The Retarding Potential Analyzer (RPA) measured the bulk ion velocity in the direction of the spacecraft motion, the constituent ion concentrations, and the ion temperature along the satellite path. These parameters were derived from a least squares fit to the ion number flux vs energy curve obtained by sweeping or stepping the voltage applied to the internal retarding grids of the RPA. In addition, a separate wide aperture sensor, a duct sensor, was flown to measure the spectral characteristics of iregularities in the total ion concentration. The measured parameters obtained from this investigation were important to the understanding of mechanisms that influence the plasma; i.e., to understand the coupling between the solar wind and the earth's atmosphere. The measurements were made with a multigridded planar retarding potential analyzer very similar in concept and geometry to the instruments carried on the AE satellites. The retarding potential was variable in the range from approximately +32 to 0 volts. The details of this voltage trace, and whether it was continuous or stepped, depended on the operating mode of the instrument. Specific parameters deduced from these measurements were ion temperature; vehicle potential; ram component of the ion drift velocity; the ion and electron concentration irregularity spectrum; and the concentration of H+, He+, O+, and Fe+, and of molecular ions near perigee. Additional details are in W. B. Hanson et al., Space Sci. Instrum., v. 5, n. 4, p. 503, 1981.