## 2 Altimetry in the GEOSS and GMES perspectives  
The Global Earth Observation System of Systems (GEOSS) is designed to be the world-wide system of atmospheric, oceanic, and terrestrial observing systems integrated so as to monitor continuously the state of the Earth, to increase understanding of dynamic Earth processes, to enhance prediction of the Earth system, and to further implement environmental treaty obligations (see [2], for instance). The GMES program (Global Monitoring for Environment and Security) launched in 1998 by the European Commission represents a concerted effort to bring data and information providers togetherwith users, so they can better understand each other and agree on how to make environmental and security-related information available to the people who need it. GMES will be the European contribution in the worldwide GEOSS. A challenge for GMES is to gather relevant data and provide innovative, cost-effective, sustainable and user-friendly services, which will enable decision-makers to better anticipate or integrate crisis situations issues relating to the management of the environment and security. In this overall framework, GMES Marine Services are proposed to support, reinforce and improve the European capacities linked to:  
* verification and enforcement of international treaties, and assessment of European policies;
* enabling of sustainable exploitation and management of ocean resources (offshore oil and gas industry, fisheries...)
* improvement of safety and efficiency of maritime transport, shipping, and naval operations, as well as of national security and reduction of public health risks;
* anticipation and mitigation of the effects of environmental hazards and pollution crisis (oil spills, harmful algal blooms);
* advanced marine research for better understanding of the ocean ecosystems and their variability;
* contribution to ocean climate variability studies;
* contribution to seasonal climate prediction and its effects on coastal populations;
* support to specific services for coastal management and planning. The sustainability of GMES Marine Core Services is dependent on the long-term availability of satellite and _in situ_ observing systems, on the European integration and organisation of the Core Service segment, as well as on close links with research and development activities. The observing systems required for Marine Core Services are integrated into a much wider context of coordinated observing systems and data providers at international level, such as the World Weather Watch (WWW), the Global Ocean Observing System (GOOS), Earth Observing Satellites, numerical weather forecasting centers, national ocean forecasting centers, etc. While some components, such as the WWW, the EUMETSAT satellites, or the Data Buoy Cooperation Panel (DBCP) are firmly established, other ocean components, endorsed by GOOS, rest on uncertain foundations, without guarantee of continuity so far. Thus an absolute prerequisite for GMES Marine Core Services is to ensure the sustainability of observing systems, including telecommunication facilities, and the centers for data management. Several demonstration satellites, e.g. ENVISAT, should reach their end of mission around 2008, at the time when the GMES Marine Services are expected to be phased in. This is of course a matter of serious concern, and it is necessary to plan for the continuity of key missions to provide global coverage of essential variables such as sea surface height, temperature, wind and waves, fluxes, ocean colour and salinity. The value of space-based measurements of SSH has been clearly demonstrated to contribute in answering a variety of ocean science questions such as in seasonal forecasting by providing realistic model initializations. The precision altimetry data from TOPEX/Poseidon (T/P) and Jason 1, when calibrated precisely with tide gauge measurements, have provided the ability to monitor the rate of sea level change with a projected accuracy of less than one millimetre per year [3].