Alaska Satellite Facility - Distributed Active Archive Center

Seasat – Technical Challenges – 7. Cleaned Swath Files

Seasat synthetic aperture radar (SAR) data holdings at ASF have been converted from their original 29 SONY SD1-1300L tapes into raw swath files with external metadata stored on disk. As detailed elsewhere in these pages, a total of 1,346 decoded, cleaned swath files were created.

The decoded data originally contained bit errors and discontinuities that had to be addressed prior to processing images. Cleaning the decoded data was an extensive process — critical metadata fields were median filtered to remove bit errors, time values were filtered into linear progressions, and discontinuities were filled with random values. In all, five separate filters were run on the telemetry data after it was decoded in order to prepare it for automated processing into image products.

The cleaned raw swath files come as file pairs. The unpacked raw data resides in a .dat file, and the decoded metadata resides in an .hdr file. The .dat files contain 13,680 byte samples of SAR data per line, so the file size for swath data files is always the number of lines in the file multiplied by 13,680 bytes.

The .hdr file contains a single entry for each line of data present in the corresponding .dat file. This entry is ASCII text formatted into 20 columns of integers recording the metadata decoded from the satellite time and status telemetry bits. If any discontinuities were found and repaired during the cleaning of the decoded data, then a third .dis file records the location and length of discontinuities.

Written by Tom Logan, July 2013