Synthetic aperture radar interferometry(2)

Synthetic aperture radar interferometry is an imaging technique for measuring the topography of a surface, its changes over time, and other changes in the detailed characteristics of the surface. By exploiting the phase of the coherent radar signal, interf

Fig.1.Typical imaging scenario for an SAR system,depicted here as a shuttle-borne radar.The platform carrying the SAR instrument follows a curvilinear track known as the “along-track,”or “azimuth,”direction.The radar antenna points to the side,imaging the terrain below.The distance from the aperture to a target on the surface in the look direction is known as the “range.”The “cross-track,”or range,direction is defined along the range and is terrain

dependent.

Fig. 2.The three-dimensional world is collapsed to two dimensions in conventional SAR imaging.After image formation,the radar return is resolved into an image in range-azimuth coordinates.This figure shows a profile of the terrain at constant azimuth,with the radar flight track into the page.The profile is cut by curves of constant range,spaced by the range resolution of

radar,defined as 1 =c=21

f

,where c is the speed of light and 1

f is the range bandwidth of the radar.The backscattered energy from all surface scatterers within a range resolution element contribute to the radar return for that element.

The relative phase changes with the topography of the surface as described below,so the fringe variations track the topographic contours.

To overcome the inherent difficulties of inverting ampli-tude fringes to obtain topography,subsequent InSAR sys-tems were developed to record the complex amplitude and phase information digitally for each antenna.In this way,the relative phase of each image point could be reconstructed di-rectly.The first demonstrations of such systems with an air-borne platform were reported by Zebker and Goldstein [10],

and with a spaceborne platform using SeaSAT data by Gold-stein and colleagues [11],[12].

Today,over a dozen airborne interferometers exist throughout the world,spurred by commercialization of InSAR-derived digital elevation products and dedicated operational needs of governments,as well as by research.Interferometry using data from spaceborne SAR instruments is also enjoying widespread application,in large part be-cause of the availability of suitable globally-acquired SAR data from the ERS-1and ERS-2satellites operated by the European Space Agency,JERS-1operated by the National Space Development Agency of Japan,RadarSAT-1operated by the Canadian Space Agency,and SIR-C/X-SAR operated by the United States,German,and Italian space agencies.This review is written in recognition of this explosion in popularity and utility of this method.

The paper is organized to first provide an overview of the concepts of InSAR (Section II),followed by more detailed discussions on InSAR theory,system issues,and examples of applications.Section III provides a consistent mathematical representation of InSAR principles,including issues that im-pact processing algorithms and phenomenology associated with InSAR data.Section IV describes the implementation approach for various types of InSAR systems with descrip-tions of some of the specific systems that are either opera-tional or planned in the next few years.Section V provides a broad overview of the applications of InSAR,including to-pographic mapping,ocean current measurement,glacier mo-tion detection,earthquake and hazard mapping,and vegeta-tion estimation and classification.Finally,Section VI pro-vides our outlook on the development and impact of InSAR in remote sensing.Appendix A defines some of the common concepts and vocabulary used in the field of synthetic aper-ture radar that appear in this paper.The tables in Appendix B list the symbols used in the equations in this paper and their definitions.

We note that four recently published review papers are complementary resources available to the reader.Gens and Vangenderen [13]and Madsen and Zebker [14]cover gen-eral theory and applications.Bamler and Hartl [15]review SAR interferometry with an emphasis on signal theoretical aspects,including mathematical imaging models,statistical properties of InSAR signals,and two-dimensional phase unwrapping.Massonnet and Feigl [16]give a comprehen-sive review of applications of interferometry to measuring changes of Earth’s surface.

II.O VERVIEW OF I NTERFEROMETRIC SAR A.Interferometry for Topography

Fig.3illustrates the InSAR system concept.While radar pulses are transmitted from the conventional SAR antenna,radar echoes are received by both the conventional and an ad-ditional SAR antenna.By coherently combining the signals from the two antennas,the interferometric phase difference between the received signals can be formed for each imaged point.In this scenario,the phase difference is essentially re-lated to the geometric path length difference to the image

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PROCEEDINGS OF THE IEEE,VOL.88,NO.3,MARCH 2000