Improved knowledge of high latitude ocean circulation with Synthetic Aperture Radar (ISAR)

The synthetic aperture radar (SAR) Doppler centroid shift has been demonstrated to contain geophysical information about sea surface wind, waves and current at an accuracy of 5 Hz and pixel spacing of 3.5 − 9 × 8 km2. This corresponds to a horizontal surface velocity of about 20 cm/s at 35◦ incidence angle. The ESA Prodex ISAR project aims to implement new and improved SAR Doppler shift processing routines to enable reprocessing of the wide swath acquisitions available from the Envisat ASAR archive (2002-2012) at higher resolution and better accuracy than previously obtained, allowing combined use with Sentinel-1 and Radarsat-2 retrievals to build timeseries of the sea surface velocity in the Nordic Seas. This project has addressed the calibration and validation of geophysical Doppler shifts from Envisat ASAR Doppler centroid shift retrievals. The main focus has been the challenges related to estimation of geometric (satellite orbit and attitude) and electronic (antenna mis-pointing) contributions and corrections. Geophysical Doppler shifts from approximately three months of data in January, February and March 2010 have been analyzed with the goal of establishing a global calibration algorithm. At 8 km pixel spacing, the uncertainty of the retrieved geophysical Doppler shift is then about 2.5 Hz in subswaths 2-5, and 3.5 Hz in subswath 1. This is half the uncertainty of previous results. Following this, new and improved routines for near-surface wind and current retrieval in the Nordic Seas have been developed, and a validation framework has been established. The preliminary results are then presented