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For more than 23 years, the multisatellite DUACS system has been providing high quality multi-mission altimetry Sea Level products for oceanographic applications, climate forecasting centers, geophysics and biology communities. They consist in directly usable and easy to manipulate Level 3 (along-track cross-calibrated Sea Level Anomaly SLA) and Level 4 (multiple sensors merged gridded gap-free) products. Global and regional datasets (Arctic Ocean, European Seas ...) are available.A full reprocessing of these products is carried out almost every 3 years, based on the state-of-the-art Level 2 to Level 4 algorithms. In Decembre 2021, a new version will be available within the Copernicus Marine Environment and Monitoring Service (CMEMS) and the Copernicus Climate Change Service (C3S) covering 28 years of altimetric data (i.e. almost a century of cumulated data using 24 altimetric missions). This version benefits from major improvements associated with new altimeter and mapping standards.Here, we report the first results of this DUACS DT2021 multi-mission reprocessing. We first describe the main steps of the altimeter production system. Then, we discuss the characteristics and limits of the different products (C3S, CMEMS) in order to help the ocean and climate communities on their optimal use for validation, assimilation activities and other scientific studies. Several comparisons with independent datasets (along-track, drifters, tide gauges) show that a significant improvement has been achieved at mesoscale with this new version: almost 20% of SLA error reduction at wavelengths [65, 500km] and around 10% of geostrophic currents error reduction compared to the previous version (DT2018). At decadal time scale, the trend of the global mean sea level has been estimated to 3.4 +/- 0.4 mm/yr. (90% Confidence Interval), in line with other estimates and previous reprocessing DUACS-DT2018. New altimeter corrections are also available for the users (internal waves, correction for the Topex-A instrumental drift, flag ice).

For more than twenty years, the multi-satellite DUACS system has been providing Near Real Time (NRT) and Delayed Time (DT) altimetric products. These data are ranging from along-track to multi-mission maps of Sea Level Anomaly (SLA) and Absolute Dynamic Topography (ADT). A reprocessing of 25 years of data, namely: DUACS DT2018, has been carried out and is available through the Copernicus Marine Environment Monitoring Service (CMEMS) and Copernicus Climate Change Service (C3S) since April 2018. Several changes have been implemented in the DT2018 processing in order to improve the quality of the products. New altimeter standards and geophysical corrections has been used, refined data selection has been implemented and Optimal Interpolation (OI) parameters have been reviewed for global and regional map generation. Through this paper, an extensive assessment has been carried out. The error budget associated to the DT2018 products at global and regional scales has been refined and the improvements compared with the previous version quantified (DT2014; Pujol et al., 2016). The DT2018 errors at mesoscales are reduced by nearly 3 to 4 % for global and regional products compared to the DT2014. This reduction is much more important in coastal areas (reduction is up to 10 %) where it is directly linked to the altimeter geophysical corrections used in the DT2018 processing. Conclusions are very similar concerning geostrophic currents, where error is reduced by 5 % and up to 10 % in coastal areas.