The Cluster project, prepared by the European Space Agency in collaboration with NASA , consists of four identical spacecraft launched in a tetrahedral configuration, flying on an orbit between 25000 and 125000 km above the Earth. There they study the interaction between the solar wind and the Earth's magnetosphere, allowing for the first time truly three-dimensional measurements of both large- and small-scale phenomena in the near-Earth environment.

The CIS (Cluster Ion Spectrometry) experiment is a comprehensive ionic plasma spectrometry package onboard the four Cluster spacecraft, capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) and with mass-per-charge composition determination. The CIS package consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion Composition Distribution Function (CODIF), plus a sophisticated dual-processor based instrument control and data processing system (DPS), which permits extensive onboard data-processing.

The CIS experiment is prepared by an international consortium, under the principal responsibility of CESR (which since 2011 is the IRAP ).

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