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13.1.1. Coordinate Reference System . Coordinate reference systems are a shorthand means of communicating location on the earth's surface. The most familiar coordinate reference system uses latitude, longitude, and elevation. Others include the Universal Transverse Mercator (UTM) and Uni-versal Polar Stereographic (UPS) grid system, which are two- dimensional. Simply put, these are grids placed on maps which allow a location to be identified without the lengthy description of degrees, minutes, and seconds of latitude and longitude. The Military Grid Reference System (MGRS) is an alphanumeric shorthand for expressing UTM and UPS coordinates with fewer numbers. Of note, a coordinate reference system always connects to a datum that defines its reference frame and point of origin; when the datum changes, so do the coordinates of the position.
13.1.1.1. Geodetic coordinates (geodetic latitude, geodetic longitude, and geodetic height) define the position of a point on the surface of the Earth with respect to the reference spheroid. Geo-graphic coordinates, on the other hand, are quantities of latitude and longitude which only define the position of a point on a reference surface .
13.2. Datums. A critical consideration often overlooked in using coordinates is the geodetic datum upon which the coordinates and stated accuracy are based. A datum is a regional or global coordinate reference system. It includes a reference ellipsoid (a mathematical representation of the size and shape of the earth) and a specific origin point. Coordinates within the same geodetic datum are directly related to the same origin point. Coordinates within different datums must be converted to a common reference before they can be used for targeting.
13.2.1. The World Geodetic System (WGS) provides the basic reference frame and geometric figure for the earth, models the earth gravimetrically, and provides the means for relating positions on vari-ous local geodetic systems to an earth- centered, earth- fixed (ECEF) coordinate system. WGS 84 cur-rently is the ECEF system officially authorized for DoD use. {WGS is the preferred designation, rather than WGS 84, which many assume is the currency date.} WGS represents NIMA's modeling of the earth from a geometric, geodetic, and gravitational standpoint. It was developed using new and more extensive data sets and improved computer software. The availability of a more extensive file of Doppler- derived station coordinates, improved sets of ground- based Doppler and laser satellite tracking data and surface gravity for local geodetic systems, and satellite radar altimetry for geoid heights resulted in significant improvements over the previous system (WGS 72). WGS parameters and models are constantly being upgraded as new information is being incorporated. NIMA currently does not plan to create another WGS- XX. 98
13.3.1. Classes of Errors. The production and presentation of geospatial information involves many steps. Numerous observations, measurements, and display operations are involved. Because of instrumental imperfections and human limitations, errors can occur at almost any point in the produc-tion process. These errors fall into three general classes: blunders, systematic errors, and random errors.
13.3.1.1. The basic definition of an error distribution assumes that systematic errors and blunders have been removed and only random errors are left. However, systematic errors cannot be removed from positional information unless some means exist for their detection, such as compar-ing this information against given control. Consequently, if systematic errors are not removed, they will have an effect, for example, on geodetic and photogrammetric measurements and the resulting positional information.
13.3.1.2. Statistical techniques are used to measure and identify these errors. These measures convey a confidence level to the user for the probably accuracy of NIMA data. Depending on the data's intended use, geospatial accuracy's are usually expressed in terms of absolute or relative accuracy, or both. Absolute accuracy tells how close each feature or data point is to the specified higher standard. It includes all random and systematic errors. Relative accuracy tells how close the measured distance or elevation is between two features or data points over a specified distance within the standard. It includes only random errors. Geospatial position accuracy is traditionally measured in feet or meters of Linear Error (LE) for heights and feet or meters of Circular Error (CE) for horizontal position, both at 90% probability. Spherical Error (SE) is the three- dimen-sional combination of horizontal and vertical errors at 90% probability and will be increasingly used as the measure of geospatial fidelity in the near future.
13.3.1.3. Certain weapons use circular measures of absolute and relative accuracy at 50% proba-bility that reflect the intended uses of these systems. The 50% Circular Error Probable (CEP) fig-ure is the radius of a circle around the target within which 50% of the weapons should fall. The remaining 50% fall outside the CEP. The Spherical Error Probable (SEP) is a three- dimensional combination of horizontal and vertical errors at 50% probability.
