## 1 Introduction  
The determination of global and regional mean sea-level variations using satellite altimetry, with an accuracy higher than 1 mm/year, is an important challenge. It is central in the current debate on climate change and its impact on the environment [1; 2]. Highly accurate time series from both satellite altimetry and tide gauges are needed to address this issue. However, the desired accuracy represents a significant challenge for the geodetic community. From the perspective of a space-borne altimetry, systematic errors from the altimeter systems are limiting factors and must be minimized in order to derive data products of the greatest geophysical value. Indeed, the objective for the overall accuracy of future altimeter systems is 1 cm, along with a stability below 1 mm/year. From the terrestrial perspective, estimating the vertical velocity of tide gauge sites with sufficient accuracy is also one of the most important and challenging problems in modern geodesy. To reach these goals, ultra-precise validation and calibration techniques, including in situ absolute calibration experiments, are needed. Most of the present calibration experiments are on or near the seacoast, reinforcing the need for developing such techniques to unify the altimetry error budget, for both open-ocean and local (coastal) conditions. The number and variety of C/V sites over the continent allows performing more robust assessment of the radar altimetry, over different types of water surface, from oceans to lakes and rivers [3; 4]. It also allows verifying whether estimation of the absolute bias is dependent on the specific water conditions. Within the framework of the Ocean Surface Topography Science Team (OSTST), a C/V project, named FOAM (From Ocean to inland wAters Monitoring [5]), including continental waters (rivers and lakes) has been setup in 2007. In collaboration with the CNES and NASA oceanographic missions--Topex/Poseidon (T/P) and Jason--the OCA (Observatoire de la Cote d'Azur) has developed a verification site in Corsica, since 1996; LEGOS has installed sites in Kerguelen in 1993 and at Vanuatu in 1999, for oceanic validation, and at lake Issykul and along the Amazon River, for continental waters [6]. Recently, new coastal sites in France (ile d'Aix and Pertuis Charentais, Arcachon Bay, and Gironde Estuary) have been developed to deal with the littoral context. Within the FOAM project we have investigated different approaches for C/V activities, including the development of dedicated instruments. This last point, which ensures consistent procedures and equipment among all different calibration sites, is crucial for a multi-mission C/V approach. C/V embraces a wide variety of activities:  
* Interpretation of information from internal-calibration modes of the sensors;
* Selection of the best algorithm in picking up the range in the radar echoes in various contexts;
* Selection of the best measurement or model to account for the delay in travelling through the atmosphere;
* Validation of the fully corrected estimates of the heights, whether it is sea-level or inland water-level. The inland sites are unique in the scientific community. They provide the only estimates of altimeter range bias and assessment of geophysical corrections for non-oceanic retracking algorithms used for continental waters or for ranges determined from ocean-like waveforms, in a context of very low Sea State Bias (SSB), as is frequently encountered over lakes. In-situ calibration of altimetric height is usually performed at the nadir of a dedicated C/V site, by direct comparison of the altimetric data with in-situ data. This configuration leads to better handling of the differences observed in the altimetric measurements system at the global scale--the geographically correlated errors at regional (orbit, SSB, atmospheric corrections, etc.)