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Chapter 9:
Radar and LIDAR
Abstract
So far, we have focused on remote sensing techniques that are associated with imaging. In this concluding chapter, well consider some remote sensing techniques that are non-imaging, particularly those used for measuring elevation. This can be done with radar of the sort described in the previous chapter or with laser radar (LIDAR). In the first case, elevation is generally developed using interferometry. Radar interferometry is the study of interference patterns created by combining two sets of radar signals. This technique allows for a number of powerful additional uses for SAR data beyond the formation of literal images. Two of the more important applications are topographic mapping and change detection. Both exploit the fundamental concept that SAR images contain both amplitude and phase information. So far, we have focused on remote sensing techniques that are associated with imaging. In this concluding chapter, well consider some remote sensing techniques that are non-imaging, particularly those used for measuring elevation. This can be done with radar of the sort described in the previous chapter or with laser radar (LIDAR). In the first case, elevation is generally developed using interferometry. 9.1 Radar Interferometry Radar interferometry is the study of interference patterns created by combining two sets of radar signals. This technique allows for a number of powerful additional uses for SAR data beyond the formation of literal images. Two of the more important applications are topographic mapping and change detection. Both exploit the fundamental concept that SAR images contain both amplitude and phase information. 9.1.1 Topographic mapping Topographic mapping makes use of pairs of images acquired over a fairly modest spatial baseline (typically on the order of kilometers), over relatively short time intervals. The latter is defined by the need to have relatively few changes in the scene between observations. For satellites such as ERS-1 and 2 and RADARSAT, these conditions are normally obtained by comparing data from observations within a few days of one another, from nearly identical orbits. The desirability of producing such products adds to the demand for strict constancy in the near-circular orbits of these satellites. The geometry is illustrated by Fig. 9.1. Targets 1 and 2 are imaged on two separate orbits as illustrated. Given the offset in the satellite location (by a distance indicated here as a baseline), there will be a relative difference in the paths to the targets (s2′ − s2 ≠s1′ − s1) that can be accurately determined to within a fraction of a wavelength. This difference in phase can then be translated into elevation differences.
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CHAPTER 9
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