Towards Absolute Positioning of InSAR Point Clouds

Kurzfassung

The German TerraSAR-X and TanDEM-X satellites are characterized by unique features such as providing images with high spatial resolution and an unprecedented geometric accuracy. The latter has been significantly improved in the recent years by quantifying and removing the most prominent error sources which affect radar range and azimuth time measurements, a method called imaging geodesy [1]. Moreover, if corrected time observations of a specific target are available from SAR acquisitions with different viewing geometries, it has been demonstrated that the stereo SAR method is capable of delivering 3-D absolute coordinates of the target with accuracies in the decimeter to centimeter regime, depending on the target being a corner reflector or an opportunistic persistent scatterer (PS) [2]. As a first step towards the inclusion of such accurately localized point targets into phase-based stacking methods, in [3] the concepts of imaging geodesy and stereo SAR were used to transform the relative estimates of SAR tomography (TomoSAR) into absolute 3-D point clouds by absolutely localizing the reference point. The improvement in the localization accuracy of the resulting point cloud has encouraged us to continue expanding the mentioned framework by automatic detection and absolute localization of useful PS candidates which are visible from SAR images acquired from different viewing geometries, either same-heading or cross-heading orbit tracks. This will generate multiple Ground Control Points (GCPs) which can be used as a reference network in multi-pass InSAR techniques for reliable estimation and removal of atmospheric phase screen and for support in phase-unwrapping. The availability of such points can also be relevant for non-InSAR applications such as detection of large magnitude motions which are invisible from InSAR time-series approaches or as tie points for improving the registration of remotely sensed optical images. This contribution is dedicated to introducing an automated processing chain for generating GCPs from SAR data. The procedures start with the identification of high quality PS candidates, in some strategies with the aid of external data, to precisely extract PS timings in a stack of non-corregistered SAR images. The subsequent steps are the correction of PS timings and the absolute localization with the Stereo SAR method. It will also be demonstrated how these GCPs can be used to improve the geocoding accuracy of InSAR, in particular PSI, point clouds leading to providing absolute positions. Finally, preliminary results based on TerraSAR-X high resolution spotlight images over the city of Oulu, Finland is reported.