Interferometric Synthetic Aperture Radar

This paper provides a brief review of interferometric synthetic aperture radar (InSAR), its history, the theory, and design / implementation / processing issues. Along-track, single-pass and repeat-pass cross-track interferometry are reviewed. Specific topics addressed include error sources and phase-unwrapping techniques. Several examples of InSAR applications are presented. Introduction For years synthetic aperture radar (SAR) has been used to produce photograph-like images of terrain features. Conventional SAR systems provide a two-dimensional map of the radar reflectivity of the illuminated scene. While complex data are collected and processed to produce the SAR image, one of the final steps in its production is to reduce a complex image (containing both magnitude and phase information) to a purely magnitude image, with the phase information being discarded. Radar interferometry, on the other hand, depends on phase information. Through interferometry, range information can be resolved to less than a wavelength. However, interferometry brings with it range ambiguities that limit its usefulness. Together, SAR and interferometry provide additional information to that of a conventional SAR. Depending on the implementation, interferometric SAR, or InSAR, can survey height information of the illuminated scene, measure the radial velocity of moving scatterers, track subtle terrain motions, or detect slight changes in scene content. History Graham [ 1974] first demonstrated interferometric SAR by using an airborne SAR system configured as a cross-track or vertical interferometer. He used two vertically separated antennas to receive simultaneously backscattered signals from the terrain. Vectoral addition of these signals produced a pattern of nulls corresponding to predetermined depression angles, which, when used in conjunction with range information, yielded elevation information. He recorded data optically from two channels: one was the normal SAR data; the other, the interferometer output containing the null patterns. He showed that since the multiple nulls were ambiguous, the elevation of at least one point within the scene must be determined by an alternate means to resolve the absolute elevation. Goldstein and Zebker [1987] first employed an along-track interferometric SAR configured to measure radial velocity. They used two horizontally separated antennas to receive backscattered signals from the moving sea surface. They processed the signals separately to form two complex images, which they then combined interferometrically; i.e., the phases were differenced pixel by pixel. They showed that since radial motion of a surface scatterer causes a phase difference between the two images that is proportional to the distance moved, scatterer motion can be measured. Gabriel and Goldstein [1988] first demonstrated single-antenna, repeat-pass interferometry by using data collected on two separate passes of the Shuttle Imaging Radar (SIR-B). Despite the fact that the orbits were skewed, through refocussing and careful image registration, they obtained an altitude map of the imaged region. Theory Whereas conventional SAR uses a single antenna, InSAR requires two antennas separated by a baseline (B). Signals from both antennas are recorded and processed to yield two complex SAR images of the same scene. Phases measured in each of the scenes are differenced on a pixel-by-pixel basis to obtain additional geometrical information about the scene. When the receive antennas are vertically separated, this phase difference can be interpreted as pixel height as illustrated in Figure 1. Pixel height, h, and phase difference, phi are related by [Li and Goldstein, 1990]: where the parameters are those shown in Fig. 1 and lambda is the radar wavelength. Equations (1) and (2) assume a single-pass system; i.e., a single transmit antenna and dual receive antennas. When repeat-pass interferometry is used, the 2pi term in (1) should be replaced by 4pi When the receive antennas are separated horizontally along the radar velocity vector (along-track InSAR), phase differences can be interpreted as scatterer motion proportional to the radial distance moved in the time required for the rear antenna to move to the position previously occupied by the forward antenna [Goldstein and Zebker 1987]. The phase difference and the radial velocity of the scatterer are related by Figure 1. Typical geometry of a cross-track interferometric SAR. Targets A and B are at the same azimuth and slant range to antenna 1. In conventional SAR, where only one antenna is used, the returns from A and B are projected into the same pixel. In the InSAR configuration both antennas are used and a phase difference between antennas 1 and 2 can he used to derive the height for each pixel. (after Li and Goldstein