oriented such as to impinge on the sea water surface at a distance higher than the target size, where the similarity or dissimilarity between the signals is used to distinguish between sea water and a target. 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.