TROPOSPHERIC CORRECTIONS OF SAR INTERFEROGRAMS WITH STRONG TOPOGRAPHY- APPLICATION TO ETNA

The accuracy of spaceborne geodetic techniques, including SAR interfer ometry, is limited by the time and spatial variation and altitude depe ndance of the propagation delay of electomagnetic waves in the lower t roposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS meas urements to correct SAR data. The differential tropospheric delay is c omputed at each pixel of the interferogram from ground temperature, hu midity and pressure using two empirical parameters calibrated from sev eral radio-soundings acquired in various latitude and climate conditio ns. It is shown that with such a model. given the 3300 meters topograp hy of Etna, tropospheric variations call generate up to 4 pi phase rot ations between the top and the bottom of the volcano. In 16 out, of th e 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the n umber of fringes associated with the 1991-93 eruption from previous es timates. The remaining deformation is consistant with a deforming sour ce located at a depth of 14+/-1 km. During the second half of the erup tion; 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 v olume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to r esume immediately after the end of the eruption at a rate of 3 mm/mont h.