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AirSAR/MASTER
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Advances in Digital Elevation Datasets for Exploration, Topographic Mapping and Disaster Management
2.2 Radar interferometry
Radar interferometry is an innovative technique that enables very high-resolution topographic maps of the earth's surface to be generated using spaceborne and airborne radar instruments. The technique has the advantages of automatic processing of the data, and operation in cloud, smoke and rainfall conditions, night and day. A transmit antenna mounted on a spacecraft or plane illuminates the terrain with a radar beam that is scattered by the surface. This radar echo has two components: amplitude (brightness) and phase (distance to the target). Two receive antennas with a fixed baseline record the radar echo from slightly different positions resulting in two different radar images. The two signals received at both ends of the baseline (referred to as the interferometric baseline) show a phase shift due to differing lengths of the signal paths. The phase difference, determined by effectively subtracting the measured phase at each end of the baseline, is sensitive to both viewing geometry and the height of the terrain. If the viewing geometry is known to sufficient accuracy, then the topography can be inferred from the phase measurement to a precision of several metres. The accompanying amplitude information is measured to construct a radar image showing details describing the surface's roughness and dielectric properties.
Interferometric data can be generated from either single-pass or repeat-pass systems. Single-pass systems such as the TOPSAR and SRTM instruments use the two-antenna system, as described above, to record both images simultaneously. Repeat-pass interferograms are generated from separate passes over the same target such as from the European ERS-1 and ERS-2 systems
http://www.eurimage.com/.
TOPSAR (TOPographic Synthetic Aperture Radar)
The JPL TOPSAR system is a 5cm wavelength, C-band interferometer operating on NASA's DC-8 research aircraft as an adjunct to the polarimetric Airborne Synthetic Aperture Radar (AIRSAR) system. TOPSAR is implemented via two antennas flush-mounted nearly vertically on the left side of the aircraft with a 2.6 m baseline spacing. The boresights of the antennas are depressed 45° with respect to the horizontal. The lower antenna is used for transmission whereas both antennas are used for reception. The height accuracy of TOPSAR digital elevation models has been shown by Madsen et al (1995) to be 1 m RMSE in flat terrain and 3 m in mountain areas with a 2 m RMSE overall. Typical data acquisitions are for areas of 10 km across-track (range direction) and 60 km along track (azimuth direction). The output of high precision TOPSAR datasets is accomplished by comprehensive navigational systems to determine the precise position of the DC-8 aircraft, and full motion compensation algorithms in the TOPSAR processor to accommodate large translational and altitude perturbations during the data recording process (Madsen et al, 1995). Apart from the digital elevation data, L- and P-band polarimetric and Cvv data are also recorded. All datasets are registered and the radar data ortho-corrected using the DEM.
SRTM (Shuttle Radar Topography Mapping Mission)
The SRTM recorded both radar images simultaneously using two antennas - a transmitting and receiving antenna in the cargo bay of the Shuttle Endeavor, and a second receiving antenna at the tip of a 60 m deployable mast. Both radar and phase data were recorded in C-band (5.56 cm) and X band (3.1 cm) frequencies. During the 11-day mission, data were acquired along 225 km wide swaths imaging all of Earth's land surface between 60° north and 56° south latitude, with data points spaced every 1 arcsecond of latitude and longitude (approximately 30 m). X-band coverage occurred along narrow 50 km wide swaths and cover 40% of the area mapped by the C-band data. The absolute horizontal and vertical accuracy of the C-band data will be 30 m and 16 m, respectively. Relative height accuracy will be 10 m. However, data of these specifications will not be readily available for public use. Within 2 years, data spatially degraded to 90x90 m horizontal resolution but retaining the initial height accuracy, will be available at the low cost of regridding the data to a 1°x1° area. The policy for distributing the higher resolution data is currently not clear. However, it is expected that data over politically insensitive areas will be accessible. X-band DEM data of the narrower 50 km wide swath have a horizontal resolution of 30 m and relative and absolute height accuracies of 6 m and 16 m, respectively. These data will be unclassified and available for public use from the German Aerospace Centre. Both C and X-band datasets will be geometrically corrected and projected to the WGS84 datum. Extensive information describing the data products and their availability is available on the SRTM Home Page
( http://www.jpl.nasa.gov/srtm/)
and DLR Home page ( http://www.jpl.nasa.gov/dlr-mirror/srtm/).
Selected Reading
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Madsen, S. N., Martin, J.M. and Zebker, H.A. 1993. Analysis and Evaluation of the NASA/JPL Across-Track Interferometric SAR System. IEEE Trans. Geosci. Remote Sensing, vol. 33, no 2, pp. 383-391.
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Thompson, T.W., Zebker, H.A., Carande, R.E., Rosen, P.A., Madsen, S.N., Hensley, S., Rodriguez, E., van Zyl, J.J., Martin, J.M. and Miller, T.W. 1994. NASA/JPL TOPSAR Interferometric SAR. NASA/JPL Imaging Radar Homepage, http://southport.jpl.nasa.gov/topsardesc.html
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Van Zyl, J.J., Zebker, H.A., Hensley, S., Haub, D. and Wiesbeck, W., 1995. The new dual frequency (C- and L-Band) TOPSAR airborne interferometric SAR.
http://www.jpl.nasa.gov/techreport/1995/95-0530.rfr.html
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Burgman, R., Rosen, P.A. and Fielding, E.J., 2000. Synthetic aperture radar interferometry to measure Earth's surface topography and its deformation. Annual Review Earth Planet Science 2000, 28:169-209.
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