Principles of Modern Radar - Volume 2 1891121537

Spotlight SyntheticAperture RadarCHAPTER6Daniel A. Cook✬Chapter Outline6.1 Introduction ..................................................................... 2116.2 Mathematical Background ...................................................... 2146.3 Spotlight SAR Nomenclature .................................................... 2206.4 Sampling Requirements and Resolution ......................................... 2256.5 Image Reconstruction........................................................... 2346.6 Image Metrics .................................................................. 2406.7 Phase Error Effects ............................................................. 2446.8 Autofocus....................................................................... 2506.9 Summary and Further Reading . . ................................................ 2536.10 References ..................................................................... 2556.11 Problems ....................................................................... 257✫✩✪6.1 INTRODUCTIONThe decades of development of synthetic aperture radar (SAR) have resulted in a familyof remarkable signal processing techniques that are capable of producing imagery whosecross-range resolution is independent of range and much finer than is possible to achievewith any practically-deployable real-beam antenna. SAR was initially conceived in the1950s by Carl Wiley in the context of stripmap collection. He was, by all accounts, atalented and colorful person [1] whose contributions include the first serious considerationof the feasibility of the solar sail [2]. This early work ignited a legacy of research,development, and practical application that remains strong to this day.Years later it was realized that under certain conditions the collected radar data canbe thought of as sampling the Fourier transform of the scene reflectivity. This thinkingproduced the spotlight mode, which is the topic of this chapter. Spotlight mode is arguablythe dominant type of SAR in current usage. While the stripmap mode is useful for imaginglarge areas, its resolution is typically more coarse. For this reason, spotlight SAR iscommonly employed in military applications where the goal is not to survey a large regionbut to produce the best possible image of a specific area of interest. This chapter emphasizesspotlight SAR. Yet, in the grand scheme it is important to realize that the various SARmodes—spotlight, stripmap, and inverse—are somewhat artificial distinctions based oncategorizing algorithms according to criteria such as application, collection geometry, and211