Abstract:
A system and method for efficient intervisibility determination. The intervisibility determination method of the present invention provides a multiple threat processing capability within a specified area of terrain using a common database. Computation is simplified through the method of processing data posts in the terrain elevation database. By taking integer steps and incrementing distance, x or y, and a predicted elevation value at each step, a small number of operations may be performed. Recomputing a change in elevation value may be reduced. An umbra database provides an enhanced look-up capability for displaying and updating the intervisibility display information. The systems and methods of the present invention may be suitable for use on a vehicle and in mission management activities.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/574,924, filed May 28, 2004, which is incorporated herein by reference. 
     
    
       [0002]     The present invention relates generally to mission management and, more particularly, to determining intervisibility.  
         [0003]     Mission management, as used herein, includes activities such as mission planning, threat avoidance, sensor coverage estimation, contingency management, prioritizing threats, route planning, and/or the like. Intervisibility, as used herein, refers to a line of sight between a selected point, such as a threat location, and an observer, such as an aircraft. An intervisibility determination may be an important consideration in the navigation of a vehicle. For example, an aircraft pilot may desire to navigate his aircraft in such a manner as to avoid the possibility of being detected by enemy radar coupled to an anti-aircraft missile or artillery unit.  
         [0004]     Typically, the word “threat” refers to a hostile or dangerous entity. A threat, as used herein, may refer to another aircraft, a vehicle, a person, or a facility that presents a danger or hostility to the observer. Additionally, a threat may also be used more generally to refer to another aircraft, a vehicle, a person or a facility that does not present a danger or hostility, but where a desire exists to determine intervisibility based on an observation point, such as, for example, an aircraft. In other words, the systems and methods of the present invention for determination of intervisibility have application in military vehicles, as well as in commercial or private vehicles. For example, a commercial airliner pilot may desire to know whether there is intervisibility between the airliner and a ground-based radar installation for purposes of navigation and communication.  
         [0005]     In addition to intervisibility determination for piloted aircraft, the intervisibility information generated by the systems and methods of the present invention may be used by an unmanned aircraft, such as, for example, an Unmanned Aerial Vehicle (UAV). Much like an operator of a manned vehicle, a UAV may use intervisibility information for multiple purposes, such as, for example, route planning, navigation, attack, reconnaissance, and/or the like.  
         [0006]     Briefly, the systems and methods of the present invention provide for efficient intervisibility determination of single or multiple threats and, optionally, display of the intervisibility information to assist an operator of a vehicle in navigation. Further, the system and methods of the present invention may be used to determine intervisibility between single or multiple observation points and single or multiple threats. As input, the systems and methods of the present invention receive a terrain elevation database. The terrain elevation database is comprised of data points, each of which correspond to a geographic location. The geographic locations are uniformly spaced in both the x and y directions. If the database is in a form that is not uniformly spaced, such as, for example, a Digital Terrain Elevation Database (DTED), then the database may be pre-processed to create an intermediate database having a uniform spacing of locations in both axes. Next, an umbra database is created, where each umbra database cell corresponds to a terrain elevation database data point. As used herein, a data element of a database may be referred to interchangeably as a data point or a database cell. Threat data comprising a threat location and a threat range capability for threats within the area of terrain covered by the terrain elevation database is received. A number of line of sight vectors are selected. Each line of sight vector emanates from a threat location and extends to the range of the threat. Each line of sight vector is traversed and a minimum visible elevation is computed for each terrain elevation database cell and stored in the corresponding umbra database location. Once all line of sight vectors for a given threat have been traversed and processed, the processing continues to a next threat, if a next threat is present. Once all threats have been processed for the given area of terrain, the umbra database may then be used in conjunction with the potential observation locations and altitudes to determine intervisibility for the observation point(s).  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will be described with reference to the accompanying drawings, wherein:  
