Comparison of ERA-Interim global meteorological reanalysis model and MERIS data to reduce the tropospheric effects in InSAR displacement velocity fields

The main limitation of radar interferometry in measuring ground displacements is due to phase propagation delays caused by the troposphere. Tropospheric water vapor is a major limitation for high precision Interferometric Synthetic Aperture Radar (InSAR)applications. Using global meteorological reanalysis models and MERISdata are two methods that can be used for correcting the effects of the troposphere. The purpose of this study is comparison of these two methods. The MERIS instrument is located on the platform of ENVISAT satellite and measures reflected solar radiation. We adopt the use of data of this sensor for our study as MERIS provides a direct estimate of atmospheric water simultaneously with Synthetic Aperture Radar (SAR) acquisition. ERA-Interim is a global atmospheric model computed by the ECMWF based on a 4D-Var assimilation of global surface and satellite meteorological data. This reanalysis provides values of several meteorological parameters on a global 70 km grid from 1989 to the present day, at 0 am, 6 am, 12 pm and 6 pm Universal Time Coordinated) UTC (daily. The vertical stratification is described on 37 pressure levels, densely spaced at low elevation (interval of 25 hPa), with the highest level around 50 km (1 hPa). For each acquisition date, we select the ERA-Interim and MERIS outputs that are of the closest time spans to the SAR acquisition time. A Kriging interpolation in the horizontal dimensions and a spline interpolation along altitude is then applied to produce a map of the predicted delay. Total delay maps at epoch of acquisitions are then combined by pairs to produce differential delay maps corresponding to each interferogram. The use of the precise formulation of the single path delay and of the profiles of temperature, water vapor and dry air partial pressure is of importance to compute an accurate delay function.We used the two ENVISAT radar acquisitions of a region in the north west of Iran. We calculated the displacement velocity field and we corrected it (by ERA-Interim and MERIS data) and then compared the results with reported displacement velocities of GPS stations. The RMSE of two methods were 1.84 mm and 2.37 mm. The maximum difference between two methods is about 7.7 mm. This difference could be due to the presence of cloudy pixels in the MERIS data. The minimum difference between two methods is about 0.3 mm. The reason for this difference is negligible horizontal changes in the tropospheric indicators. The results show that cloudy weather and changes in the troposphere indicators, are the most important factors in the accuracy of the results of the two methods.