README.TXT This file includes general information about the SIR-C/X-SAR mission as well as information on how to use the CD-ROM. This CD-ROM is a sample CD containing SIR-C survey images processed during the Flight 1 mission. Each image will be reprocessed during Phase 1 operations and redistributed on a Phase 1 CD-ROM. This CD is the second SIR-C sample CD-ROM. The first sample, distributed in January 1994, contains data acquired by SEASAT and SIR-B processed by SIR-C. Mission Summary: The Shuttle Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) is an imaging radar system scheduled for launch aboard the space shuttle in the mid 1990s with two planned reflights in different seasons. Flying at an altitude of 215 km in a circular orbit and a 57 degree inclination, the three-frequency, multipolarization radar performs a variety of Earth environmental observations that will improve our understanding of the Earth's carbon, water, and energy cycles. SIR-C/X-SAR is a joint project of the National Aeronautics and Space Administration (NASA), the German Space Agency (DARA), and the Italian Space Agency (ASI). SIR-C/X-SAR is the next step in a series of spaceborne imaging radars, beginning with Seasat in 1978 and continuing with SIR-A (1981), Germany's Microwave Remote Sensing Experiment (1983), and SIR-B (1984). It is the potential precursor for the Earth Observing System (EOS) SAR. A total of about 50 hours of data, corresponding to roughly 50 million square kilometers of ground coverage, is collected during the first mission. The ground swath width varies from 15 to 90 kilometers depending on the imaging mode and incidence angles of the radar beams. Mission Objectives: SIR-C/X-SAR mission objectives are to: - conduct a long term research program in earth sciences using remote sensing data acquired from spaceborne imaging radar sensors - develop quantitative tools to extract geophysical parameters from radar data - develop the sensor, processing technology, and quantitative tools in preparation for the next generation of multifrequency SAR systems Instrument Description: The SIR-C instrument was built by the Jet Propulsion Laboratory (JPL) and the Ball Communication System Division for NASA and is a two-frequency radar including L-band (23 cm wavelength) and C-band (6 cm wavelength) with four polarizations (HH, HV, VH, VV). The L- and C-bands use distributed phased-array antennas with electronic beam steering radars. The X-SAR instrument was built by the Dornier and Alenia Spazio companies for DARA and ASI and is a single-frequency radar with X-band (3 cm wavelength) and vertical polarization (VV). X-SAR uses a passive slotted waveguide antenna and a tilt mechanism to point the radar beam in the same location as SIR-C's beam. The science data are recorded aboard the space shuttle by the Payload High Rate Recorder. The data is played back from the High Density Digital Cassettes and processed into images by the Ground Data Processing System. Data Products: There are three ground data processing systems for the SIR-C/X-SAR mission. JPL is responsible for survey and precision processing of the SIR-C data. ASI is responsible for survey processing of the X-SAR data. Precision processing of the X-SAR data is shared between the ASI and the DLR. Two types of image products are produced by the ground data processing systems: the survey product and the precision product. All of the science data collected on HDDCs are processed into survey products. The survey product is intended as a "quick look" browsing tool for viewing the areas imaged by SIR-C/X-SAR. It is not designed to be used for quantitative scientific analysis. Survey data products are stored on this CD-ROM. The survey product is a strip image of the entire data take with the following properties: four-look detected (SIR-C) eight-look detected (X-SAR) ground range 50 m pixel spacing approximately 100 m resolution single radar channel The precision product is a frame image of a subset of the data take. Only a small percentage of the acquired data will be processed into precision products. Processing requests are to be submitted to the appropriate ground data processing system for their generation. The precision product has the following properties: Multi-look Precision Product: multi-look ground range 12.5 m pixel spacing approximately 25 m resolution all radar channels frame size is 100 km SIR-C Single-look Precision Product: single-look slant range natural pixel spacing full resolution all radar channels frame size is 50 km X-SAR Single-look Precision Product: single-look slant range complex natural pixel spacing full resolution frame size is 100 km Survey Processing Algorithm: The SIR-C survey processor design is based on a burst mode