         [0008]      FIG. 1  is a diagram of a portion of an exemplary terrain elevation cell array with a threat overlaid;  
         [0009]      FIG. 2  is a diagram of a portion of an exemplary terrain elevation database cell array showing a line of sight vector;  
         [0010]      FIG. 3  is a diagram of a portion of an exemplary terrain elevation database cell array showing incremental distances;  
         [0011]      FIG. 4  is a diagram of a portion of an exemplary terrain elevation database cell array showing the size of a cell;  
         [0012]      FIG. 5  is a diagram showing a profile view of an exemplary line of sight vector and the terrain values associated therewith;  
         [0013]      FIG. 6  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a first intervisibility calculation;  
         [0014]      FIG. 7  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a second intervisibility calculation;  
         [0015]      FIG. 8  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a preliminary third intervisibility calculation;  
         [0016]      FIG. 9  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a re-computed third intervisibility calculation;  
         [0017]      FIG. 10  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a fourth intervisibility calculation;  
         [0018]      FIG. 11  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a preliminary fifth intervisibility calculation;  
         [0019]      FIG. 12  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a re-computed fifth intervisibility calculation;  
         [0020]      FIG. 13  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a preliminary sixth intervisibility calculation;  
         [0021]      FIG. 14  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a re-computed sixth intervisibility calculation;  
         [0022]      FIG. 15  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a seventh intervisibility calculation;  
         [0023]      FIG. 16  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing an eighth intervisibility calculation;  
         [0024]      FIG. 17  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a ninth intervisibility calculation;  
         [0025]      FIG. 18  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing an exemplary change in elevation calculation;  
         [0026]      FIG. 19  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing an initial umbra database value;  
         [0027]      FIG. 20  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing exemplary intervisibility values for an exemplary first threat;  
         [0028]      FIG. 21  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing mean sea level altitude intervisibility values;  
         [0029]      FIG. 22  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing above ground level altitude intervisibility values;  
         [0030]      FIG. 23  is a diagram of a portion of an exemplary terrain elevation database cell array showing multiple threats;  
         [0031]      FIG. 24  is a flowchart of a method for determining intervisibility in accordance with the present invention; and  
         [0032]      FIG. 25  is a diagram of a system for determining and displaying intervisibility in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0033]     In general terms, the input data used for determining intervisibility in accordance with the present invention comprises a terrain elevation database, threat data, an umbra database, and observation point data.  
         [0034]     In an exemplary embodiment, the terrain elevation database contains data representative of the terrain elevation features of a particular region or area, such as, for example, a Digital Terrain Elevation Database (DTED). Each element of data in the database, commonly referred to as a cell or data post, stores terrain data that is based on the scale of the digital map. In other words, each cell in the database is representative of a certain area of terrain.  
         [0035]     In an exemplary embodiment, the threat data contains a threat location and a threat range. Further, the systems and methods of the present invention are capable of processing multiple threats for a given area. Therefore, threat data may contain threat information for multiple threats.  
         [0036]     In an exemplary embodiment, the umbra database contains an array of cells that corresponds to the array of cells in the terrain elevation database. The cells in the umbra database are used to store the minimum visible elevation values that are calculated during intervisibility processing in accordance with the present invention. Umbra, as defined by Webster&#39;s Revised Unabridged dictionary, refers to the conical shadow projected from a planet or satellite, on the side opposite to the sun, within which a spectator could see no portion of the sun&#39;s disk. However, as used herein, umbra is used to indicate association with computed intervisibility data. The phrase “umbra value” is used herein to refer to an intervisibility value, such as, for example, minimum visible elevation, for a particular terrain elevation database cell. The phrase “umbra database” is used herein to refer to a database containing umbra values.  
