Developments in SAR Altimetry over Coastal and Open Ocean: A Retrospective of Developments in SAR Altimetry Processing and the Improvements Achieved through the SAMOSA, CP4O and SCOOP Projects

The European CryoSat-2 and Sentinel-3A satellites, soon to be followed by Sentinel-3B, were the first to operate SAR mode altimeters. These missions initiated a new era in satellite altimetry over the oceans, in which a step change in improvement in measurements over the open ocean and coastal zone has been achieved, in terms of accuracy of measurement, the capability to map features that could not previously be resolved, and to provide measurements closer to the coast than ever before. This presentation looks back at a series of three rojects initiated by ESA: SAMOSA, CP4O and SCOOP, reviews the development and assessment of new processing schemes for SAR mode altimeter measurements, plots the performance improvements achieved and then looks ahead to further developments. SAMOSA (SAR Altimetry Mode Studies and Applications) was an ESA-funded project initiated in 2007 to investigate the improvements offered by SAR mode altimetry over ocean, coastal and inland water surfaces, developing practical implementation of new theoretical models for the SAR echo waveform as part of this process. SAMOSA developed physical based models for SAR altimeter ocean waveforms, and applied them to develop re-trackers for SAR mode products, which were further developed and tested on simulated data, airborne data, and then on real satellite data from CryoSat-2. Approaches to reduce SAR mode data to “RDSAR” data were also investigated, to infer the statistical equivalence between SAR mode and the traditional low-resolution altimetry. Subsequent work further developed the “SAMOSA” model, balancing the aim to improve the modelling of the waveform against efficiency of processing to achieve a practical scheme that could be implemented in the Sentinel-3 Ground Segment. CP4O (CryoSat Plus for Oceans) was a project supported under the ESA Programme Element coined “Support to Science Element” (STSE) with the objectives to build a sound scientific basis for new scientific and operational applications of CryoSat-2 data, generate and evaluate new methods and products to enable the full exploitation of the CryoSat-2 SIRAL altimeter, extending their application beyond the initial mission objectives, and to ensure that the scientific return of the CryoSat-2 mission is maximised. Within CP4O, processing schemes for CryoSat-2 data were developed and valuated for SAR and RDSAR products over the open ocean, and SAR and SARin modes data over the coastal ocean. New geophysical correction products and models (Wet Troposphere, Ionosphere, and Regional Tide Models) were developed and assessed, as was specialized processing of SAR mode data to improve Polar Ocean bathymetry and tidal modelling. SCOOP (SAR Altimetry Coastal & Open Ocean Performance) is a project funded under the ESA SEOM (Scientific Exploitation of Operational Missions) Programme Element, to characterise the expected performance of Sentinel-3 SRAL SAR mode altimeter products, in the coastal zone and open ocean, and then to develop and evaluate enhancements to the baseline processing scheme in terms of improvements to ocean measurements. Another objective is to develop and evaluate an improved Wet Troposphere correction for Sentinel-3, and 1provide recommendations for use. Recommendations for further developments and implementations are provided through a scientific roadmap.