Principles of Modern Radar - Volume 2 1891121537

8.1 Introduction 3398.1.1 OrganizationThis chapter begins with an introduction to digital elevation models, the desired outputof the InSAR process, in Section 8.1.3. Section 8.3 discusses how elevation is measuredin InSAR using pairs of range measurements. Analysis of the accuracy requirementsestablishes the need for using phase to measure range. The differences between one-passand repeat-pass operation and between airborne and spaceborne InSARs are consideredin Section 8.4.Section 8.5 introduces the major processing steps needed to actually form DEMs usingInSAR, with an emphasis on the problem of phase unwrapping in two dimensions. Briefdiscussions of image registration, absolute elevation estimation, and orthorectification andgeocoding are included. Error sources in InSAR are discussed in Section 8.6, which alsointroduces the important concept of coherence. Selected notable airborne and spaceborneInSAR systems are described in Section 8.7. Although this chapter focuses strongly on 3-DInSAR elevation profile mapping, Section 8.8 provides a short introduction to the relatedmethods of terrain motion mapping, change detection, and along-track interferometry.Finally, Sections 8.9 and 8.10 summarize the chapter and provide suggestions for furtherreading.8.1.2 Key Points• Coherent imaging radars, both airborne and spaceborne, can generate very high-qualityDEMs covering large areas on dense sampling grids at day or night and in all weather.• Terrain elevation profile variations can be measured using pairs of range measurements,but achieving sufficient elevation profile accuracy requires that range be measured tosmall fractions of a typical radar wavelength; hence, radar echo phase variations areused to measure small range variations.• Formation of InSAR elevation maps requires formation of pairs of high-quality SARimages, followed by a complicated series of signal processing operations to developthe DEM such as subpixel image registration, two-dimensional phase unwrapping, andorthorectification to correct geometric distortions.• A variety of sensor configurations and operational concepts can be used to acquireInSAR data, including single- and multiple-receiver radars, one- and repeat-pass collectionprotocols, and multiplatform formation flying protocols.• The coherence of an InSAR image pair is the primary determinant of the elevationestimate quality. It is affected by a variety of statistical and systematic error sources.• A variety of airborne and spaceborne InSAR systems have been placed in service, mostsince about 1995. New systems continue to be developed and refined.• Using appropriate sensor configurations, the same basic technology can be used to mapscatterer and terrain motion and reflectivity changes over time scales from fractions ofa second to years.8.1.3 NotationFollowing are the principal notations used in this chapter:x ∗ complex conjugate of x〈 · 〉 2π evaluation modulo 2πα IF interferometric scale factor