         [0037]     In an exemplary embodiment, the observation point data comprises an observer location and observer altitude. However, it should be appreciated that the systems and methods of the present invention may be used for determining intervisibility between single or multiple threats and single or multiple observation points. The observation point data may be used in a last step of intervisibility determination. The umbra database is used in conjunction with the observation point data to determine for a given observation point location and altitude, or elevation, each point in the umbra database, or selected portion of the umbra database, where the observer may be visible to a threat. Once the determination of intervisibility is computed, the results may be displayed to an operator, such as, for example, a pilot of an aircraft, so that the operator may take intervisibility information into account when navigating the vehicle. In addition the results of intervisibility determination may be used for other mission management activities, such as, for example, automated mission management or mission planning. Wherein the vehicle may be an aircraft, a terrestrial vehicle, a boat, a spacecraft, or other vehicle or platform capable of housing or using the systems and methods of the present invention.  
         [0038]      FIG. 1  is a diagram of a portion of an exemplary terrain elevation database with an exemplary threat overlaid. In particular, a cell array  108  represents a portion of an exemplary terrain elevation database comprised of an array of cells, wherein each cell is used to store a value that is representative of the terrain elevation, such as, for example, the elevation of that point, the elevation representative of an average or composite of the surrounding area, or the highest elevation within the area represented by the cell. The cell array  108  is shown with a threat location  102  overlaid. The threat located at threat location  102  has a threat range circle  106 . The threat range circle  106  encompasses an area in which the threat may have capabilities for sensing, action, and/or the like. The radius of the threat range circle  106  is equal to the threat range  110 .  
         [0039]     In order to compute umbra values, a number of line of sight vectors  112  emanating from the threat location  102  are used as reference lines. The number of line of sight vectors may be predetermined, or dynamically determined. In an exemplary embodiment, the number of line of sight vectors used may be a function of threat range, terrain data, and/or the like. In another exemplary embodiment, the number of line of sight vectors is a fixed, predetermined number. The method of selecting the number of vectors to be used may depend on the contemplated use of the invention in a particular embodiment. The terrain database comprising a cell  108  array corresponds to geographic locations. In an exemplary embodiment, the distance between geographic locations represented by each terrain elevation database cell  108  is constant in both x and y dimensions. That is, geographic locations corresponding to adjacent data points of the database are uniformly spaced in a first direction (x axis) and a second direction perpendicular to the first (y axis). However, it should be appreciated that other scales and methods of representing the terrain in the terrain elevation database may be used.  
         [0040]      FIG. 2  is a diagram of a portion of an exemplary terrain elevation database cell array showing a line of sight vector. In particular, a line of sight vector  112  extends from a threat location  102  to an endpoint  202 . The endpoint  202  represents a point at the range extent of the threat. The line of sight vector  112  has a change in x-axis position  206  and a change in y-axis position  204 .  
         [0041]      FIG. 3  is a diagram of a portion of an exemplary terrain elevation database cell array showing incremental distances. In particular, the line of sight vector  112  emanating from a threat location  102  has associated incremental distances for each step traversed along the line of sight  112 . The incremental distances are an x-axis distance  302 , a y-axis distance  304 , and a hypotenuse distance  306 .  
         [0042]      FIG. 4  is a diagram of a portion of an exemplary terrain elevation database cell array showing the size of a cell. In particular, in an exemplary embodiment, each cell in the terrain elevation database has an even spacing in the x-axis  402  and an even spacing in the y-axis  404  over the entire terrain elevation database.  
         [0043]     In order to illustrate the methods and systems of the present invention, an example intervisibility determination along a line of sight vector will be described. It should be appreciated that this is an example for illustration purposes and represents an exemplary terrain elevation database and an exemplary embodiment of the present invention.  
         [0044]      FIG. 5  is a diagram showing a profile view of an exemplary line of sight vector and the terrain values associated therewith. In particular, a threat location  102  is shown at a threat elevation  502 . A terrain profile  504  has an elevation at each terrain elevation database cell ( 506 - 512 ).  
         [0045]      FIG. 6  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a first intervisibility calculation. In particular, a threat location  102 , a threat elevation  502  and a terrain elevation of the first cell  506  are shown.  
         [0046]     In operation, the umbra value for the first cell at the threat location  102  is calculated as the terrain elevation of the first cell  506  along the line of sight vector.  
