Built by the Mullard Space Science Laboratory, this comprised two electrostatic analysers.

    The instrument measured ions in the energy range 10 eV to 20 keV. The energy range was divided up into either 30 or 60 logarithmically space energy levels. The instrument achieved all-sky coverage each spin, returning data from 4 polar sectors and 8 azimuthal sectors (i.e. 32 patches of the sky). High telemetry rates allowed the transmission of fully 3-d ion distributions every 5s spin period. Thus the instrument provided high time, energy and angular resolution for its time, and even by todays standards. 
    Different operational modes modified the angular and energy resolution to suit the type of observation required and telemetry limitations. A special solar wind mode concentrated measurements around the solar direction, while still monitoring the rest of the sky in the usual way. Thus the instrument could make good quality measurements of both foreshock ions and the solar wind simultaneously. The instrument was used principally for observations of the low-latitude boundary layer, magnetopause, magnetosheath, bow shock, foreshock and solar wind and observations related to the chemical releases.