This investigation uses a planar retarding potential analyzer, designed to obtain measurements of relative thermal-ion velocity, plasma density, and ion temperature. The ion angle of arrival can be determined by use of a square aperture collimator and a split collector. Together with knowledge of spacecraft motion, this allows computation of the three-dimensional thermal-ion motion along the orbital path. Plasma density and temperature are calculated by interpretation of the volt-ampere profile produced by the instrument for a given impressed voltage pattern on the grids and collector. Ion velocity measurements are obtained once each spacecraft spin period (10 s).