         [0047]      FIG. 7  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a second intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a distance increment  702 , a change in elevation  704 , a viewing angle vector  706  and an elevation at a second cell  508  are shown.  
         [0048]     In operation, the umbra value for the second location is calculated as the elevation of the second cell  508  and is stored in the umbra database at the corresponding location. The change in elevation value  704  is calculated as the difference between the threat elevation  502  and the elevation of the second cell  508  divided by the number of steps taken along the line of sight vector. Further, the change in elevation  704  is stored for future use in a next cell calculation.  
         [0049]      FIG. 8  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a preliminary third intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , an umbra value  510 , a third umbra value starting point  802  and a difference in elevation factor  704  are shown.  
         [0050]     In operation, the calculation of the umbra value at the third step along the line of sight vector begins by establishing an elevation of the previous umbra value. In this case, the value of the second umbra value  508  is used as a third umbra value starting point  802 . Next, the difference in elevation factor  704  is applied. The resulting predicted umbra value  510  is the difference between the third umbra value starting point  802  and the difference in elevation factor  704 . However, the predicted umbra value  510  is below the terrain elevation for the third data post and a corrected umbra value must be calculated.  
         [0051]      FIG. 9  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a re-computed third intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , an umbra value  510 , a terrain reference point  902  and a difference in elevation value  904  are shown.  
         [0052]     In operation, the umbra value must be recalculated because the value first arrived at was below the terrain elevation value. The new umbra value  510  is recalculated as the terrain elevation at the reference point  902 . The new difference in elevation factor  904  is computed as the change in elevation between the threat location  102  and the reference point  902  divided by the number of steps taken along the line of sight vector. In this case the number of steps is two. In other words, the new difference in elevation factor  904  is a change in elevation per step value. This change in elevation per step value is used in order to predict an umbra value for one or more succeeding steps. If the predicted umbra value is reasonable (i.e. above the terrain), then the change in elevation value will continue to be used. If the predicted umbra value is determined to be unreasonable (i.e. below the terrain), then the change in elevation value is recomputed at that point, according to the method described above.  
         [0053]      FIG. 10  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a fourth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a predicted umbra value  1002  and a difference in elevation factor  904  are shown.  
         [0054]     In operation, the predicted umbra value has been calculated at the third step along the line of sight vector begins by establishing an elevation of the previous umbra value. In this case, the value of the fourth umbra value  510  is used as a starting point. Next, the difference in elevation factor  904  is applied. The resulting predicted umbra value  1002  is the difference between the third umbra value and the difference in elevation factor  904  (or an incrementing of the previous predicted umbra value by the change in elevation per step value). The predicted umbra value  512  is at a point  1002  above the terrain elevation for the fourth data post and is stored as the umbra value for the third step along the line of sight vector. The predicted umbra value is then incremented by the difference in elevation factor  904  to arrive at a new predicted umbra value to be used in a next step.  
         [0055]      FIG. 11  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a preliminary fifth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a predicted umbra value  1102 , and a difference in elevation factor  904  are shown.  
         [0056]     In operation, the calculation of the umbra value at a fourth step along the line of sight vector begins by establishing an elevation of the previous umbra value. In this case, the value of the fourth umbra value  512  is used as a starting point. Next, the difference in elevation factor  904  is applied. The resulting predicted umbra value  1106  is the difference between the fourth umbra value starting point  512  and the difference in elevation factor  904 . However, the predicted umbra value  1106  is at a point  1102  below the terrain elevation for the fifth data post and a corrected umbra value must be calculated.  
         [0057]      FIG. 12  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a re-computed fifth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a recalculated umbra value  1202 , and a difference in elevation factor  1204  are shown.  
         [0058]     In operation, the umbra value must be recalculated because the value first arrived at was below the terrain elevation value. The new umbra value  1206  is recalculated as the terrain elevation at the fifth data post  1202 . The recomputed difference in elevation factor  1204  is calculated as the change in elevation between the elevation at the fifth data post  1202  and the threat location  502 , divided by the number of steps taken along the line of sight vector. In this case the number of steps is four. The recomputed difference in elevation value  1204  is stored for use in a next umbra calculation.  
