Abstract
Given the considerable range of applications within the European Union Copernicus system, sustained satellite altimetry missions are required to address operational, science and societal needs. This article describes the Copernicus Sentinel-6 mission that is designed to provide precision sea level, sea surface height, significant wave height, inland water heights and other products tailored to operational services in the ocean, climate, atmospheric and land Copernicus Services. Sentinel-6 provides enhanced continuity to the very stable time series of mean sea level measurements and ocean sea state started in 1992 by the TOPEX/Poseidon mission and follow-on Jason-1, Jason-2 and Jason-3 satellite missions. The mission is implemented through a unique international partnership with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union (EU). It includes two satellites that will fly sequentially (separated in time by 5 years). The first satellite, named Sentinel-6 Michael Freilich, launched from Vandenburg Air Force Base, USA on 21st November 2020. The satellite and payload elements are explained including required performance and their operation. The main payload is the Poseidon-4 dual frequency (C/Ku-band) nadir-pointing radar altimeter that uses an innovative interleaved mode. This enables radar data processing on two parallel chains the first provides synthetic aperture radar (SAR) processing in Ku-band to improve the received altimeter echoes through better along-track sampling and reduced measurement noise; the second provides a Low Resolution Mode that is fully backward-compatible with the historical reference altimetry measurements, allowing a complete inter-calibration between the state-of-the-art data and the historical record. A three-channel Advanced Microwave Radiometer for Climate (AMRC) provides measurements of atmospheric water vapour to mitigate degradation of the radar altimeter measurements. The main data products are explained and preliminary in-orbit Poseidon-4 altimeter data performance data are presented that demonstrate the altimeter to be performing within expectations.
Highlights
Introduction and background to the Sentinel6 missionSatellite altimetry is a fundamental tool for the European Copernicus services providing measurements over the global ocean and, increas ingly in the coastal zones and inland waters
We note that new analyses (e.g. WCRP, 2018; Ablain et al, 2019) and the assessments made in the European Space Agency (ESA) Climate Change Initiative (CCI) sea Level project, (Legeais et al, 2018) suggest that even stricter re quirements are needed for future missions
The Copernicus Sentinel-6 Mission has been designed to continue the heritage satellite altimeter missions occupying the specific ‘reference orbit’ that have supplied the long-term reference data set to accurately monitor Mean Sea Level change that is recognised as a key indicator of climate change
Summary
Satellite altimetry is a fundamental tool for the European Copernicus services providing measurements over the global ocean and, increas ingly in the coastal zones and inland waters. Et al, 2000) including high orbit inclination missions such as CryoSat-2 (Wingham et al, 2006), and polar orbiting mission such as SARAL/ AltiKa, (Steunou et al, 2015) and Copernicus Sentinel-3 ((Donlon, 2011) Donlon et al, 2012) This is important, since the small measure ment footprint of nadir viewing altimeters (between ~2 to ~15 km depending on sea state and radar frequency) severely limits sampling of the ocean both in time and in space and that multiple missions must be used together. New efforts are in progress (e.g. Dodet et al, 2020) to homogenize satellite altimeter Hs data sets from different missions and provide a stable, well calibrated and quality-controlled sea state record as a contribution to the Global Climate Observing System (GCOS, 2016) Essential Climate Variables ECV (GCOS, 2011; NRC, 2004). We conclude with an assessment of early in-orbit performance for the first Sentinel-6 satellite successfully launched on 21st November 2020
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