## 1 Introduction  
Earth observation is the gathering of information about planet Earth's physical, chemical and biological systems via remote sensing technologies. It is meant to observe our environment, to collect, store and analyze Earth data in order to furnish products for effective decisions on potential risks and vulnerabilities of our planet and its ecosystems. Satellite altimetry falls in that category of Earth observation. International missions have been measuring geocentric sea level as of 1992 (TOPEX/Poseidon and Jason [1]), with an accuracy, at present, of \(\pm\)1-2 cm. Altimetry measurements coupled with other data from various sea-level observing systems (i.e., tide gauges) have established a long-term record of sea level on global scale. The trend and acceleration of sea level have been found to be +3.35 \(\pm\) 0.40 mm/yr and 0.12 \(\pm\) 0.07 mm/yr\({}^{2}\), respectively, for 1992-2017 [2]. In addition, other acceleration rates of 0.08 \(\pm\) 0.008 mm/yr\({}^{2}\)[3] and 0.084 \(\pm\) 0.025 mm/yr\({}^{2}\)[4] have been reported. Sea level rise threatens coasts, and certainly people, through flooding, coastline erosion and submergence, salinization of soil, contamination of underground water, destruction of shoreline defenses, etc. [5; 6; 7]. More than 190 million people are at present vulnerable to sea level rise as a result of coastal flooding even under an optimistic scenario of climate change (increase in global warming by 1.0 \({}^{\circ}\)C and in sea level by +0.24 m over the period 2046-2065) [8]. Europe, and in particular the European Space Agency (ESA) together with the European Union and Eumetsat, has implemented a large, ambitious and sustained space-based Earth Observation Program, with Copernicus being one of its main components. Copernicus ([[http://www.copernicus.eu/](http://www.copernicus.eu/)]([http://www.copernicus.eu/](http://www.copernicus.eu/))) is a system for monitoring the Earth in support of European policy. It includes Earth Observation satellites (notably the Sentinel series developed by ESA), ground-based measurements, and services to processes data to provide users with reliable and up-to-date information through a set of Copernicus operational services related to environmental and security issues. These include:  
* Copernicus Marine Environmental Monitoring Service (CMEMS ([[http://marine.copernicus.eu](http://marine.copernicus.eu)]([http://marine.copernicus.eu](http://marine.copernicus.eu))));
* Copernicus Land Monitoring Service (CLMS ([[http://land.copernicus.eu/](http://land.copernicus.eu/)]([http://land.copernicus.eu/](http://land.copernicus.eu/))));
* Copernicus Atmospheric Monitoring Service (CAMS ([[https://atmosphere.copernicus.eu/](https://atmosphere.copernicus.eu/)]([https://atmosphere.copernicus.eu/](https://atmosphere.copernicus.eu/))));
* Copernicus Emergency Management Service (CEMS) ([[http://emergency.copernicus.eu/](http://emergency.copernicus.eu/)]([http://emergency.copernicus.eu/](http://emergency.copernicus.eu/)));
* Copernicus Climate Change Service (C3S) ([[http://climate.copernicus.eu](http://climate.copernicus.eu)]([http://climate.copernicus.eu](http://climate.copernicus.eu))). Copernicus services provide critical information to support a wide range of applications, including environment protection, management of urban areas, regional and local planning, agriculture, forestry, fisheries, health, transport, climate change, sustainable development, civil protection and tourism. Copernicus satellite missions are designed to provide 'upstream' inputs to all Copernicus Services as systematic measurements of Earth's oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. The primary users of Copernicus services are policymakers and public authorities that need information to develop environmental legislation and policies or to take critical decisions in the event of an emergency, such as a natural disaster or a humanitarian crisis. The Copernicus programme is coordinated and managed by the European Commission.