         [0059]      FIG. 13  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a preliminary sixth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a fifth umbra value  1206 , a predicted elevation point  1302 , a predicted umbra value  1306 , and a difference in elevation factor  1204  are shown.  
         [0060]     In operation, the calculation of the umbra value at a fifth step along the line of sight vector begins by establishing an elevation of the previous umbra value. In this case, the value of the fifth umbra value  1206  is used as a starting point. Next, the difference in elevation factor  1204  is applied. The resulting predicted umbra value  1306  is the difference between the fifth umbra value starting point  1206  and the difference in elevation factor  1204 . However, the predicted umbra value  1306  is at a point  1302  below the terrain elevation for the sixth data post and a corrected umbra value must be calculated.  
         [0061]      FIG. 14  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a re-computed sixth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a fifth umbra value  1206 , a recomputed difference in elevation value  1404  and a recomputed umbra value  1402  are shown.  
         [0062]     In operation, the recomputed sixth umbra value  1402  has been calculated as the terrain elevation at the sixth data post on the line of sight vector. The recomputed sixth umbra value is stored in the umbra database at the location corresponding to the sixth data post location. The recomputed difference in elevation value  1404  has been calculated as the difference between the terrain elevation at the sixth data post  1402  and the threat elevation  502 , divided by the number of steps taken along the line of sight vector (i.e. five, in this case). The recomputed difference in elevation value  1404  is stored for use in a next umbra calculation. A next predicted umbra value is computed by summing the current umbra value and the difference in elevation value  1404 .  
         [0063]      FIG. 15  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a seventh intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a fifth umbra value  1206 , a sixth umbra value  1402 , a seventh umbra value  1502  are shown.  
         [0064]     In operation, the predicted umbra value is calculated as the elevation value of the sixth umbra value  1402  plus the stored change in elevation value  1404  (from  FIG. 14 ). The resulting sum is the seventh umbra value  1502 , which is above the terrain elevation at a seventh data post. Accordingly, the seventh umbra value  1502  is stored in the umbra database and the difference in elevation value  1404  continues to be stored for use in a next umbra calculation. A next predicted umbra value is computed by summing the current umbra value ( 1502 ) and the difference in elevation value  1404 .  
         [0065]      FIG. 16  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing an eighth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a fifth umbra value  1206 , a sixth umbra value  1402 , a seventh umbra value  1502  and a predicted umbra value  1602  are shown.  
         [0066]     In operation, the predicted umbra value  1602  is calculated as the elevation value of the seventh umbra value  1502  plus the stored change in elevation value  1404  (from  FIG. 14 ). The resulting sum is the eighth umbra value  1602 , which is above the terrain elevation at an eighth data post. Accordingly, the eighth umbra value  1602  is stored in the umbra database and the difference in elevation value  1404  continues to be stored for use in a next umbra calculation. A next predicted umbra value is computed by summing the current umbra value ( 1602 ) and the difference in elevation value  1404 .  
         [0067]      FIG. 17  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a ninth intervisibility calculation. In particular, a threat location  102 , a terrain profile  504 , a threat elevation  502 , a first umbra value  506 , a second umbra value  508 , a third umbra value  510 , a fourth umbra value  512 , a fifth umbra value  1206 , a sixth umbra value  1402 , a seventh umbra value  1502 , an eighth umbra value  1602  and a ninth umbra value  1702  are shown.  
         [0068]     In operation, the ninth umbra value  1702  is calculated as the elevation value of the eighth umbra value  1602  plus the stored change in elevation value  1404  (from  FIG. 14 ). The resulting sum is the ninth umbra value  1702 , which is above the terrain elevation at a ninth data post. Accordingly, the ninth umbra value  1702  is stored in the umbra database and the difference in elevation value  1404  continues to be stored for use in a next umbra calculation.  
