Geometric Validation of Low and High-resolution Asar Imagery

We present issues and solutions concerning the calibration and validation of ENVISAT ASAR image products – both their geometry and radiometry are investigated. Concerning geometric calibration, the slant range products (IMS, APS) offer the best available resolution. We tabulate differences between measured (in image products) and predicted (via geodetic position, precise state vectors, and range and Doppler equations) positions of calibration targets such as transponders and corner reflectors. The differences are interpreted and used to calibrate and validate timing annotation conventions in both the range and azimuth dimensions. For 7 IMS products covering corner reflectors deployed outside Zürich, Switzerland, differences between predictions and measurements were below 3m. The ENVISAT ASAR modes include single-beam image (IM) and alternating polarisation (AP) modes, as well as wide swath (WS), and global monitoring (GM): ScanSAR modes that cycle through swaths at different incidence angles. The geometry of each mode is validated. We also review validation results for ground range precision products (IMP, APP) and medium resolution products (IMM, APM, WSM) where multiple versions of the polynomial slant/ground range transformation are annotated. We conclude with a demonstration and discussion of the improved retrieval of radiometry that is possible when a well-validated geometry is available.