INTERFEROGRAM: A Bold New Direction
by Gwendolyn R. Bryson, Alaska Satellite Facility
The ASF SAR Data Center (SDC) now provides users the capability to search-and-order Interferometric SAR (InSAR) pairs through the online User Remote-Sensing Access (URSA) interface and has expanded the data-product offerings to include custom InSAR products. Users are now capable of identifying stacks of SAR granules suitable for interferometric processing, assess the perpendicular and temporal baseline distribution of a stack by interacting with the online baseline plot tool (Figure 1), and select pairs to order a number of interferometric products.
By exploiting phase differences between two SAR images, interferometric processing techniques provide estimates of topographic changes to a subcentimeter level. In principle, data from satellites in the ASF archive are suitable for InSAR, as long as the image pair adheres to some very basic rules. The data needs to be acquired by the same satellite, in the same beam mode, and with the same look direction. There are a few exceptions to this rule. The tandem mode of the European Remote-Sensing Satellite-1 (ERS-1) and ERS-2 satellites provides data very suitable for InSAR applications because both satellites are identical and meet the criteria above with a favorable temporal baseline of one day. ScanSAR data, available for RADARSAT-1 and Advanced Land Observing Satellite Phased Array L-band SAR (ALOS PALSAR), are another special case as these are formed by combining several beam modes in one swath. The interferometric processing of ScanSAR data is still on a research level that does not allow the use in a production environment. This data type has not been included in the InSAR capable datasets.
In the InSAR tab of the search results in URSA, users will find granules organized into lists of interferometrically-suitable datasets, referred to as InSAR stacks. InSAR stacks are composed of granules that cover the same geographic region, are from the same platform, and were acquired with the same beam mode, look angle, and bandwidth. Theoretically, two granules in an InSAR stack may be used to create an interferogram, however, image selection depends entirely upon the specific application of study. Often researchers will explore the distribution of baseline lengths to determine the feasibility of interferometry for a given pair and application. Baseline is the temporal and spatial distance between two satellite observations and its length provides an indication of the sensitivity to topographic height. URSA users may access baseline plots to view granule acquisition dates, parallel, perpendicular, and temporal baselines, as well as to filter InSAR stacks and observe only those granules that are within specified baseline lengths.
Users who order an interferogram from ASF will receive a beta product that has been processed using the Repeat Orbit Interferometry Package (ROI PAC). ROI PAC is an open-source software package developed and distributed by the Jet Propulsion Laboratory, with contributions from the user community, for the purpose of creating SAR interferograms (Figure 2).
ASF offers users four processing options: UTM_GEOTIFF, COMPLEX_PAIR, COMPLEX_IGRAM, and COMPLEX_PAIR_AND_IGRAM. The UTM_GEOTIFF is the basic InSAR product and includes an interferogram, coherence image, and amplitude image geocoded to UTM (Universal Transverse Mercator) projection and stored in GeoTIFF format. InSAR product packages produced at ASF include the contents of the UTM_GEOTIFF package. For users interested in completing their own ROI-PAC processing, the COMPLEX_PAIR package contains the complex Master and Slave data in ROI-PAC format and the COMPLEX_IGRAM package provides the complex interferograms in ROI-PAC format. For those interested in the additional items contained in both the COMPLEX_PAIR and COMPLEX_IGRAM product packages, the COMPLEX_PAIR_AND_IGRAM option is offered.
As the ASF SDC strives to promote, facilitate, and participate in the advancement of remote-sensing data, ASF is pleased to announce the beta release of the InSAR product packages. Whether your interest in InSAR is for topographic mapping, deformation, glacier monitoring, volcanic observation, or other applications, ASF invites your feedback on their InSAR tools and products, and looks forward to making improvements that will better serve your research and scientific needs.
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