This experiment was designed to determine vector ion drift velocities, ion concentration
and temperature, total ion concentration roughness, and spacecraft potential. The experiment consisted
of a retarding potential analyzer with four planar sensor heads. The sensor heads were spaced nearly
equally around the satellite equator. Since the satellite spin axis was perpendicular to the orbit plane,
these heads could observe along the spacecraft velocity vector in either the spin or despun mode of
the spacecraft. Three of the sensor heads were similar. They had two grounded entrance grids, two
retarding grids, a suppressor grid, a shield grid, and a collector. A linear sweep voltage (32 or 22
to 0 V, up or down) was normally applied to the retarding grids in 0.75 s. Interpretation of the
resulting current-voltage profiles provided the ion temperature, the ion and electron concentration,
some ion composition information, and vehicle potential and plasma drift velocity parallel to the
velocity vector. With the retarding grid at constant zero volts, current changes could be observed for
3-s periods to obtain gradients of ion concentration. Electron parameters were measured in a manner
similar to ions. Ions in mass ranges 1 to 4, 14 to 16, 24 to 32 and greater than 40 atomic mass units
could be identified. The fourth sensor head was for the ion-drift velocity measurements, and consisted
of four grounded grids, a negatively biased suppressor grid, and a four-segment collector. Differences
in the collector segments' currents provided ion-drift directional component information. More details
of this experiment are available in W. B. Hanson et al., Radio Sci., v. 8, n. 4, p. 333,