Correction of atmospheric water vapour effects on repeat-pass SAR interferometry using GPS, MODIS and MERIS data
Over the last two decades, repeat-pass Interferometric Synthetic Aperture Radar (InSAR) has been a widely used geodetic technique for measuring the Earth's surface, including topography and deformation, with a spatial resolution of tens of metres. Like other astronomical and space geodetic techniques, repeat-pass InSAR is limited by the variable spatial and temporal distribution of atmospheric water vapour. The purpose of this thesis is to seek to understand and quantify the spatial and temporal variations in water vapour and to reduce its effects on repeat-pass InSAR using independent datasets such as Global Positioning System (GPS), the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and the ESA's Medium Resolution Imaging Spectrometer (MERIS) measurements. The performance of different techniques including radiosondes, GPS, MODIS and MERIS for measuring precipitable water vapour (PWV) is assessed through inter-comparisons. It is shown that MODIS appears to overestimate water vapour against GPS and radiosondes. For the first time a GPS-derived correction model has been developed to calibrate the scale uncertainty of MODIS near IR water vapour product, and regional 1 km x 1 km water vapour fields have been produced with a standard deviation of up to 1.6 mm using a GPS/MODIS integrated approach, from which a zenith-path-delay difference map (ZPDDM) can be derived with an accuracy of 5 mm and a spatial resolution of 2 km. Based on analyses of the spatial structure of water vapour using spatial structure function, a GPS topography- dependent turbulence model (GTTM) has been developed to produce ZPDDMs with a standard deviation of 6.3 mm. A water vapour correction approach has been successfully designed and incorporated into the ROI PAC (version 2.3) software using the ZPDDMs provided by the GTTM and GPS/MODIS integrated models. The application of both correction models to ERS data over the Southern California Integrated GPS Network (SCIGN) shows that the order of water vapour effects on interferograms can be reduced from 10 mm to 5 mm using the GTTM or the GPS/MODIS integrated models. It is also demonstrated that the application of both correction models can improve InSAR processing such as phase unwrapping.