COASTAL SAR AND PLRM ALTIMETRY IN GERMAN BIGHT AND WEST BALTIC SEA

摘要

The CryoSat-2 altimeter (SIRAL) features a novel Synthetic Aperture Radar (SAR) mode that allows higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. This study is a regional analysis and validation of CryoSat-2 SAR altimeter products along the German coasts at distance to shore smaller than 10 km. The validation in performed against regional models and an in-situ network of tide gauges and Global Navigation Satellite Systems (GNSS) stations. The goal is an assessment of the geophysical altimeter parameters sea surface height above the ellipsoid (SSH), significant sea wave height (SWH) and wind speed (U10), all estimated at 20 Hz over the time interval from October 2010 to July 2015. We have carried out from FBR (L1a) data a Delay-Doppler processing and waveform retracking tailored to coastal zone by a dedicated SAMOSA-based coastal retracker (SAMOSA+). SAMOSA+ accepts mean square slope as free parameter and the epoch’s first guess fitting value is decided according to the peak in correlation between 20 consecutive waveforms to reduce land off-ranging effects. Since the remaining uncertainty in the altimeter products for coastal shallow waters arises mainly from residual errors in the applied corrections, we use the regional model TPXO8 for tides, EGM 2008 for geoid and DTU13 for mean sea surface and the regionally improved wet tropospheric correction GNSS-based Path Delay Plus (GPD+) computed at University of Porto. To quantify the improvement with respect to pulse-limited altimetry, we build 20 Hz PLRM (pseudo-LRM) data from FBR as a proxy for real pulse-limited products (LRM). They are retracked with a numerical Brown-based retracker. The L2 SAR ocean data products are generated and extracted from ESA-ESRIN GPOD service (named SARvatore) while the PLRM data are built and retracked by Technical University of Darmstadt (TUDa). The validation shows that the accuracy of sea level in coastal zone is higher in SAR than in PLRM and that is comparable to the accuracy of SAR in open sea until 3 km from coast. The standard deviations of sea level difference with in-situ data in open ocean and coastal zone are 4.1 cm and 5.1 cm respectively.