Abstract:
An alarm system for military objects of a laser-radiating hostile weapon system. A plurality of units for detecting impinging laser radiation are mounted on an airplane, for example. Each detection unit is arranged to detect in one of six directions along three mutually perpendicular axes. The detection units are coupled to output signals representing the direction of incoming laser radiation to processing electronics. Each detection unit has an array of detector elements in a first plane and a diaphragm equal in size to the array in a second plane parallel to the first plane. The diaphragm and array are arranged so that the direction of the laser radiation can be determined from the pattern of detector elements activated in response to laser radiation passing through the diaphragm.

Description:
FIELD OF THE INVENTION 
     The invention relates to an alarm system for military objects for dectecting laser-radiating hostile weapon systems. 
     Such a detection system is shown in DE-AS No. 2852224 for the visible and near-IR region, that is, for an Nd-YAG laser with a wavelength of 1.06 microns. 
     The object of the invention is to improve the known system to discriminate the greatest number of solid angle intervals with the fewest possible individual detector elements and a space-saving concept. 
     A plurality of units for detecting impinging laser radiation are mounted on an airplane, for example. Each detection unit is arranged to detect in one of six directions along three mutually perpendicular axes. The detection units are coupled to output signals representing the direction of incoming laser radiation to processing electronics. Each detection unit has an array of detector elements in a first plane and a diaphragm equal in size to the array in a second plane parallel to the first plane. The diaphragm and array are arranged so that the direction of the laser radiation can be determined from the pattern of detector elements activated in response to laser radiation passing through the diaphragm. 
     By such an arrangement, the employment of a lens can be avoided, thereby resulting in an economic and space-saving construction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the invention will be described in detail with reference to the drawings, wherein: 
     FIG. 1 is a sectional view in azimuth (respectively, elevation) through the solid sphere and a cube inscribed in the sphere for the purpose of subdivision. 
     FIGS. 2a-2c are side views of the laser radiation, diaphragm and detector array (left-hand side) and front views of the array and cross section of the laser radiation (right-hand side) for different angles of incidence. FIG. 3 shows the coordinates of the direction vector connecting the middle point of the diaphragm with the middle point of the cross section of the laser radiation taken in the plane of the detector array. 
     FIG. 4a shows a side view of a two-channel array, each channel having a two-color sandwich. 
     FIG. 4b shows a front view of a four-channel array. 
     FIG. 5 shows a perspective view of a plurality of detection units arranged on a cube. 
     FIG. 6 shows the arrangement of a plurality of detection units on an airplane. 
     FIG. 7 shows a 5×5 array of detector elements. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to determine the direction of the laser radiation 7, which can enter over the entire sphere, the entire sphere is subdivided into six regions. 
     The solid angle of the six regions correspond to the six side faces 2 of the cube 1. The corresponding great circle of the sphere is accordingly subdivided in azimuth and elevation into four 90-degree sections, as illustrated in FIG. 1. Each of the six regions is further subdivided into 5×5 subregions each being 18° azimuth and elevation. An arrangement as shown in FIG. 2 is used as direction detector for each of the six regions. The detector comprises in principle a diaphragm 4 with a quadrate, in particular, a square, window, a detector array 5 comprising n D  ×n D  detector elements 6 as well as associated processing electronics (not shown in the drawings). The variable n D  is preferably set equal to three. If the array dimensions x M  and y M  in the x and y directions are respectively defined as a multiple n M  of the detector diminsions x D  and y D  respectively, that is, x M  =n M  ·x D  and y M  =n M  ·y D , then the predetermined window dimensions x B  and y B  are given by the relations x B  =n B  ·x D  and y B  =n B  ·y D  for the purpose of maintaining uniform discriminating intervals, wherein n B  =n M  +n A  -1 and n A  =(n M  -n D )/2 for the 3×3 array. The variable n A  represents the relationship of the interval x A  between detector elements and x D . A corresponding relationship applies in the Y direction. The invention is not limited to this embodiment. For other values of n D , corresponding measures apply. 
     The square window of diaphragm 4 allows passage of a square cross section of the incoming laser radiation. The position of the middle point of this cross section in the detecting plane parallel to the diaphragm plane is a measure of the laser radiation direction. FIG. 2A depicts the case where α=ε=0 degrees. With an angle of incidence of the laser radiation 7 such that azimuth α=0 and elevation ε=0, the cross section 8 of the laser radiation covers all nine detector elements 6, that is, elements a-i. Each of the detector elements is connected to a logic circuit. In response to all of the detector elements being activated, the directional data α=ε=0 degrees is recorded by the logic circuit. 
     FIG. 2b depicts the arbitrary case wherein α=0 degrees, ε=17 degrees. The cross section 8 of the laser radiation is correspondingly displaced, so that the detector elements 6, that is, elements d-i, supply directional data signals with the precision to be discussed below. 
     FIG. 2c depicts the boundary case where α=0 degrees, ε=45 degrees. In this case none of the detector elements are activated, while shortly before, that is, when α=0 degrees, ε=44 degrees, elements g, h and i each supply a signal. 
     Corresponding cases apply for arbitrary values of angles α and ε smaller than or equal to 45 degrees. A precise study of this arrangement shows that five ranges of the azimuth and elevation angles can be descriminated. Consequently, a directional resolution of 90/5=18 degrees in azimuth and elevation is possible. 
     The direction vector r from the middle point of the diaphragm 4 to the middle point of the cross section 8 of the laser radiation in the plane of the detectors, depicted in FIG. 3, is defined by the relationships: 
     
