Synthetic Aperture Radar Products
The Alaska Satellite Facility acquires, archives, processes, and distributes SAR data from the ERS-1, ERS-2, JERS-1, and RADARSAT-1 polar-orbiting satellites. The ERS (European Remote Sensing) satellites are operated by the European Space Agency. The ERS SARs use C-Band, 5.66 cm radar wavelength. The ERS-1 data have been archived since 7 September 1991, and ERS-1 operations went into stand-by (campaign) mode in June 1996. The ERS-1 satellite was retired on March 10, 2000. ERS-2 data collection began in October 1995 and is ongoing. JERS (Japanese Earth Resource Satellite, L-Band / 23.5 cm wavelength) data coverage began 20 May 1992 and runs through October 11, 1998. Routine C-Band data collection from Canada's RADARSAT-1 satellite began in June 1996 and is ongoing. Most data cover ASF's station mask, approximated by a circle with radius 3000 km centered at Fairbanks, Alaska. The JERS-1 and RADARSAT-1 satellites carry tape recorders, however, making world-wide coverage possible. Examples of archived out-of-mask data include two full JERS-1 mappings of the Amazon basin (see JAMMS homepage and Antarctic Mapping Mission data. ASF also archives SAR data obtained by the McMurdo, Antarctica ground station.
ASF primarily provides SAR products to the U.S. government, research, and education communities, as per the agreements between NASA and the foreign flight agencies. Commercial users are encouraged to order the satellite SAR data through MDA Geospatial Services , the North American commercial distributor of ERS, and RADARSAT-1 SAR products.
Standard SAR Images
All standard data are processed to a 100 km x 100 km scene. If the actual swath is less than 100 km, such as JERS at 75 km, the remaining pixels are 0-filled. Nominally, the full-resolution images have 12.5 m pixel spacing and 30 m resolution, and the low-resolution images have 100 m pixel spacing and 240 m resolution. Each full-resolution image is 8192 x 8192 x 8 bits or 67 MB in size, and each low-resolution image is 1024 x 1024 x 8 bits or 1 MB in size. These image products represent how brightly the viewed ground targets backscattered the SARs' radar pulses. Metadata distributed with these images allows users to compute various other parameters, such as sigma-naught, from the pixel values.
For further information, see the detailed documentation for the ERS-1 and ERS-2 SAR images, JERS-1 SAR images, RADARSAT-1 Standard Beam SAR Images in the ASF document archive.
RADARSAT-1 Antarctic Mapping Mission data includes both Standard Beam data and Extended High Incidence Beam data. These data were processed in a special format call "RAMP". For details about the RAMP data see the document RADARSAT-1 Left Looking RAMP SAR Images.
The RADARSAT-1 ScanSAR images are somewhat different. The RADARSAT-1 SAR utilizes beam forming techniques in order to provide several different beam modes. The modes may be capable of imaging closer to or farther from the satellite, with finer resolution and accordingly smaller areas or wide areas with worse resolution. The ScanSAR mode actually combines a few of these other modes in order to obtain a very wide swath - approximately 500 km, depending on which ScanSAR sub-mode is chosen. Three resolutions are provided: 75, 150, and 600 meters. ASF can also process these data to a Polar Stereographic map projection. Please see the documentation on RADARSAT ScanSAR Images for further information.
SAR Data, Phase Information Retained (Complex Data):
These data are left in their natural spacing (slant range, i.e., set time intervals) and have approximately 8 m resolution in that direction. The complex data format implies real ('I') and imaginary ('Q') components. In this case, 'I' stands for the backscattered radar's in-phase or cosine component, while 'Q' represents the quadrature or sine component. Therefore, these products' 'I' and 'Q' values can be used to regenerate the backscattered radar's amplitude and phase by the following equations: amplitude = sqrt(I^2 + Q^2), phase = atan(Q/I).
These data are often used for interferometry.
The SLC (Single-Look Complex) product was developed primarily to improve the support of interferometry research and projects. The CPX products were processed leaving the image skew and Doppler inherent in the actual acqusition geometry, in which the antenna beam has a certain squint angle. The SLC products are processed as though the data was acquired with zero squint angle (in the Doppler domain), and hence acquired at zero Doppler. In the processing the data is deskewed to the zero Doppler line, the data is shift in the frequency domain from the acquisition Doppler centroid to zero (as a function of range), the phase is modified to obtain the correct target phase at zero Doppler, and the range of the pixels is adjusted to the zero Doppler slant range. This processing to zero Doppler results in congruent geometry for repeat orbit images and siginificantly simplifies the interferometry processing. In the CPX products, the image skew will, in general, be different between repeat orbit images, and one image must be interpolated to fit the geometry of the other.
For further information, see the detailed guide documentation for the ERS-1 and ERS-2 SAR complex images and the JERS-1 SAR complex images. Also see the Complex SAR Data Example.
Level 0 Data products:
Level-0 data is now available in two formats, STF and CEOS, to all ASF NASA-approved investigators.
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