Review of state of knowledge for SAR altimetry over ocean. Report of the EUMETSAT JASON-CS SAR mode error budget study

Abstract

SAR altimetry over the ocean has attracted considerable attention in the past three years and remarkable progress has been made in a short space of time. Cryosat-2 is the first satellite to provide SAR altimeter data over the ocean, and the datahelped to demonstrate the significant benefits of SAR mode for ocean altimetry compared to conventional low-resolution mode(LRM). This document provides an overview of the state of knowledge for SAR altimetry over the ocean based on research reported between 2010 and 2013. There is increasing consensus between various independent investigation teams that SAR altimetry over the ocean leads to significant performance improvements when compared to even the best available conventional radar altimetry. The results are evident in reduced measurement noise, improved performance in coastal regions and improved spectral information content for Sea Level Anomaly at the ocean mesoscale. The convergence of results from different groups using different SAR waveform retrackers indicates that there is now a high level of confidence in the ability to retrieve geophysical data from SAR mode altimetry over ocean. Several issues particular to SAR altimetry remain open, specifically the sensitivity to platform mispointing, the lack of a sea state bias model in SAR mode, and the effects of swell and swell direction on SAR waveforms. It is noted that these issues disappear with SAR interleaved mode since the resulting SAR mode data can be transformed seamlessly into LRM data for self-calibration. The document discusses differences between SAR closed- burst and SAR interleaved mode and the transformation of SAR data into pseudo?LRM waveforms. Closed-burst SAR used on CryoSat-2 and Sentinel-3 can be transformed into pseudo-LRM waveforms that look similar to LRM but are not statistically equivalent to real LRM data. Lack of equivalent P-LRM waveforms from closed burst SAR mode data precludes direct SAR/LRM cross-calibration. This fact jeopardizes the self-calibration potential of a closed burst SAR mode altimeter, and compromises attempts to relate with sufficient confidence and precision the closed-burst SAR sea level measurements to the existing sea level record. Adopting closed-burst SAR on Jason-CS would compromise the continuity of the high-precision sea level 20-year time series. In contrast, SAR interleaved would realize the theoretically optimal performance expected from a SAR mode altimeter, while ensuring continuity with conventional altimeters. This report explains that the SAR interleaved mode is essential since it is the only method that would assure continuity between the SAR mode aboard Jason-CS, and contemporaneous and prior conventional altimetric missions.

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