Recently, interferometric SAR processing has become very common as a tool for the surface deformation detection. For the basis of the data interpretation, we would like to introduce a processing principle of the synthetic aperture radar data in this paper.

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Synthetic aperture radar (SAR) interferometry from space has become an important and powerful tool for measuring dense spatial distribution of surface deformation of the Earth. However, this technique has not widely used by geodetic researchers in Japan because it is quite new and needs a lot of knowledge of 2-D image analyses which have not been used in crustal deformation study. This paper is an introductory guide for the geodetic researchers who will use SAR interferometry to detect surface deformation, and shows its principle, analysis processes and its error sources.

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A good coherence of a SAR interferogram gives precise measurement of surface change. The quality of coregistration between two SLC (Single Look Complex) images influences the coherence. A new coregistration algorithm is proposed. A pixel that has the maximum amplitude is automatically selected to be a center of each correlation window in a master SLC image. This method and employing a large size (128 x 128 pixel) of correlation windows decrease the number of bad offset data. Moreover, 32 times FFT over-sampling of offset field is effective to get subpixel (1/32 pixels) resolution of the pixel offset between two SLC images. The algorithm is tested with JERS-1 SAR images of North Sakhalin to evaluate the performance. The result shows that it enables us to get enough tie points over the whole images with a precision of 0.1 pixel without interactive sampling of them.

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Making SAR interferograms for the Earth's surface change detection, determination of a baseline between positions of a repeat-pass satellite severely affects its precision. The baseline is automatically estimated using differential phase in the interferogram and a DEM (Digital Elevation Model). It is essential to use as wide area as possible for the determination of the baseline. In the SAR interferograms, we often find error correlation between differential changes and the topography (elevation), so apparent error surface deformation will appear. This error is due to the differential tropospheric delay between the two SAR scenes in different atmospheric condition. We applied a simple and effective correction method that the error is removed by subtracting the DEM multiplied a coefficient.

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In use of the repeat pass SAR interferometry, errors in the satellite's state vector and the atmospheric excess path delay degrade the accuracy of measured geodetic information (topography and surface deformation). This information requires centi-meter order in phase measurement, thus the accuracy of current state vector does not meet the requirement . Since the path delay caused by water vapor reaches several tens centi-meter as a two day difference, the correction is essential. Here, a method to correct the atmospheric excess path delay using the global objective analysis data (CANAL) and a method to estimate the state vector using ground control points are investigated. This method was evaluated by using Mt . Fuji and Mt. Iwate data both observed by JERS-1 and this method was shown very effective .

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We succeeded to detect land subsidence in northern Kanto area by JERS-1 SAR interferometry. The amount of the subsidence was calculated from the SAR interferogram and compared with the leveling data. We found that the subsidence that is calculated from SAR interferogram has a bias, which is considered to come from the baseline estimation in SAR interferometry process.

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We acquired European Remote Sensing Satellite 1 and 2 (ERS-1/2) synthetic aperture radar (SAR) data at an ending stage of the tandem mission over Antarctica from February 15 through June 3, 1996, using the S/X band antenna at Syowa Station, Antarctica. The number of acquired pair scenes attained more than 1100. We processed two tandem pair scenes as examples; one over the Shirase Glacier which flows down to Lutzow-Holm Bay, and the other over the Baudouin Ice Shelf which lies in the eastern part of the Princess Ragnhild coast. We successfully generated interferograms for the both scenes. Because of short (13 m and 17m) satellite baselines and rather flat topographies over the two scenes, the resultant dense fringes within each scene can be considered as reflecting mainly displacements/deformations. The detection of clear fringes over the fast-flowing (7 m/ day) Shirase Glacier shows the effectiveness of the 1-day apart tandem mission. The dense fringes around the Delwell Ice Rise in the Baudouin Ice Shelf scene must be related to the oceanic tidal motion, delineating steep deformation bands near the grounding line of the ice shelf.

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Spaceborne repeat-pass SAR interferometry (RP-INSAR) is an excellent tool to detect the very slight earth crustal movements caused by an earthquake, a volcanic activity, and so on. In principal, an airborne SAR system has an ability to detect the movement as well. In order to realize an airborne RP-INSAR, the airplane must be flown on almost the same pass repeatedly and very precise airplane motion data are required to process the RP-INSAR data. We developed an X-band airborne RP-INSAR system with a D-GPS navigation system, and carried out the data correction using the phase information from corner reflectors set on the ground. As a result, we succeeded the airborne RP-INSAR processing. Furthermore, we tried to investigate an L-band airborne RP-INSAR system, and showed that L-band system is better than X-band system on the points of spatial decorrelation and surveying without reflectors in order to detect an earth crustal movement.

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