A Coordinated International Satellite Altimetry Virtual Constellation: Towards 2050

The “Assmannshausen Report” (Escudier and Fellous, 2009) stood for 15 years as the CEOS reference document for the international altimetry community, gathering user requirements for the altimetry satellite constellation for the years 2010 to 2025. It is encouraging to see that all the high-level requirements from that document have since been implemented in the various recent satellite missions, their products, or dissemination strategies. This new report builds on the progress made over those 15 years, in terms of satellite instrumentation as well as the user uptake, new and developing applications, and the outlook of user needs for the next decades. The Ocean Surface Topography Virtual Constellation (OST-VC) working group, co- chaired by CNES and EUMETSAT, took on the task, incorporating contributions from all involved space agencies and a wide range of experts and end users, to build the new reference for a better international coordination of the altimeter constellation for the years 2025-2050. After some preliminary context about altimetry and its community, the report lists the user needs including emerging ones, then focus on the gaps in the current international space constellation, gaps that space agencies must consider in their future program prospectives. This report highlights the importance to continue the ocean observation including waves and currents. Considering the climate change impact on the evolution of the water cycle, the monitoring of inland waters, coastal waters and ice is crucial and must be elevated as key objective. Swath altimetry, successfully demonstrated by SWOT mission, will benefit numerous applications and is on good track to be implemented on future space missions (such as the Sentinel-3-Next Generation Topography mission). The interest in smaller scale and rapidly evolving phenomena, justify the recurring user need to improve both the revisiting times and the spatial sampling. On the global and basin scale, it remains imperative to continue and to improve the climate time series of sea level change with missions such as Sentinel-6. The report further emphasizes the importance to create higher level thematic products by collocating high-resolution altimetry with other satellite data (ocean color, wind speed, salinity, or others). It is also essential to continue the open and free data policy, to support and develop user uptake and to take advantage of the future computing capacities. Thanks to a huge implication of the altimetry community (International Altimetry Team, 2021), the current altimetry constellation is currently consolidated by Copernicus missions until 2040, with still some enhancements to be done. Open data polices, widespread collaboration and support from multiple agencies and nations will be necessary for the continued success of these missions and especially if the enhancements are to be realized.