This experiment was designed to determine vector ion drift velocities, ion concentration and
temperature, and spacecraft potential. An ionospheric irregularity index was also obtained from the ion
concentration sensor. The experiment consisted of a retarding potential analyzer with four planar sensor
heads. The sensor head used for ion drift measurements was co-located with another head, and all were spaced
nearly equally, looking outward from 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. The primary objective of this experiment was to provide accurate ion
temperatures with other measurements being of secondary importance. 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, vehicle potential and plasma drift velocity parallel to
the velocity vector. Two of the three similar sensors had an additional grid between the entrance and
retarding grids in order to protect inner grids from ion bombardment during electron measurements. The
other significant feature of these two sensors was that a small positive collector bias could be applied
to assure adequate access of thermal electrons to the collector. 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 various collector segment 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, 1973. NSSDC has all the useful data that exist
from this investigation.