The development and operation of space vehicles, launchers and instruments are highly technical endeavours that are generally delegated by national governments to specialised space agencies. A typical space agency has responsibility for overseeing all aspects of the space activities of its host national or regional government. Applications of Earth observation satellite programmes are typically numerous and diverse, including, but not limited to, studies of climate, environmental issues, agriculture, meteorology, and natural disasters.

More information on the important role of CEOS in this framework is provided in section 3. In broad terms, CEOS membership comprises two kinds of space agencies:

— research agencies, which typically undertake cutting edge R&D activities, often involving ‘one-off’ Earth observation missions that are intended to demonstrate a technical concept of measurement capability in support of well-identified science objectives; and

— operational agencies, which are funded by governments to make continuous and time-critical observations, ensuring that there are no temporal or spatial gaps in coverage. A limited number of space agencies fall into this operational category.

Clearly a climate data record requires a commitment to stable and continuous measurements over long time periods, but, to date, issues such as data and mission continuity, overlap, and cross-calibration have been undertaken by research agencies on a ‘best efforts’ basis. While a typical mission involves considerable effort dedicated to these activities, there remains a need to ensure that this happens systematically. In recent years, agencies have endeavoured to ensure continuity of some key measurements (e.g. ocean surface altimetry) that have become established as near-operational within some user communities. This remains, however, the exception rather than the rule for research-oriented space agencies, which are neither mandated nor funded to provide operational services.

In contrast to research-focused satellite programmes, the satellite programmes of operational agencies have many of the characteristics required by GCOS for climate applications, such as constant interaction with operational user communities and adaptation to their needs, as well as sustained, overlapping and coordinated coverage. Although recognition is growing of the need to transition research satellites that provide observations required by the GCOS IP into operational systems, constructing such a migration path in the planning for current and future systems remains difficult. To support this aim, and with the objective of creating an operational system that monitors and evaluates the calibration of the global meteorological satellite observing system in a coherent and systematic manner, the Coordination Group for Meteorological Satellites (CGMS) has recently started the Global Space-based Intercalibration System (GSICS) initiative.

Despite the utility of satellite Earth observations for climate, it also should be understood that there is presently no overall strategy across nations for a comprehensive design of these systems.

Most contributing missions were neither intended nor optimised for climate purposes. Therefore, gaps and needed improvements have been identified to realise the ambitions expressed in the GCOS IP.

The role of CEOS in helping to provide the coordination necessary to address such issues is explained in section 3. Section 4 highlights the challenges which lie ahead if we are to successfully implement the system specified by GCOS.

Some examples of the vital contribution of Earth observation satellites to the development of our climate data records are explored in the Case Studies in Part II of this document. Part III explores in considerably more detail the adequacy of planned and existing satellite systems to meet the needs for a particular measurement or Essential Climate Variable.