The CEP consisted of two identical instruments designed to measure electron temperatures,
electron and ion concentrations, ion mass, and spacecraft potential. One probe was oriented along the
spin axis of the spacecraft (normally perpendicular to the orbit plane), and the other radially so
that it could observe in the direction of the velocity vector once each 15-s spin period. Each
instrument was a retarding potential Langmuir probe device that produced a current-voltage (I-V)
curve for a known voltage pattern placed on the collector. Electrometers were used to measure the
current. There were two systems of operation (one with two modes and another with three modes)
using collector voltage patterns between plus and minus 5 volts. Most modes involved an automatic or
fixed adjustment of collector voltage limits (and/or electrometer output) such that the region of
interest on the I-V profile provided high resolution. Each system was designed for use with only one
of the probes, but they could be interswitched to provide backup redundancy. The best measurements
in the most favorable modes provided 1-s time resolution; electron temperature between 300 and 1.0E4
deg K (10% accuracy); ion density between 1.0E4 and 1.0E7 ions/cc (10-20% accuracy); electron
density between 50 and 1.0E6 electrons/cc; and ion mass at ion densities above 1.0E4 ions/cc. Each
probe had a collector electrode extending from the central axis of a cylindrical guard ring. The
2.5-cm-long guard ring was at the end of a 25-cm boom, and the collector extended another 7.5 cm
beyond the guard ring. The boom, guard, and collector were 0.2 cm in diameter. More detailed
information can be found in L. H. Brace et al., Radio Sci., v. 8, n. 4, p. 341, 1973. NSSDC has
all the useful data that exist from this investigation.