Synthetic aperture radar interferometry(3)

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.3.Interferometric SAR for topographic mapping uses two apertures separated by a“baseline”to image the surface.The phase difference between the apertures for each image point,along with

the range and knowledge of the baseline,can be used to infer the precise shape of the imaging triangle to derive the topographic height of the image point.

point,which depends on the topography.With knowledge of the interferometer geometry,the phase difference can be con-verted into an altitude for each image point.In essence,the phase difference provides a third measurement,in addition to the along and cross track location of the image point,or “target,”to allow a reconstruction of the three-dimensional location of the targets.

The InSAR approach for topographic mapping is similar in principle to the conventional stereoscopic approach.In stere-oscopy,a pair of images of the terrain are obtained from two displaced imaging positions.The“parallax”obtained from the displacement allows the retrieval of topography because targets at different heights are displaced relative to each other in the two images by an amount related to their altitudes[17]. The major difference between the InSAR technique and stereoscopy is that,for InSAR,the“parallax”measurements between the SAR images are obtained by measuring the phase difference between the signals received by two InSAR antennas.These phase differences can be used to determine the angle of the target relative to the baseline of the interfer-ometric SAR directly.The accuracy of the InSAR parallax measurement is typically several millimeters to centimeters, being a fraction of the SAR wavelength,whereas the par-allax measurement accuracy of the stereoscopic approach is usually on the order of the resolution of the imagery(several meters or more).

Typically,the post spacing of the InSAR topographic data is comparable to the fine spatial resolution of SAR imagery, while the altitude measurement accuracy generally exceeds stereoscopic accuracy at comparable resolutions.The regis-tration of the two SAR images for the interferometric mea-surement,the retrieval of the interferometric phase differ-ence,and subsequent conversion of the results into digital el-evation models of the terrain can be highly automated,repre-senting an intrinsic advantage of the InSAR approach.As dis-cussed in the sections below,the performance of InSAR sys-tems is largely understood both theoretically and experimen-tally.These developments have led to airborne and space-borne InSAR systems for routine topographic mapping.

The InSAR technique just described,using two apertures on a single platform,is often called“cross-track interferom-etry”(XTI)in the literature.Other terms are“single-track”and“single-pass”interferometry.

B.Interferometry for Surface Change

Another interferometric SAR technique was advanced by Goldstein and Zebker[18]for measurement of surface mo-tion by imaging the surface at multiple times(Fig.4).The time separation between the imaging can be a fraction of a second to years.The multiple images can be thought of as “time-lapse”imagery.A target movement will be detected by comparing the images.Unlike conventional schemes in which motion is detected only when the targets move more than a significant fraction of the resolution of the imagery, this technique measures the phase differences of the pixels in each pair of the multiple SAR images.If the flight path and imaging geometries of all the SAR observations are identical, any interferometric phase difference is due to changes over time of the SAR system clock,variable propagation delay,or surface motion in the direction of the radar line of sight.

In the first application of this technique described in the open literature,Goldstein and Zebker[18]augmented a con-ventional airborne SAR system with an additional aperture, separated along the length of the aircraft fuselage from the conventional SAR antenna.Given an antenna separation of roughly20m and an aircraft speed of about200m/s,the time between target observations made by the two antennas was about100ms.Over this time interval,clock drift and propa-gation delay variations are negligible.Goldstein and Zebker showed that this system was capable of measuring tidal mo-tions in the San Francisco bay area with an accuracy of sev-eral cm/s.This technique has been dubbed“along-track in-terferometry”(ATI)because of the arrangement of two an-tennas along the flight track on a single platform.In the ideal case,there is no cross-track separation of the apertures,and therefore no sensitivity to topography.

C.General Interferometry:Topography and Change

ATI is merely a special case of“repeat-track interferom-etry”(RTI),which can be used to generate topography and motion.The orbits of several spaceborne SAR satellites have been controlled in such a way that they nearly retrace them-selves after several days.Aircraft can also be controlled to repeat flight paths accurately.If the repeat flight paths result in a cross-track separation and the surface has not changed between observations,then the repeat-track observation pair can act as an interferometer for topography measurement. For spaceborne systems,RTI is usually termed“repeat-pass interferometry”in the literature.

If the flight track is repeated perfectly such that there is no cross-track separation,then there is no sensitivity to topog-raphy,and radial motions can be measured directly as with an ATI system.Since the temporal separation between the ob-servations is typically hours to days,however,the ability to detect small radial velocities is substantially be …… 此处隐藏：12203字，全部文档内容请下载后查看。喜欢就下载吧 ……