USGS EROS Archive - Radar - NASA SIR-C Guide

Science Center Objects

 NASA Spaceborne Imaging Radar-C (SIR-C) Guide

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Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) was a joint project of the National Aeronautics and Space Administration (NASA), the German Space Agency (DARA) and the Italian Space Agency (ASI). An imaging radar system launched aboard the NASA Space Shuttle twice in 1994, SIR-C/X- SAR's unique contributions to Earth observation and monitoring were its capability to measure, from space, the radar signature of the surface at three different wavelengths and make measurements for different polarizations at two of those wavelengths. SIR-C image data will help scientists understand the physics behind some of the phenomena seen in radar images at just one wavelength/polarization, such as those produced by SeaSAT. Investigators on the SIR-C/X-SAR Science team will use the radar image data to make measurements of: Vegetation type, extent and deforestation; Soil moisture content; Ocean dynamics, wave and surface wind speeds and directions; Volcanism and tectonic activity; Soil erosion and desertification.

X-SAR provides single frequency (3.1 cm), single polarization (vertical) data and SIR-C provides multi-frequency, multi-polarization radar data. The SIR-C antenna was composed of two planar arrays, one for L-band (23.5 cm) and one for C-band (5.8 cm). Each array was composed of a uniform grid of dual- polarized microstrip antenna radiators, with each polarization port fed by a separate corporate feed network. These vertically- and horizontally-polarized transmitted waves were received on two separate channels so that images of the magnitude of radar backscatter were acquired in four polarization combinations: HH (Horizontally-transmitted, Horizontally-received), VV (Vertically-transmitted, Vertically-received), HV, and VH. Data was also acquired on the relative phase difference between the HH, VV, VH, and HV returns. By properly phasing the array, the beam could be electronically steered in the range direction up to 23 degrees from the nominal 40 degrees off nadir position without physically moving the antenna. The X-SAR antenna was mechanically tilted to angles between 15 and 60 degrees.

Both SIR-C and X-SAR could be operated as either stand alone radars or together. Roll and yaw maneuvers of the shuttle will allow data to be acquired on either side of the shuttle nadir (ground) track. The width of the imaged swath on the ground varies from 15 to 90 kilometers (9 to 56 miles) depending on the orientation of the antenna beams and the operational mode. The data collected was processed into images with resolution selectable from 10 to 200 meters.

Two types of image products were produced by the ground data processing systems: the Survey product and the Precision (Standard) product. JPL was originally responsible for Survey and Precision processing of the SIR-C data, while ASI was responsible for Survey processing of the X-SAR data. Precessing processing of the X-SAR data was shared between the ASI and the DLR. The Precision product is a frame image of a subset of the data take (swath). Only a small percentage of the acquired data has been processed into Precision products.

All science data were 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. The Survey product is a strip image of the entire data take (swath) with the following properties:

  • four-look detected (SIR-C)
  • eight-look detected (X-SAR)
  • ground range
  • 50 m pixel spacing
  • approximately 100 m resolution
  • L-Band or C-Band single radar channel

Additional Information

Access Data

As of June 2005, unprocessed precision and interferometry data is no longer available from the EROS archive as the SIR-C processor is inoperable and not repairable. Repair parts and maintenance support of critical system components are no longer available to continue SIR-C system operations.

Precision and interferometry data that has been previously processed can be downloaded through EarthExplorer.