processing algorithm. To attain a high throughput rate, the data is bursted in azimuth (slow time) with a one-quarter duty cycle factor. The data volume is 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 is performed using the spectral analysis (SPECAN) algorithm. Following azimuth compression, radiometric correction is applied to compensate for the cross-track and along track radiometric modulation. This is followed by a geometric rectification step that resamples the slant range-Doppler image into the ground range cross-track and along-track domain. The rectified burst images are then overlaid to produce the final multi-look image. The initial Doppler centroid frequency is determined using a clutterlock routine and a PRF ambiguity resolution technique. The unambiguous Doppler centroid frequency is 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 is based on a range-doppler algorithm. To attain a high throughput rate, the data volume is reduced by applying a variable presumming and undersampling to the range lines to meet the range resolution requirement of 100 m (ground range). The range processing is performed by using the consequently reduced chirp bandwidth. The azimuth processing is 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 is 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 is applied for the Doppler ambiguity resolution. A raw data quality analysis is 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. Using the CD-ROM: The data capacity of a CD-ROM is about 650 MB. Consequently, up to 20 survey data takes are stored on one CD-ROM. The number of data takes per CD-ROM depends on the length and radar mode of each data take. The total number of SIR-C CD-ROMs is about 50 and no more for X-SAR. The CD-ROM is organized into two directory levels. The top-level directory contains two text files and the names of each data take directory. Top-Level CD-ROM Directory: README.TXT This file. DTREF.TXT Data Take Cross-Reference File: a table with one entry per data take on this CD-ROM which includes data take ID, start/stop GMT, and start/stop latitude and longitude. Also includes a master index of data takes available on CD-ROM. DTxxx_yy Directory name for data take ID xxx.yy The second-level directories, or data take directories, contain text and image files pertaining to the specified data take. Each survey strip image is divided into 100 km image segments numbered from 01. When displayed, each image segment is oriented such that the near range pixels are at the top of the image. For right-looking images, the spacecraft flight direction is from left to right. For left-looking images, the spacecraft flight direction is from right to left. No image display software is provided on this CD-ROM. You must provide your own display software appropriate to the type of computer (Sun, VAX, IBM PC, Macintosh, etc.) you are using. Your display software should permit you to scroll up and down, right and left. You will need to know how many image lines (records) are in each segment, and how many bytes (pixels) are in each image line. You will find the number of records and number of bytes per record in the DTHDR.TXT file. For all images, the number of bytes per record is fixed at 2000 pixels (100 km). The number of records, however, depends on the swath width. But all image segments for a given data take are of the same dimensions. Data Take Level Directory: DTHDR.TXT Contains radar, image, and processing parameters of the corresponding data take. Lists which precision products may be ordered. Also contains a table describing each image segment in terms of GMT, latitude, and longitude at image center. QA.TXT Contains Q/A parameters of the corresponding data take. DOPPLR.TXT Contains Doppler centroid estimates of the corresponding data take. ROLL.TXT (SIR-C data only) Contains roll angle estimates of the corresponding data take. SEG_nn.IMG Image data for Image Segment nn. Annotated with one-second tick marks, center GMT, center latitude and longitude, and track angle. This file is a simple pixmap; there are no GIF, TIFF, etc. header records embedded in the image. Note that the SEG_nn.IMG file contains both image data and annotation. It may be desirable to extract the image data from the file. In all data takes, the annotation consumes the bottom 150 image lines (records). So, the read in just the image, calculate the number of bytes to read from the beginning of the file as: number of bytes to read = X * (Y - 150) where X = number of bytes per record. Y = number of records per segment. Values for X and Y are found in DTHDR.TXT and also in DTREF.TXT. The data on the CD-ROM is formatted according to the ISO 9660 specification. ISO 9660 is the international standard format for CD-ROMs.