13.3.1.4. Target Location Error (TLE) is the difference between the actual location of the target and the expected location. Understanding and predicting TLE is particularly crucial to autono-mous weapons development because of low CEP objectives. The total overall error is a statistical combination of TLE and the errors associated with the weapon (e. g., INS, GPS, aircraft, and oper-ator).
13.3.2. Precision and Accuracy. Although the terms precision and accuracy are often used inter-changeably, there is an important difference between them. "Precision" is the closeness with which repeated measurements made under similar conditions are grouped together, and "accuracy" is the closeness of the best estimated value obtained by the measurements to the "true" value of the quantity measured. 99
13.3.2.2. Coordinates that are developed, transmitted and used should have a format (capability) to support measurements to a deci- foot (equivalent to DDD MM SS. SSS or thousandths of an arc second). The associated accuracy of the coordinates should also be stated. The User can deter-mine from these parameters whether the coordinate data will meet User requirements. {Note: This does not mean all coordinates must be derived to that level of precision or that the position be accurate to that level. Example: Measurements of an object on the source may be precise to the 6 inches. The positional accuracy of the object itself may within 100 feet. If the intended use is to measure the object, this precision may support the process. If the intended use is to verify the object and to bomb the object, the precision is superfluous and the accuracy may or may not be adequate based on the bombing scenario.
13.3.3. Error Budget Concept . When the strike system is not terminally guided, or when the crew member does not acquire the target visually, the aimpoint coordinate is a critical component of system accuracy. Total system accuracy may be viewed as an "error budget" in considering each source con-tributing to the total error. Conceptually, the CEP "error budget" is a set of systematically defined error sources, each of which contributes some identifiable portion to total system inaccuracy. The "error budget" concept allows us to systematically address error contribution to ensure that no single component is excessive. Targeting personnel must ensure that lack of target location accuracy does not degrade the overall system accuracy.
13.3.4. Precise Geopositioning Capability. Coordinate derivation is the process of generating geo-detic coordinates that precisely identifies the position of a point or target. Due to the development of systems with greater precision, it is critical that the accuracy of the target coordinate be commensurate with the strike system CEP. Furthermore, the target coordinate must be described in terms common to the strike. Accurate coordinates and their conversion to a frame of reference usable by the strike system are required because it affects the system and its means of employment (tactics). Targeteers are responsible for converting target coordinates into these common terms and generating the required offset data (i. e., OAP to target, range, and bearing, or rectangular coordinates). Accuracy in describ-ing target position, a desired ground zero (DGZ), or desired point of impact within a common refer-ence system is a key element in targeting and is an important strike/ attack cycle mission function.
13.3.4.1. Point Positioning Data Base (PPDB). PPDBs are sets of geodetically controlled pho-tographic materials, accompanying data, and computer programs which enable trained personnel to derive accurate coordinates for any identifiable ground feature within the database area. PPDB accuracy is estimated for the entire coverage. Derivation of target or point coordinates from PPDBs requires the use of the Analytical Photogrammetric Positioning System (APPS) for men-suration and geopositioning. The APPS is a manual system in which the operator selects the APPROPRIATE stereo pair, locates the target optically, and determines the geoposition of the point. NIMA began phasing out hardcopy PPDB's production in FY96. The use of APPS and PPDBs is decreasing and will culminate as other PGCs mature.
13.3.4.2. Digital Point Positioning Data Base (DPPDB). DPPDB is a classified image product consisting of high- resolution digital stereo image pairs and replaces the hardcopy PPDB. The DPPDB provides warfighters with a deployable product from which latitude, longitude, and eleva-tion can be quickly and accurately derived on digital exploitation workstations with stereo capa- 100
JOHN P. JUMPER, Lt Gen, USAF
DCS, Air & Space Operations
101
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