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Passive determination of ground target locationPassive determination of ground target location description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080074639, Passive determination of ground target location. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001]The invention relates generally to navigation and target location systems. More particularly, this invention relates to a passive target identification systems. BACKGROUND [0002]Prior art target location systems exist, which can identify the latitude, longitude and elevation of a distant target using a laser beam that is directed at the target from an observation point, the latitude, longitude and elevation of which is known. If the latitude, longitude and elevation of an observation point are known, the latitude, longitude and elevation of a point in space that is distant from the observation point can be easily determined using simple trigonometric formulae on the azimuth, elevation angle and distance between the observation point and the target. [0003]Distance to a target point from an observation point can be easily and accurately determined by measuring the time required for a pulse of laser light to traverse the distance between the observation point where a laser is located and the target point. Laser beam elevation angle (positive and negative) and the beam's azimuth angle to a target point are also easily determined. Prior art target location systems that use a laser or other detectable signal are referred to herein as active target location systems. [0004]A problem with prior art active target location systems is that laser light or other electromagnetic energy used to locate or mark a target can be detected. Sighting a target using laser light energy therefore announces to the world that the target is being lased. Using a laser to mark a target and/or determine a target's location therefore has drawbacks. Thus, a need exists for a method and apparatus by which coordinates of a target or other destination point in a three-dimensional space can be determined without having to announce to all those concerned that the target is being marked or lased. SUMMARY [0005]The invention in one implementation encompasses a passive method for locating a distant target from a known location. The method is to iteratively determine latitude and longitude coordinates for locations at, around or near a target. The latitude and longitude coordinates around the target are determined using a line-of-sight elevation angle between an observation point to the target. The observation point is elevated above the ground, which determines a reference point elevation. Azimuth angles are also used to calculate latitude and longitude coordinates of the target from the observation point. [0006]When latitude and longitude coordinates are first determined, the actual elevation of the terrain immediately above the location they define is read from a digital terrain elevation database or DTED. If the terrain elevation above the calculated latitude and longitude is different than the elevation of the reference point elevation, the reference point elevation is adjusted and the calculations repeated until the reference point elevation matches or substantially matches the elevation of the coordinates of the target, in which case the target's latitude, longitude and elevation are precisely determined. No detectable radiation is needed to mark or locate the target. DESCRIPTION OF THE DRAWINGS [0007]Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: [0008]FIG. 1 is a representation of one implementation of a method for passively determining a ground target's location in the simple case where the target is coplanar with a reference point; [0009]FIGS. 2-5 depict a second implementation of a method for passively determining a ground target's location by iteratively adjusting a reference point elevation until it is determined to be at the same elevation of the target; and [0010]FIG. 6 depicts an apparatus for passively determining a ground target's location. DETAILED DESCRIPTION [0011]FIG. 1 demonstrates a method 10 for passively determining or identifying the location 12 of a ground target 14 from an observation point 16 for the trivial case where the observation point 16 and the target 14 are on the same flat, horizontal plane. As shown in the figure, the observation point 16 is directly above a first reference point 20 by a height or elevation 18. As is also known, the first reference point 20 will itself have an elevation with respect to sea level. The elevation of the first reference point could of course be greater than or less than or equal to sea level. [0012]It is well known that every point on the Earth's surface 22 can be precisely identified by latitude and longitude coordinates. Since the first reference point 20 is located on the Earth's surface 22, it is unambiguously identified by latitude and longitude coordinates. Since the observation point 16 is directly above the first reference point 20 by a known elevation 18, the latitude and longitude of the first reference point 22 is known from the latitude and longitude of the first reference point 22 and the elevation 18 of the first observation point 16 above the first reference point 22. These three scalars (latitude and longitude of first reference point 22 and elevation of the observation point 18) unambiguously identify the observation point 16. [0013]In light of the foregoing, a first step of passively determining the location of a remote or distant target 14 is to determine the latitude and longitude of the first reference point 22, which is depicted in FIG. 1 as being on the Earth's surface 22. A second step of passively determining the location of a remote target 14 is to determine the elevation 18 of the observation point 16 above the first reference point 22. [0014]After the observation point 18 elevation 22 is determined, the next step of the method is to determine the line-of-sight elevation angle from the observation point 18 to the ground target 14. In addition to determining the line-of-sight elevation from the observation point 18 to the target 14, an additional step of determining the target location is to determine the azimuth angle from true north to the ground target 14 (not shown in FIG. 1). As used herein, azimuth is the angle measured about (i.e., around) the observation point 18 or the first reference point 22 between true north (or other reference point) and a vertical line passing through the center of the ground target 14. Those of ordinary skill in navigation usually measure azimuth angle in a direction that is clockwise from the north point (or other reference point) through 360 degrees. [0015]Once the elevation 18 of the observation point 16 is known and the line-of-sight elevation angle and azimuth are known, latitude and longitude coordinates of the ground target 14 can be determined in the trivial case shown in FIG. 1 by using basic trigonometry. FIG. 1 is considered to depict a trivial or simple case of locating a remote target 14 because FIG. 1 depicts the target 14 and the first reference point 20 to be on a flat plane in which the Earth's curvature is ignored. [0016]In the simple case shown in FIG. 1, the tangent of the line of sight elevation angle A, is determined by the quotient of the horizontal distance (along the x-axis) between the first reference point 22 and the elevation 18 of the observation point 18. The horizontal distance (along the x-axis) between the first reference point 22 and the target 14 is therefore determined by dividing the elevation distance 18 by the tangent of the elevation angle `A.` [0017]Once the distance between the first reference point 22 and the target 14 is known, the latitude and longitude coordinates of the target 14 can be determined from the horizontal distance to the target 14 from the first reference point 20 and by dividing the distance between the first reference point 22 and the target 14 by either the sine or cosine of the azimuth angle. [0018]It should be noted that the line of sight distance 22 between the target 14 and the observation point 16 can also be determined by dividing the elevation 18 of the observation point 16 by the cosine of the angle A. [0019]FIGS. 2-5 depict steps of a method 10 for passively determining or identifying the location 12 of a ground target 14 from an observation point 16 in the general case where the elevation of the target 14 is unknown. Key to the general case method, however, is a terrain database of actual elevations of points around the target 14 wherein there are stored, the actual elevation of the Earth's surface at various latitude and longitude coordinates. In some circles, a database wherein elevations of latitude and longitude coordinates are stored is referred to or known as a digital terrain elevations database or DTED. A DTED, which is not shown in the figures, is a database of latitude and longitude coordinates throughout a geographic region (or world wide) and the ground's elevation at each latitude and longitude coordinate pair. Continue reading about Passive determination of ground target location... 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