The instrument consisted of dual 90-deg quadraspherical electrostatic analyzers, one with 26 individual particle detectors and the other with 5 current collectors. The system was capable of measuring incident plasma distribution parameters over the energy range 0.1 to 18 keV for protons and approximately 1-500 eV for electrons. The high-resolution analyzer, with a constant of 9 keV/Q per kV applied to the plates, had a mean plate radius of 9 cm and separation of 0.5 cm. This analyzer which was used to measure ions only and had 26 channeltrons mounted on the semicircular exit to the analyzer. The aperture pointed through a wide slit in the back of the spacecraft high-gain antenna reflector and pointed along the spin axis toward the earth (and therefore the sun). The edges of the antenna reflector limited the viewing of the instrument to 73 deg with respect to the spin axis. The channeltrons covered a range of plus or minus 51 deg. Each channeltron near the center covered 3 deg, and approximately 8 deg near the edges of the analyzer. The angular width perpendicular to the long angular width was about 2 deg. In one half the spin period, the whole cone of half-angle 51 deg, centered on the sun, was swept out. A medium-energy analyzer with a mean radius of 12 cm and a 1-cm plate separation (constant of 6 keV/Q per kV applied) was used to detect both ions and electrons. The detectors were five flat-surface current collectors. The three center collectors each covered 15 deg and covered the angular range of plus or minus 22.5 deg from the spin axis. The two outside collectors had an angular width of 47.5 deg and were located at plus or minus 46.25 deg from the center of the analyzer. There were a variety of possible operating modes for the experiment; however, the principal mode utilized during the encounter phase was one in which the analyzer plate potential was stepped through its range every one-half revolution of the spacecraft, and all current collectors or channeltrons were read out at the peak flux roll angle. The high- and medium-resolution analyzers operated independently, so that a cross-check between these analyzers was possible. The dynamic range for the particle fluxes was from 1.0E+2 to 3.0E+9/(sq cm-s) and the proton temperature could be ascertained down to 2.0E+3 deg K. This instrument experienced a partial failure in February 1990 when it began reporting erroneous voltage data. Information on solar wind bulk flow speed was still available, due to a ground software work-around, until September 1995 when the instrument was turned off to conserve power for other experiments and subsystems.