GAS: Generic Atmosphere Solutions for radar measurements

Land surface deformation is a major worldwide hazard that can result from natural processes such as landslides, earthquakes, and volcanoes, or from anthropogenic processes including extraction of groundwater, oil and coal. Repeat-pass Interferometric Synthetic Aperture Radar (InSAR) has been widely used to map land surface deformation in the past two decades (Massonnet and Feigl, 1998). However, change in the atmosphere (especially tropospheric water vapour) is one of the major sources of uncertainty in determining surface deformation using InSAR, and correcting for such errors is key to the detection of subtle deformation signals. The PI has considerable experience in InSAR atmospheric correction models. Indeed, the PI has been at the forefront of InSAR atmospheric correction by developing five models to reduce water vapour effects on InSAR measurements using remotely-sensed water vapour data (e.g. Global Positioning System(GPS), NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and ESA Medium Resolution Imaging Spectrometer (MERIS)): Each model is capable of improving the accuracy of InSAR derived deformation signals from 10 mm before correction to 5 mm after correction (e.g. Li et al., 2009, Li et al., 2006c, Li et al., 2006b, Li et al., 2006a, Li et al., 2005). The PI is determined to capitalize on it in this research. The objectives of this proposed research are as follows: (i) to develop atmospheric model based correction technique(s) to make our correction models globally applicable; (ii) to demonstrate the ease of application of the atmospheric model based correction technique(s) under any possible scenario: (a) with a single InSAR pair, or (b) with multiple InSAR pairs; (iii) to develop atmospheric estimation techniques in InSAR time series analysis so that deformation signals can be extracted without any assumption on deformation models or any requirement on atmosphere data. This is desirable for users who are not able to access any atmospheric model. Methodology: All the above-mentioned correction models are inherently limited by data availability, e.g. the lack of a dense GPS network (for GPS-based model) and the sensitivity to the presence of clouds (for MODIS and MERIS). The Unified Model developed by the UK MET Office provides high spatial resolution (e.g. 1 km) estimates of tropospheric path delays with a global coverage, 24 hours a day in all weather, which is highly desirable for InSAR atmospheric correction, and will be investigated for the correction of atmospheric effects on a single-pair and/or multi-pairs of SAR interferograms in this research. A prior deformation model and/or some spatial/temporal filtering techniques are commonly assumed in most of current InSAR time series techniques to separate deformation signals from atmospheric effects, which is evidently not optimal. Based on three key physical features of atmospheric signals identified by previous studies, an iteration process is proposed to estimate atmospheric signals from multiple InSAR pairs and then to extract deformation signals without any assumption of deformation models and any requirement on atmosphere data.