A LASER THREAT WARNING SYSTEM

Abstract: A laser threat warning system particularly for ship defence, in which directional sensors are directed to nearer and farther bands or swathes of the sea surface. Scattering of an enemy laser beam from points in the two bands enables the direction of the laser source to be determined.

Description: This invention relates to a laser threat warning system. In moern warfare a laser beam may itself constitute a weapon or may be the bearing and range-finding prelude to an attack by other weapons. In either event it is highly desirable that the presence of a laser beam and its source direction be detected as soon as possible so that a pre-emptive attack can be made if considered necessary or evasive action taken. A number of laser threat warning receiver architectures and designs have been previously disclosed. However, because of the rapid decrease of intensity in the ′skirts′ of a laser beam, even a highly sensitive receiver will not normally detect a beam which is targeted more than a very few metres from the receiver. Thus, a single receiver may be sufficient to protect a small vehicle such as a tank or helicopter, but a ship or large ground installation would require an inconveniently large number of receivers (with attendant unacceptable complexity and cost) to provide adequate threat warning capability. An object of the present invention is thus to provide a system which can watch a relatively large sweep of territory, on land or sea, with relatively few, or even only one, receiver. According to the present invention, a laser threat warning system comprises sensor means providing directional discrimination, the sensor means being directed at a local extended scattering body so as to receive laser radiation scattered from the body and comprising an array of sensor elements and an optical focussing system defining relatively near and far swathes of elemental areas of the body extending transversely between the sensor means and the source of a potential threat, the sensor means being simultaneously receptive to radiation scattered from elemental areas of the relatively near and far swathes, the directions of the near and far scattering elemental areas determined from a particular laser beam scattering providing an indication of the direction of the source of the laser beam. According to another aspect of the invention, in a method of determining the direction of the source of a laser threat, sensor means providing directional discrimination are directed at a local extended scattering body, the sensor means comprising an aray of sensor elements and an optical focussing system defining relatively near and far swathes of elemental areas of the body extending transversely between the sensor means and the source of a potential threat, the particular elements of the swathes from which scattered radiation is received are identified, and the direction of the source is determined from the identity of the identified elements. There may be more than two of the near and far swathes each of which is relatively nearer to or farther from the sensor means than each other swathe. When the surface is substantially horizontal, the array of sensor elements comprise a respective linear array for each swathe, each linear array being curved in the focal plane of the optical system so that the swathes are of arcuate form around the sensor means. The sensor means may be mounted on a ship and directed downwardly but having no view of the ship's structure. There may be included a plurality of the sensor means, each providing a plurality of swathes which together substantially surround the ship. There may also be included means for dispensing a cloud of aerosol to constitute the scattering body. There is preferably included processor means arranged to receive data from the sensor means and determine therefrom the direction of the source of the laser beam. A laser threat warning system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a perspective view of a ship carrying the system; Figure 2 is a plan view of the ship of Figure 1 showing swathes of scatter areas on the sea surface around the ship; Figure 3(a) shows a laser warning receiver mounted on the side of a mast of the ship and Figure 3(b) shows, diagrammatically, the constitutents of the laser warning receiver. Referring to Figures 1 & 2 of the drawings, four laser warning receivers 1, 2, 3 and 4, each providing sensor means, are mounted high up on masts 5 and 6 of the ship and directed at the sea surface around the ship. If more convenient the laser warning receivers can be mounted on pylons attached to the masts. The receiver is directed downwardly as shown in Figure 3(a). As shown in Figure 3(b) each laser warning receiver includes two linear arrays 7 and 9 of sensor elements responsive to the expected laser frequency. The two linear arrays, in conjunction with an optical system shown for simplicity as a single lens 11, define two swathes of elemental areas on the sea surface, 13 and 15 for receiver 1, and 17 and 19 for receiver 3 (the remaining swathes not being shown). The receiver optics are provided with baffling 12 providing protection against direct laser illumination and also to limit the area of the scattering surface. Each swathe comprises a large number, for example, one hundred, elemental areas in a single line, each elemental area being defined by the optical system and a corresponding sensor element of one of the arrays 7 and 9. Thus any radiation scattered from a particular elemental area of the sea surface within one of the swathes will be focussed on a corresponding one of the sensor elements and the particular elemental area can be identified by the particular sensor element activated. With a large number of sensor elements and corresponding small extent of each elemental area the ′scatter′ will not be from one elemental area exclusively. The centre point of the scatter is then obtained by comparing the energy received by eaCh sensor element against a threshold to eliminate virginal effects and then averaging the position of the remaining axtivated sensors. From the identity of the two scatter centres, such as the pixels (elemental areas) A and B of Figure 2, the direction of the source of the laser beam can be determined easily by a processor 21 coupled to the sensor array (with intermediate pre-amplifiers and interface electronic circuitry 23). Clearly, the direction finding accuracy improves with the number of sensor elements in the array 7, 9. An accuracy of 3° to 5° is quite practicable. While two swathes are illustrated above for each sensor, there may be three or more, provided by a corresponding increase in linear arrays. The further swathes give ′redundant′ information and correspondingly improved estimates of threat direction. The system is particularly suited to naval operation, where the threat source is likely to be distant, the source low on the horizon and the level of scattered laser energy consequently significant. Alternatively however, the system may be used to protect a land based installation, the local extended surface then being the surrounding ground area. In any event, a significant advantage of the system is that the sensor array is not directed toward the potential threat but obliquely to it so as to receive only scattered and not direct energy. The sensor elements are not therefore subject to damage from high energy levels. While the system has been described for use with an existing natural feature (the sea or land surface) constituting the scattering body, in certain circumstances it may be desirable to dispose an artificial ′body′ to provide the scattering effect. Thus an aerosol cloud by be disposed in a suitable position offset from the supposed enemy direction by means of rockets for very rapid disposal, or by periodic dispensing from a helicopter.

