The RAS is the central processing and analysis facility for the Ultra Low Energy Isotope Spectrometer (ULEIS) particle instrument, one of six sensors and three instruments in the Advanced Composition Explorer (ACE) mission.

ULEIS measures ion fluxes over the charge range from He through Ni from about 20 keV/nucleon to 10 MeV/nucleon, thus covering both suprathermal and energetic particle energy ranges. Exploratory measurements of ultra-heavy species (mass range above Ni) will also be performed in a more limited energy range near 0.5 MeV/nucleon. ULEIS will be studying the elemental and isotopic composition of solar energetic particles, and the mechanisms by which these particles are energized in the solar corona. ULEIS will also investigate mechanisms by which supersonic interplanetary shock waves energize ions.