Principles of Modern Radar - Volume 2 1891121537

342 CHAPTER 8 Interferometric SAR and Coherent ExploitationSIR-CSMTISRTMSTAPUTMWGSshuttle imaging radar Csurface moving target indicationshuttle radar topography missionspace-time adaptive processinguniversal transverse Mercatorworld geodetic system8.2 DIGITAL TERRAIN MODELSA digital terrain model (DTM) is a digital representation of the elevation of a portion ofthe Earth’s surface [2]. It typically is composed of elevation measurements for specifiedpoints, lines, and surface elements and may also include an interpolation scheme forestimating elevation between sample points and descriptive metadata such as the latitudeand longitude of a cell, its estimated height error, the data source, and so forth. The termDEM, also sometimes called a digital height model (DHM), usually implies a simplearray of regularly spaced elevation values referenced to a standard geographic coordinatesystem [3]. A DHM or DEM is a special case of a DTM. The term DEM also refers to aspecific class of data products available from the U.S. Geological Survey. The data in aDTM or DEM are usually intended to represent the elevation of the bare or bald earth. Incontrast, a digital surface model (DSM) is a representation of the top of the terrain ratherthan the bare earth. In a forested area, a DSM estimates the elevation of the forest canopy,while a DEM estimates the elevation of the forest floor.Digital terrain models have an expanding variety of uses. The most obvious andimportant is topographic mapping, which in turn is useful for such diverse applicationsas three-dimensional visualization, terrain analysis for precision agriculture, line-of-sight(LOS) mapping for telecommunications tower siting and utilities routing, disaster analysis(e.g., flood mapping), and navigation. A less obvious example is the use of DTMs toenhance radar GMTI and STAP performance by incorporating knowledge of the terrain intothe clutter statistics estimation procedures at the core of GMTI and STAP algorithms [4].What degree of accuracy makes for a useful DEM? The quality of a DEM is determinedby the spacing of the grid points (the denser, the better) and the accuracy of the individualelevation values. A commonly cited DEM standard is the Digital Terrain ElevationData (DTED) specification developed by the U.S. National Geospatial Intelligence Agency(NGA) and its predecessors [5]. DTED DEM data are classified into six DTED levels numbered0 through 5, though only levels 0 through 2 are standardized [6]. Table 8-1 shows theTABLE 8-1Selected DTED SpecificationsAbsolute Vertical Relative VerticalDTED Level Post Spacing ‡ Accuracy Accuracy0 30.0 arc sec ≈ 928 m < 30 m < 20 m1 3.0 arc sec ≈ 93 m < 30 m < 20 m2 1.0 arc sec ≈ 31 m < 18 m < 12–15 m3 † 0.3333 arc sec ≈ 10 m < 10 † m < 1–3 † m4 † 0.1111 arc sec ≈ 3m < 5 † m < 0.8 † m5 † 0.0370 arc sec ≈ 1.1 m < 5 † m < 0.33 † m† Accuracies for DTED levels 3–5 are proposed but not final and not included in MIL-PRF-89020B. Various sourcesreport varying values for the proposed accuracy.‡ Post spacings in meters are based on a nominal earth radius of 6378.1 km.