Claims: 1. A laser threat warning system characterised by sensor means (1, 2, 3 & 4) providing directional discrimination, the sensor means being directed at a local extended scattering body (13, 15, 17 & 19) so as to receive laser radiation scattered from the body and comprising an array of sensor elements (7/9) and an optical focussing system (11) defining relatively near (13, 17) and far (15, 19) swathes of elemental areas of said body extending transversely between the sensor means (1, 3) and the source of a potential threat, said sensor means (1, 2, 3 & 4) being simultaneously receptive to radiation scattered from elemental areas (B, A) of said relatively near (13, 17) and far (15, 19) swathes, the directions of said near (B) and far (A) elemental areas determined from a particular laser beam scattering providing an indication of the direction (S) of the source of the laser beam. 2. A system according to Claim 1, characterised in that there are more than two of said near (13, 17) and far (15, 19) swathes each of which is relatively nearer to or farther from the sensor means (1, 3) than each other swathe. 3. A system according to Claim 1 or Claim 2, characterised in that said array (7, 9) of sensor elements comprises a respective linear array (7, 9) for each swathe (13, 15), and each linear array (7, 9) is curved in the focal plane of the optical system (11) so that the swathes (13, 15) are of arcuate form in a horizontal plane around the sensor means (1). 4. A system according to Claim 3, characterised in that said sensor means is mounted on a ship and directed downwardly but having no view of the ship's structure. 5. A system according to Claim 4, characterised by a plurality of said sensor means (1, 2, 3 & 4), each providing a plurality of said swathes (13, 15, 17 & 19) which, together, substantially surround the ship. 6. A system according to Claim 1 or Claim 2, characterised by means for dispensing a cloud of aerosol to constitute said scattering body. 7. A system according to any preceding claim, including processor means (21) arranged to receive data from said sensor means (7/9) and determine therefrom the direction (S) of the source of the laser beam. 8. A method of determining the direction of the source of a laser threat, characterised in that sensor means (1, 2, 3 & 4) providing directional discrimination are directed at a local extended scattering body (13, 15, 17, 19), said sensor means comprising an array of sensor elements (7/9) and an optical focussing system (11) defining relatively near (13, 17) and far (15, 19) swathes of elemental areas of said body extending transversely between the sensor means (1, 3) and the source of a potential threat, the particular elements (A, B) of said swathes from which scattered radiation is received are identified, and the direction of said source is determined from the identity of the identified elements (A, B). 9. A method according to Claim 8 for determining the direction of a laser threat to a ship, wherein said sensor means (1, 2, 3 & 4) are directed to define swathes (13, 15, 17 & 19) of said elemental areas on the sea surface.

Inventors: COLEMAN, CLIVE IAN
Applicants: GEC-MARCONI LIMITED
Application year: 1987
Main classification: G01S3
IPC codes: G01S3, G01S7