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SIR-C Interferometry
BackgroundA portion of the second flight of SIR-C/X-SAR (Mission Elapsed Time days 7, 8, 9, and part of day 10) was devoted to collecting a series of repeat pass interferometry data_takes collected at sites all over the world. In addition, nine data_takes were collected that were repeat passes of data_takes collected on the first flight of SIR-C/X-SAR in April 1994. All the interferometry data_takes are summarized in a table. In order to collect these interferometry data, the shuttle's orbit was circularized so that the same targets would be passed over with the same viewing geometry every 24 hours. The radar configuration (look angle, radar mode, bandwidth, pulse repetition frequency, etc.) was identical for each data_take over a site. The data that were collected on most of the SIR-C interferometry data_takes are Mode 20: L-band VV and C-band VV, both collected at 40 mHz bandwidth. Dual and quad polarization data (HH, HV, VV, and VH) were collected on some of the interferometry passes. X-SAR data (X-band VV polarization) were also collected as part of the repeat pass interferometry experiment. The passes that were collected are quite long, some crossing whole continents. This was done to increase the chances that at least a portion of the data pass would yield successful interferometry results. Results and ReferencesSince these data were collected on an experimental basis, the data set and interferometry products generated by the SIR-C processor are not well documented. However, the SIR-C/X-SAR repeat pass interferometry experiment is considered a success and many results have been obtained to date. Most of the research on this dataset has been done at the Jet Propulsion Laboratory, the DLR in Germany, and also at a few universities. Some references, a report on interferometry, and a short DEM generation tutorial are available. Products
The SIR-C processor can produce two types of interferometry data products: interferometric single-look complex (iSLC) products and raw interferograms. In addition, some researchers use the Reformatted Signal Data (RSD) product to generate interferograms. At least two data_takes from a repeat pass suite of data_takes (2 to 4 data_takes) must be identified for each interferometry data product. Pre-processing baseline estimates are not available and the customer may not be able to select the optimal pair of data_takes for processing a priori, but all of the baselines for scenes processed so far are less than 400 meters and most are less than 200 meters. It is recommended that data processing requests be submitted by specifying the center latitude and longitude of a scene. Using the center latitude/longitude, rather than the time, increases the amount of overlap between the two scenes. Interferometric Single Look Complex (iSLC) Products:
iSLC products are frame products and can be processed for the same area from two or more repeat pass data_takes from a given suite. The iSLC product is a CEOS-formatted frame image that is 50 km along the flight track by the swath width (which varies with the viewing geometry and the radar mode). iSLC's are distributed in CEOS format on high- or low-density 8 mm tapes. As with all CEOS distribution tapes, CEOS_reader software or direct import capability is required. The image is in slant range with natural spacing and is full resolution. Typically the files are hundreds of megabytes in size for each scene. iSLC's are uncalibrated SLC images that are processed with the same Doppler centroid (for all data_takes requested) to allow interferometric processing. Reformatted Signal Data (RSD) Products:RSD products are not technically interferogram products, however some researchers have used them to generate interferograms. RSD products are frame products (100 km along track) in the CEOS format and can be processed for the same area from two or more repeat pass data_takes from a given suite. Raw Interferogram (RIn) Products:
The raw interferogram products are distributed on high-density 8 mm tape in TAR format and contain the files listed below. Unlike the previous two products (iSLC and RSD), the customer can specify no more than two data_takes for a raw interferogram product. Two processing runs are combined for each frequency to produce interferograms for L-band and C-band. The interferograms and the detected (backscatter amplitude) images have had 4 looks in the azimuth direction (four pixels averaged). The following files are included in the raw interferogram data product (all file names start with a 5 character processing run number):
CaveatsThe following caveats apply to these data products.
Ordering SIR-C Interferometry Data ProductsIf you are interested in interferometry data over a specific site, first check the survey data for an interferometry data_take to determine if your site was covered. Then contact DAAC User Services with the following information:
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