Principles of Modern Radar - Volume 2 1891121537

344 CHAPTER 8 Interferometric SAR and Coherent ExploitationTABLE 8-2Selected HRE SpecificationsHRE Product Post Absolute Vertical Relative VerticalLevel Name Spacing Accuracy LE90 † (goal) Accuracy1 HREGP 0.4 arc-sec 12.4 m 6.2 m(≈12.4 m)2 HRE80 8 m 8 m 4 m3 HRE40 4 m 4 m 2 m4 HRE20 2 m 2 m 1 m5 HRE10 1 m 1 m 0.5 m6 HRE05 0.5 m 0.5 m 0.25 m7 HRE02 0.25 m 0.25 m 0.12 m8 HRE01 0.125 m 0.12 m 0.06 m† LE90 = linear error at 90% probability level. See [11].Development of an updated version of the DTED specification, called high-resolutionterrain information (HRTI), was initiated by NGA as standard MIL-PRF-89048. However,more recently NGA has superseded that effort with development of the high-resolutionelevation (HRE) DEM standard [11], which provides eight levels and is suitable for newer,finer resolution sensors. Table 8-2 lists some of the HRE specifications. NGA has statedthat it will continue to support DTED levels 0, 1, and 2 but not levels 3–5 or the HRTI draftspecification. DTED levels 0–2 and HRE levels 1–8 collectively cover relative (point-topoint)vertical accuracies ranging from approximately 20 m to 0.06 m (about 2.4 inches).8.3 ESTIMATING ELEVATION PROFILES USINGRADAR ECHO PHASESAR systems form fine resolution 2-D images using various approximations to matchedfiltering of the point target response of terrain scatterers, as discussed in the previous twochapters. In InSAR, the third dimension of elevation is generally obtained not by matchedfiltering, but by phase interferometry techniques. Some newer, more advanced systemsare beginning to explore matched filtering in elevation, as described in Section 8.3.8.8.3.1 The Effect of Scatterer Elevation on 2-D SAR ImagesA general stereo imaging geometry is shown in Figure 8-4. For simplicity, we consider sidelookingradar only (no squint). A sensor platform such as an aircraft or spacecraft travelsin the x-dimension (normal to the plane of the figure). A coherent pair of imaging sensors,denoted a and b, are separated by a spatial baseline B orthogonal to the velocity vector(thus in the y–z plane) at an angle β with respect to horizontal. The angle β is often approximatelyeither 0 ◦ (horizontal displacement of sensors) or 90 ◦ (vertical displacement). Thesensors are located at an arbitrary position x 0 in the cross-range (also called along-track)coordinate x, which is not shown in the figure. The center of the baseline is located at anelevation z = H relative to the x–y ground plane and ground range position y = 0. 33 We assume a flat earth (flat ground plane) for simplicity. In practice, spaceborne InSAR systems mustaccount for Earth curvature.