Tropospheric corrections of SAR interferograms with strong topography. Application to Etna

The accuracy of spaceborne geodetic techniques, including SAR interferometry, is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS measurements to correct SAR data. The differential tropospheric delay is computed at each pixel of the interferogram from ground temperature, humidity and pressure using two empirical parameters calibrated from several radio-soundings acquired in various latitude and climate conditions. It is shown that with such a model. given the 3300 meters topography of Etna, tropospheric variations call generate up to 4 pi phase rotations between the top and the bottom of the volcano. In 16 out, of the 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the number of fringes associated with the 1991-93 eruption from previous estimates. The remaining deformation is consistant with a deforming source located at a depth of 14+/-1 km. During the second half of the eruption; the subsidence rate at the top of the volcano is roughly stable at 13+/-3 mm/month. These values are in good agreement with tiltmeter data collected on Etna during the same period and with the estimated volume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm/month.