         [0069]      FIG. 18  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing an exemplary change in elevation calculation. In particular, a threat  1802 , a threat elevation  1804 , a first step  1806 , a second step  1808 , a third step  1810 , an elevation at a fourth step  1812 , a change in elevation  1814  and a change in elevation per step  1816  are shown.  
         [0070]     In operation, the change in elevation per step may be calculated as the difference between the elevation at a fourth step  1812  and the threat elevation  1804 , divided by the number of steps (i.e. four, in this example). The result is a change in elevation per step  1816 . This change in elevation per step may be used in order to predict an umbra value for a succeeding step. If the predicted umbra value is reasonable (i.e. above the terrain), then the change in elevation value will continue to be used. If the predicted umbra value is determined to be unreasonable (i.e. below the terrain), then the change in elevation value is recomputed at that point, according to the method described above.  
         [0071]      FIG. 19  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing an initial umbra database value. In particular, an umbra database is initialized to a large value, for example, the maximum value for the data type being used, such as, for example, 32767, in the case of a 16-bit signed integer data type. This initial value may be stored in all umbra database locations, including those along the line of sight vector (U 0 -U 8 ).  
         [0072]      FIG. 20  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing exemplary intervisibility values for an exemplary first threat. In particular, umbra values have been calculated for the data posts within a threat range (U 0 -U 6 ), while the remaining data posts U 7  and U 8  contain an initial value, for example, 32767.  
         [0073]      FIG. 21  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing a mean sea level elevation used for intervisibility determination. In particular, an MSL elevation  2102  and umbra values for nine data posts ( 2104 - 2120 ) are shown. In operation, intervisibility information may be displayed to an aircraft pilot. The aircraft is potentially visible to the threat at the area of the map corresponding to data posts  2104 - 2112 . Accordingly, the display may show an indication of intervisibility for this area that may alert the pilot to the intervisibility condition, so that the pilot may take appropriate action.  
         [0074]      FIG. 22  is a diagram of a profile view of an exemplary line of sight vector and associated terrain elevation values showing above ground level (AGL) altitude intervisibility values. In particular, an AGL elevation  2202  is shown, along with umbra values for nine data posts ( 2204 - 2220 ). In operation, intervisibility information may be displayed to an aircraft pilot. Further, if the intervisibility determination is being used in the context of a mission management activity, the intervisibility data may be used for that purpose, such as, for example, in a route-planning algorithm. The aircraft is potentially visible to the threat at the area of the map corresponding to data posts  2204 - 2208 , and  2214 , because the AGL elevation of the aircraft determining intervisibility at these data posts is greater than the umbra value at the data posts. Accordingly, the display may show an indication of intervisibility for these areas that may alert the pilot to the intervisibility condition and the pilot may take appropriate action.  
         [0075]      FIG. 23  is a diagram of a portion of an exemplary terrain elevation database cell array showing multiple threats. In particular, four threats  2302 - 2308  are shown overlaid on an exemplary terrain elevation database  2310 . The systems and methods of the present invention may be capable of processing multiple threats into an umbra database that contains a composite of the threat intervisibility information.  
         [0076]      FIG. 24  is a flowchart of an exemplary embodiment of a method for determining intervisibility in accordance with the present invention. In particular, control begins at step  2402  and proceeds to step  2404 . In step  2404 , a terrain elevation database comprising an array of cells corresponding to geographical locations is received. Each terrain elevation database cell contains a value corresponding to terrain elevation at a geographical location represented by the terrain elevation database cell and the terrain represented by each terrain elevation database cell may be constant in both x and y dimensions. Control then continues to step  2406 .  
         [0077]     In step  2406 , an umbra database is created comprising umbra database cells. Each umbra database cell corresponds to a terrain elevation database cell and the terrain represented by each umbra database cell may be constant in both x and y dimensions. The umbra database may correspond to a portion, or all, of a terrain elevation database. In an embodiment where the umbra database covers a portion of the terrain elevation database, the portion may be an area of interest. Control then continues to step  2408 . In step  2408 , a threat location and a threat range capability is received for all known threats within the area of terrain covered by the terrain elevation database. Control then continues to step  2410 .  
