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 heads were spaced almost 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 purpose 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 4-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.