The CORAL instrument measures quasi-3D ion distribution in the range 30-24200 eV/q without separation of species. It is a magnetospheric experiment : it does not sample along the spacecraft spin axis, which is towards the Sun. The sensor is a hemispherical electrostatic analyser.

The instrument counts particles arriving within a fan-shaped cone which is divided into five 35° polar sectors centred respectively at the polar angles of 42°, 66°, 90°, 114° and 138° (i.e. latitudes 0°, 24°, and 48° with respect to the spin plane). The azimuthal angle is scanned by the rotation of the spacecraft. One complete rotation is divided into 32 angular sectors of 11.25° each. The distribution of particle flux in 5x32 = 160 directions covering approximately 4pi x sin(60°) = 10.9 ster is thus obtained every 2 min. The cones of aperture of 0.84 ster centred at sunward and anti-sunward directions are not sampled.

The energy range is selected by changing the potential on the hemispherical plates. There are 32 energy steps distributed logarithmically in the range of 50 eV to 25 keV centred approximately at 50 x 1.223^n eV for 0 < n < 31. The energy range sampled on each step is deltaE ~ E/20 which is about one quarter of the energy step : the energy spectrum is thus very undersampled.