The dual-sensor spectrometer RAPID (Research with Adaptive Particle Imaging Detectors) analyzes suprathermal plasma distributions in the energy range from 20--400 KeV for electrons and from 2 KeV/nucleon to 1.50 MeV/nucleon for ions. Angular distributions are measured over a range of 180 degrees in polar angle for either species. Electrons are identified by the well-known energy-range relationship. The detection principle for ions is based on a two-dimensional analysis of a particle's velocity and energy. Moderate mass resolution of about four (for oxygen ions) permits the identification of ions and ion groups of significance. Particle species identified are electrons, protons, He and CNO group ions, and energetic neutral atoms of energies 10--100 KeV. RAPID science data are transmitted through a complex data format, using 17 Kbps in the telemetry nominal mode. Time resolution is as high as 16 samples/spin for certain elements. The nominal spin period for the spacecraft is 4 s. The RAPID field of view covers a range of 180 degrees with respect to the spin vector. The sensor systems, IIMS and IES, scan the full range of the solid angle as the spacecraft rotates. The data returned will be used to project the intensity distribution in distant particle sources on a spherical or plane image area with 192 pixels for IIMS and 144 pixels for IES. The Imaging Ion Mass Spectrometer (IIMS) sensor system is composed of three identical SCENIC (Spectroscopic Camera for Electrons, Neutral and Ion Composition) heads. The sensor systems of IIMS are identical to those in the HEP-LD instrument included in the Japanese Geotail spacecraft. The IIMS uses two-dimensional time-of-flight/energy analysis to determine particle mass. The addition of position-sensing techniques on the entrance foil, in combination with a geometrically small stop detector at the end of the flight path provides directional sensitivity. This approach divides the 180 degree polar segment into twelve contiguous angular intervals. Sectoring the spacecraft spin plane with a maximum of sixteen azimuthal intervals covers the unit sphere in velocity space completely with a total of 192 angular bins. The Imaging Electron Spectrometer (IES) achieves directional sensitivity by using novel microstrip solid-state devices in combination with a pin-hole acceptance. This technique divides the 180 degree polar range into nine angular intervals which, with sixteen sectors in the spin plane, corresponds to a total of 144 bins on the unit sphere. The RAPID spectrometer is connected to the magnetic field instrument FGM through the inter-experiment link. FGM sends 64 uncorrected magnetic field vectors per spacecraft rotation. These are used by the data processor to determine, for each of the sixteen azimuthal sectors, which look directions in the IIMS and IES fans are perpendicular to the current magnetic field vector. For more details of the Cluster mission, the spacecraft, and its instruments, see the report Cluster: mission, payload and supporting activities,'' March 1993, ESA SP-1159, and the included articleRAPID: The Imaging Energetic Particle Spectrometer on Cluster,'' by B. Wilken et al., from which this information was obtained.