         r=(x.sup.2 +y.sup.2 +z.sub.2)1/2 
    
     
         tan α=x/z 
    
     
         tan ε=y/(x.sup.2 +z.sup.2)1/2 
    
     wherein x, y and z are the coordinates of the direction vector. 
     The nonlinear relationship between α and x, that is between x and y on the one hand and ε on the other hand, causes a certain, yet relatively small deviation of the corresponding solid angle interval from the desired theoretical value mainly in the diagonal peripheral area of the array formed by the plurality of detector elements 6. This can, for example, be corrected by altering the height y D  and width x D  (FIG. 2c) of all detector elements 6 arranged at the corners of the array. A second possibility entails extending the array by adding four additional detector elements along the two diagonals. 
     Ultimately, a 5×5 element array can be employed. Such a 5×5 array is depicted in FIG. 7, with detector elements 6 being designated by letters a through y. With such an array, (2·5-1) 2  =81 solid angle intervals can be discriminated if the size of the diaphragm conforms to the size of the 5×5 element array. However, if the window size corresponds to that used with the 3×3 element array, (2·3-1) 2  =25 solid angle intervals can be discriminated as before, but now the α and ε corrections are inherently realized by the extension of the detector arrangement. For the sake of completeness, it should be mentioned that a detector array can be employed in which the interval between elements is not, as indicated in FIG. 2, equal to the element dimension, but rather equal to n A  ·x D  and n A  ·y D  respectively, where n A  &lt;1 or substantially greater than one. 
     The above described concept of direction detection by means of a detector array and a diaphragm having a window of a size relatively greater than that of an individual detector element makes it possible to discriminate relatively many solid angle intervals (that is, (2n D  -0.1) 2 ) without discontinuity using an n D  ×n D  array of relatively small detector elements. 
     The diaphragm 5, which here serves the function of a lens, need not be greater in size than the detector array itself. Therefore for different spectral ranges (not diagrammatically depicted) a plurality of channels can be constructed relatively close to each other, so that different types of lasers can be detected. In order to reduce the number of different wavelength range channels, a so-called two-color array in a sandwich construction is used for every two ranges. The interval a (see FIG. 4a) between two chips amounts in this case to, for example, 0.5 mm, so that the above-described concept, according to which a predetermined interval b between the detector and diaphragm planes must be provided, can also apply in this sandwich arrangement, whereby for the corresponding construction of the array dimensions this interval b is great in comparison to the 0.5-mm interval between sandwich chips. 
     As mentioned at the outset, the arrangement according to FIg. 2 is intended to cover one of the six regions into which the sphere is divided. In order to monitor all six regions, either six similar sectors 9 can be installed on the six sides of an airplane, for example, or a different number of sectors, for example, up to five, can be combined in a single instrument, which then cover five sides of the aforementioned cube. The embodiment with six sectors mounted on an airplane 12 is depicted in a side view in FIG. 6. Only five sectors 10a through 10e are visible in this view, the sixth sector on the left side of the plane being hidden. The embodiment with five sectors mounted as a single instrument is depicted in the perspective view of FIG. 5. Only three sectors 10a, 10b and 10c are visible in this view, each with a 2×2 array of detector elements 6. The opposing installed instrument can then comprise not only the sixth still missing sector, but also four additional sectors which, in common with the first five-unit instrument, provide redundant monitoring. 
     The above-described concept for radiation detection without utilizing a radiation focusing lens assumes that the sensitivity of the individual detector elements is sufficient to convert a predetermined amount of impinging radiation energy from the laser beam source into a signal. 
     In practice, the path of the laser radiation 7 between the diaphragm 4 and the array of detector element 6, which are found, for example, inside the airplane, can be bridged by a correspondingly arrayed fiber optic bundle. Using this technique allows the radiation to be directed to an arbitrarily arranged central instrument and achieves another space-saving, so that the processing electronics can be located after the central instrument.