Principles of Modern Radar - Volume 2 1891121537

358 CHAPTER 8 Interferometric SAR and Coherent ExploitationFIGURE 8-11SAR image of theU.S. Capitol showinglayover distortion ofthe dome. (Courtesyof Sandia NationalLaboratories.)Figure 8-10b is the same scene with foreshortening distortion accounted for. This isthe slant range SAR image naturally generated by the radar. Foreshortening of the brightbands is clearly evident in the beginning and end of the hill and in the edge of the plateauas well as in the shortening of the range extent of the image from 400 m to about 392 m,as shown by the end of the data at a range of +192 m.Figure 8-11 is an airborne image of the U.S. Capitol. The radar is imaging the scenefrom a position above the top of the image. Layover toward the top of the image is clearlyevident in the distorted dome. Figure 8-12a is a satellite SAR image of the volcano MountVesuvius in Italy collected by the ERS-2 SAR, again with the radar viewing the scenefrom a position above the top of the image [22]. Compared with the satellite photo inFigure 8-12b, foreshortening and brightening of the slopes of the volcano facing the radarare both evident.8.3.8 Other Three-Dimensional Coherent SAR TechniquesThe InSAR approach that is the focus of this chapter, and of virtually all operational 3-DSARs, is based on using a two-element interferometer orthogonal to the flight path toestimate angle of arrival and thus scatterer elevation. More advanced approaches usingmultiple spatial samples orthogonal to the flight path are being investigated for obtainingmultiple-resolution cell imagery in the vertical dimension. An introduction to the twomajor multiple sample methods is given in [23].The first, called tomographic SAR, uses some number N of vertical apertures insteadof the two used in standard InSAR. These could be obtained using multiple flight passes orby building a multiple-aperture system. Analysis of the phase signature observed across the