Initial Assessment of the Applicability of TerraSAR-X for Repeat-Track Interferometry

Initial Assessment of the Applicability of TerraSAR-X for Repeat-Track Interferometry,10.1109/IGARSS.2008.4779045,Urs Wegmüller,Charles Werner,Andreas

Initial Assessment of the Applicability of TerraSAR-X for Repeat-Track Interferometry   (Citations: 1)
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The TerraSAR-X satellite, launched 15 June this year, is an X-Band SAR operating at 9.65 GHz (X-Band) in spot-light, stripmap, and ScanSAR modes. In this paper we present our initial assessment of the applicability of TerraSAR-X for differential interferometric applications. While the high frequency of operation leads to reduced interferometric correlation several aspects of the TerraSAR sensor and mission are mitigators. One of the main goals of the TerraSAR mission is to produce high resolution imagery with near optical quality. Consequently, the frequency of operation was chosen at X-Band due to the available radar bandwidth of up to 300 MHz. The previous success of the X-SAR instrument on SRTM was an additional motivation (1). The antenna on TerraSAR can be steered in both elevation and azimuth and can be used to generate spotlight, stripmap and ScanSAR images. Spotlight images have a resolution of about 1-meter, while stripmap data has a resolution in the range of 3-6 meters, and ScanSAR images have a resolution of approximately 16 meters. The satellite operates in an 11 day repeat orbit at an altitude of 514 km (2). 2. DIFFERENTIAL INSAR APPLICABILITY Applicability in the context of repeat-track interferometry means the potential to obtain interpretable phase information that can be related to surface deformation. Repeat-track interferometry utilizes multiple data SAR acquisitions from closely spaced satellite tracks to measure small-scale deformation of the surface. Important considerations include the frequency, spatial resolution or equivalently, radar bandwidth, look-angle, perpendicular repeat-track baseline, and the time interval between observations. Based on wavelength alone, observations at X-Band (O9.65 GHz = 3.13 cm) are almost twice as sensitive to deformation as at C-Band (O5.3 GHz = 5.66 cm). However factors such as the temporal decorrelation of the surface, accuracy of the DEM used for creation of the differential interferogram, and size of the perpendicular baseline, are significant. In particular, the temporal decorrelation of the surface will determine if TerraSAR-X data can be used in a particular area. The temporal correlation is related to the stability of scatterers in the scene that are on the scale of the wavelength or larger. In vegetated areas, surfaces tend to decorate rapidly at X-band. Correlation in arid regions with little or no vegetative ground cover can be high. The same can be expected in urban areas. Differential interferometric studies are still possible in areas with some stable scatterers using point-based analysis methods (3)(4). The high resolution increases the number of possible point candidates in an area that can fulfill the requirements for phase stability. Hence, even if the deformation is relatively large, the phase differences between adjacent point scatters can remain small due to the higher point density. Furthermore, the chance that the scattering within a resolution element is dominated by a single stable scatterer is increased due to the smaller resolution element size. The short time repeat interval of TerraSAR is significant when observing continuous deformation processes such as landslides or subsidence due to pumping of water or oil. While Envisat has a 35 day repeat, the differential interferometric phase due to deformation during the TerraSAR-X 11 day interval will be less. The shorter time interval should also lead to less temporal decorrelation for some surfaces. Another aspect of the short time interval between observations is the ability to reduce the error contribution due to tropospheric water vapor through averaging multiple observations. Variations of the path delay due to the troposphere are usually on the order of a few cm in a particular scene. Hence the greater number of observations can therefore be averaged over any particular time interval.
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