Successful Application of GPS-derived Water Vapor to the Improvement of the Estimation of Surface Deformation from InSAR

A major source of error for repeat-pass InSAR comes from the phase delay in radio signal propagation through the atmosphere (especially the part due to tropospheric water vapor). It was in 1997 that Bock and Williams first proposed to use GPS data to reduce water vapor effects on InSAR measurements [Block and Williams, 1997]. However, there had been few satisfactory results for the reduction of atmospheric effects on interferograms using GPS data until it was found that it is crucial to apply spatial interpolation to zenith total delay (ZTD) differences from different times instead of ZTD values themselves [Li et al., 2005a] . Application of the topography-dependent turbulence model (GTTM for short) developed at University College London (UCL) to ERS Tandem data (i.e. one day apart) over the Los Angeles region has shown that use of the GTTM can reduce water vapor effects on interferograms from ∼10 mm down to ∼5 mm [Li et al., 2005]. The major objective of this paper is to further validate GTTM using ESA's ENVISAT Advanced Synthetic Aperture Radar (ASAR) data (particularly under cloudy conditions). It is shown that the GTTM model can significantly reduce water vapor effects even under cloudy conditions, which is a major advantage over existing MODIS and MERIS water vapor correction models [Li, 2005; Li et al., 2005b; Li et al., manuscript in preparation, 2005] . After correction, the RMS differences between GPS-derived and InSAR-derived range changes in the LOS direction varied from 0.57 cm to 0.80 cm with a reduction of up to 0.55 cm.