         [0078]     In step  2410 , a number of line of sight vectors to compute for each threat are selected. The number of line of sight vectors may be determined based on the number of cells forming the perimeter of a rectangle (or in a preferred embodiment, a square) bounding the threat and having a length of two times the threat range divided by the distance covered per cell in each of the X and Y axes. This number of line of sight vectors provides an adequate number to process in order to determine threat intervisibility. Control then continues to step  2412 . In step  2412 , cell coordinate variables X and Y are initialized to the cell coordinate representing a threat location. Also, a distance variable and a step count variable is initialized. Control then continues to step  2414 .  
         [0079]     In step  2414 , an initial umbra value is calculated, wherein the initial umbra value is calculated as the elevation at the location of the threat and stored in the umbra database at the corresponding cell. Control then continues to step  2416 .  
         [0080]     In step  2416 , the cell coordinate variables X and Y are incremented to reference a next cell on the line of sight vector. Depending on the quadrant that the line of sight vector lies in relative to the threat location, the X (or Y) cell coordinate variable may be incremented by 1.0 and the Y (or X) variable may be incremented by a value ≦1.0. For example, in a line of sight vector extending in an easterly direction, the X increment value is 1.0 and the Y increment value is 0.0. In another example, in a line of sight vector extending in a southwesterly direction, the X increment is −1.0 and the Y increment is −1.0. Control then continues to step  2418 . In step  2418 , a distance variable is incremented by the value representing the distance taken in each step along the line of sight vector. Control then continues to step  2420 .  
         [0081]     In step  2420 , a variable storing the step count is incremented. Control then continues to step  2422 . In step  2422 , a predicted umbra value is calculated by summing the present umbra value with the change in elevation value. Control then continues to step  2424 . In step  2424 , the predicted umbra value is evaluated in order to determine if it is at or above the terrain elevation value. If the predicted umbra value is at or above the terrain elevation value the predicted umbra value is stored in the umbra database and control proceeds to step  2428 . However, if the predicted umbra value is not at or above the terrain elevation value, control proceeds to step  2426 . In step  2426 , the change in elevation factor is recomputed by subtracting the initial elevation value from the current elevation value and dividing the resulting difference by the step count traversed. The new change in elevation value is stored. The terrain elevation value is stored in the umbra database. Control then continues to step  2428 .  
         [0082]     In step  2428 , the distance variable is evaluated. If the distance traversed along the line of sight vector is equal to or greater than the threat range capability of the threat being processed then control continues to step  2430 . Otherwise, control continues to step  2416  to continue the cell processing for the current line of sight vector.  
         [0083]     In step  2430 , the processing ends for the current line of sight vector. The line of sight vectors are evaluated. If all line of sight vectors have been processed, control continues to step  2432 . Otherwise, control continues to step  2412  and a next line of sight vector is processed.  
         [0084]     In step  2432 , the threats received are evaluated. If another threat is available for processing, then control continues to step  2408 , where a next threat is processed by repeating the steps  2408 - 2432 , described above. If there are no other threats to process, control continues to step  2434 , where intervisibility processing ends for this area of terrain. In addition, steps may be repeated as necessary when a threat moves or a different observation location or altitude is desired as a result of movement by the observer.  
         [0085]     In an exemplary embodiment of the present invention, the umbra database may be used to display intervisibility data in an aircraft by coupling the umbra database to a display device of an aircraft. As the altitude and/or location of the aircraft changes, the umbra database may be re-computed. The umbra database may also be re-computed when the presence of threats changes, such as, for example, when a threat enters the area or when a threat is neutralized. The umbra database may also be re-computed in response to an operator command, or in the case of a mission management application, any time a new determination is desired or required, such as, for example, when a new automatic route plan is desired. Once the umbra database has been re-computed, the intervisibility display may be updated in response to the umbra database update.  
