## 1 Introduction
The development of the observation infrastructure is performed under the aegis of the European Space Agency for the space component and of the European Environment Agency and the Member States for a separate, but important, in-situ measurement component. Copernicus is a programme mandated to make all data products available to subscribing scientists, policy makers, entrepreneurs and the citizens in a full, free, and open access policy. The space element of Copernicus comprises a family of satellites called \"Sentinels\", yet exploits data coming from other Earth Observation satellites developed by other agencies. Of particular note are Sentinel-3 and Sentinel-6 that carry a Ku- and C-band radar altimeter and other microwave instruments, designed to observe the topography of the ocean surface, in support of operational oceanographyand monitoring changes in sea level, a key indicator of climate change. These missions also provide information to derive ocean currents, wind speed, ocean heat content and wave height, contributing to maritime safety. Copernicus has implemented a sustained operational Earth Observation System for the next 20 years with Sentinel satellites, now in place with multiple satellites in each family. For example, Sentinel-3 operates in a sun-synchronous orbit at an altitude of \(\sim\)800 km, while Sentinel-6 continues the legacy of the TOPEX/Poseidon and Jason series of missions flying at an altitude of \(\sim\)1300 km at an inclined orbit of 66 degrees--referred to as the 'altimeter reference orbit'. The Senitinel-6 satellites have been developed by the ESA together with NASA, EUMETSAT, NOAA, and the European Commission with the support of CNES. Still, Jason-3 currently provides operational altimeter measurements following preceding satellite missions occupying the same ground track (including TOPEX/Poseidon, Jason-1 and OSTM/Jason-2). A specific set of design and performance requirements is associated with each separate satellite mission to address their primary mission objectives and performance needs. In addition to the Sentinel-3, Sentinel-6 and Jason-3 missions, a number of additional international satellite altimeters either on orbit or shortly to be launched (e.g., CryoSat-2, IceSat-2, SWOT, HY-2C and Quanalan), implement diverse measuring technology (interferometry, wide swath and different frequencies), and will contribute to the monitoring of geocentric sea level. Small differences between successive satellite missions of the same design may be anticipated. If fundamental changes are implemented using new measurement approaches such as synthetic aperture radar (SAR) altimetry, different supporting radiometer channels, or even different choices of microwave radiometer channels, great care must be taken to ensure that such changes do not introduce artificial artefacts into the multi-mission altimetry time series. Furthermore, since range calibration of a satellite altimeter can only be derived effectively once in orbit, a dedicated program of ground-based validation and verification is required. All satellite altimeters are required to rely upon reference standards and calibration/validation (Cal/Val) infrastructures on the ground to understand relative and absolute differences due to design choices or in-flight degradation of specific components (e.g., antenna, electronics). To maintain an understanding of in-flight performance and maintain the quality of satellite altimetry products, each mission must be regularly monitored using independent data in an objective and absolute sense [9]. This obligation has been implemented using the strategy of Fiducial Reference Measurements (FRM) [9; 10] for satellite observations and is supported by the European Space Agency for all its Sentinels. The FRM strategy addresses the need, at first, for Cal/Val facilities to report their results along with a realistic, exhaustive and trustworthy uncertainty budget.