SAR satellite imagery applications

Full text: Differential radar imagery of images regularity acquired by synthetic radar aperture instrument onboard commercial satellite plateforms (ENVISAT, ERS, RADARSAT, etc.) allows to monitor evolution of ground subsidences with an accuracy typically better than few millimeters and independent measurement every about 20 m. The technique of SAR interferometry is described and illustrated on some test cases. Interferograms are computed from the phase difference of SAR images matched with an sub-pixel accuracy. The phase difference includes information about the distance between orbits, atmospheric phase delays, topography and the component of the ground displacement along the line of sight of the satellite. The displacement of the ground is estimated with an accuracy better than the radar wavelength (5.6 cm in C band) using standard Digital Elevation model (SRTM) because the sensitivity to displacement of the interferogram is far better than their sensitivity to topography. An end-to-end operational software has been developed at CEA. It allows to compute whole archives of images and thus provides information about the evolution of sites with time within the past decade. Especially a procedure has been developed that allows to estimate subsidence rates from images presenting a non uniform date distribution. The characteristics of the software are presented. Interferograms also include residual atmospheric artefact resulting from the interaction of the radar beam with both the ionospheric plasma and the atmospheric water vapor. The atmospheric delays may prevent accurate estimation of subsidence at some places. A filter is presented that use both the meteorological meso-scale model MM5 and radiometric measurements acquired simultaneously with the radar images. The meso-scale model allows to estimate the atmospheric delays at wavelengths larger than about 2km. Radiometric images acquired within the water vapor absorption bands provide an estimate of the atmospheric delays at high spatial resolution below cloud free areas. Some results over sites of interest are presented and discussed. (author)