         [0086]     Threat intervisibility may be computed in terms of altitude above ground level (AGL) or in terms of altitude above mean sea level (MSL).  
         [0087]     Threat data may be received from a variety of sources, such as, for example, from a pre-loaded threat information database, from sensors, from a communications message received by the aircraft, and/or the like. Further, the systems and methods of intervisibility determination may be used to compute instantaneous sensor coverage, for example, from an aircraft to the surrounding terrain. In a sensor coverage scenario, no actual threats are involved and the intervisibility calculation is directed from point of view of the aircraft as the “threat” with the surrounding terrain points as the “observation points.” Thus, the sensor coverage map depicts the areas where the sensor on board the aircraft is able to detect.  
         [0088]      FIG. 25  is a diagram of a system for determining and displaying intervisibility in accordance with the present invention. In particular, a processor  2502  is coupled to a memory  2504 . The memory  2504  may be a component of the processor or may be a peripheral device of the processor. The memory  2504  comprises at least a portion of a terrain elevation database  2506 , a software program  2507  for determining intervisibility in accordance with the present invention and at least a portion of an umbra database  2508 . Also shown are a threat information source  2510  and a vehicle location and altitude information source  2512 . In addition, an optional display  2514  and an optional mission management module  2516  are shown coupled to the processor  2502 .  
         [0089]     In operation, the processor  2502  loads the software program  2507 , and the terrain elevation database  2506 . The processor receives threat information from a threat information source  2510  and stores the threat information in memory. The software program  2507  accesses the terrain elevation database  2506  and the threat information (not shown) in order to calculate the umbra values for storing into the umbra database  2508 , for example, by using the exemplary embodiment of the method described above. Once the umbra database has been populated with umbra values, the processor  2502  receives the vehicle and location information from the vehicle location and altitude source  2512 . The software program  2507  uses the vehicle location and altitude information (not shown) along with the umbra database  2508  in order to determine the intervisibility for the vehicle. Optionally, the intervisibility information is displayed on the display  2514 . Also, the intervisibility information may optionally be sent to the mission management module  2516  for use in various mission management applications in accordance with a contemplated use of the invention.  
         [0090]     The intervisibility determination methods and systems, as shown in the above figures, may be implemented on a general-purpose computer, a special-purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit element, and ASIC or other integrated circuit, a digital signal processor, a hardwired electronic or logic circuit such as a discrete element circuit, a programmed logic device such as a PLD, PLA, FPGA, PAL, or the like. In general, any process capable of implementing the functions described herein can be used to implement a system for determining intervisibility according to this invention.  
         [0091]     Furthermore, the disclosed system may be readily implemented in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer platforms. Alternatively, the disclosed system for intervisibility determination may be implemented partially or fully in hardware using standard logic circuits or a VLSI design. Other hardware or software can be used to implement the systems in accordance with this invention depending on the speed and/or efficiency requirements of the systems, the particular function, and/or a particular software or hardware system, microprocessor, or microcomputer system being utilized. The intervisibility determination system illustrated herein can readily be implemented in hardware and/or software using any known or later developed systems or structures, devices and/or software by those of ordinary skill in the applicable art from the functional description provided herein and with a general basic knowledge of the computer and mark-up language arts.  
         [0092]     Moreover, the disclosed methods may be readily implemented in software executed on programmed general-purpose computer, a special purpose computer, a microprocessor, or the like. In these instances, the systems and methods of this invention can be implemented as program embedded on personal computer such as JAVA® or CGI script, as a resource residing on a server or graphics workstation, as a routine embedded in a dedicated encoding/decoding system, or the like. The system can also be implemented by physically incorporating the system and method into a software and/or hardware system, such as the hardware and software systems of an image processor.  
         [0093]     It is, therefore, apparent that there is provided in accordance with the present invention, systems and methods for intervisibility determination and display. While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, applicants intend to embrace all such alternatives, modifications, equivalents and variations that are within the spirit and scope of this invention.