NASA SIR-C Data
Further Information on SIR-C Products
Selected data takes were processed as Precision products. Precision products are a subset of a data take and contain high resolution multi- frequency and multi-polarization data. A multi-look Precision product has a frame size of 100 Km with 25 m resolution and 12.5 m pixel spacing for all radar channels. A SIR-C single-look Precision product has a 50 Km frame size with natural (i.e. not resampled to a regular ground sample distance interval) pixel spacing at full resolution for all radar channels. An X- SAR single-look Precision product has a frame size of 100 Km with natural pixel spacing at full resolution. Flight 1 data processed before October 1, 1994 are not calibrated and flight 2 data processed before February 1, 1995 are not calibrated. However, the data can still be used for qualitative analysis. For additional information on Precision product characteristics see the JPL Imaging Radar Home Page, or the SIR-C Education CD-ROM for a tutorial on the radar data characteristics.
SIR-C survey processing is based on a burst mode processing algorithm. To attain a high throughput rate, the data were bursted in azimuth (slow time) with a one-quarter duty cycle factor. The data volume was further reduced by a factor of four in range (fast time) by processing the data using only one- quarter of the range chirp bandwidth. The azimuth compression was performed using the spectral analysis (SPECAN) algorithm. Following azimuth compression, radiometric correction was applied to compensate for the cross- track and along track radiometric modulation. This was followed by a geometric rectification step that resamples the slant range-Doppler image into the ground range cross-track and along-trac domain. The rectified burst images are then overlaid to produce the final multi-look image.
The initial doppler centroid frequency was determined using a clutterlock routine and a Pulse Repetition Frequency (PRF) ambiguity resolution technique. The unambiguous doppler centroid frequency was then tracked by a burst mode clutterlock algorithm during the data processing. The roll angle estimates are derived from the null-line data at one-second intervals. The quality analysis functions, which include raw data histogram, range spectra, and azimuth spectra, are performed at one-minute intervals.
The X-SAR survey processor design was based on a range-doppler algorithm. To attain a high throughput rate, the data volume was reduced by applying a variable pre-summing and undersampling to the range lines to meet the range resolution requirement of 100 m (ground range). The range processing was performed by using the consequently reduced chirp bandwidth. The azimuth processing was performed by first using the full azimuth bandwidth and then by applying, after detection and resampling, an azimuth multi-look filtering for speckle reduction.
The doppler centroid frequency estimation was performed by applying the CDE algorithm every one second of acquisition. At the beginning and at the end of the data take, the multiple PRF algorithm was applied for the doppler ambiguity resolution. A raw data quality analysis was performed every 15 seconds of data. For both the SIR-C and X-SAR survey products, the doppler centroid estimates and the Q/A results are included in the CD-ROM product. Additionally, for the SIR-C product only, the roll angle estimates are included.
All land coverage and some ocean coverage has been processed to Survey data. Data contained on this FTP server is also available on CD-ROM at a nominal charge. Information on obtaining Survey Data CD-ROMs is available by contacting the EDC Customer Services at firstname.lastname@example.org .
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