This experiment measured in situ the spatial distribution and temporal changes of the
concentrations of the neutral atmospheric species. In addition, new insight into in situ measurement
techniques was obtained from comparisons of these measurements with those obtained from other on-board
experiments; namely, open source spectrometer (73-101A-07), solar EUV spectrophotometer (73-101A-06),
and density accelerometer (73-101A-02). The mass-spectrometer sensor had a gold-plated stainless steel
thermalizing chamber and ion source, a hyperbolic-rod quadrupole analyzer, and an off-axis electron
multiplier. Five different sequences of mass selections were available and, expressed in atomic mass
units (u), were (a) geophysical -1, 2, 4, total, 16, 28, 32, selected, 40, (b) analytical -12, 14,
18, 20, 22, 30, 44, calibrate, zero, (c) individual -selected, selected, selected, ... (any mass 1 to 44),
(d) sweep digital -1, 2, 3, 4, 5, ... 45 (in 3/16 u steps), (e) sweep analog -2, 3, 4, 5, ... 45
(continuous). Five operational formats were available and selected by ground command, and each one
contained a different combination of the five mass selection sequences listed above. When operating in
the "normal format," the analyzer measured all masses in the range 1 to 44 with emphasis on hydrogen,
helium, oxygen, nitrogen, and argon. Another format was optimized for minor constituent studies of any
individual gas species in the measured range. Spatial resolution was determined primarily by the mode
of spacecraft operation. In orbit, the presealed spectrometer was opened, and the atmospheric
constituents passed through a knife-edged orifice into the thermalization chamber and ion source.
Selected ions left the quadrupole analyzer through a weak focusing lens and were accelerated into a
14-stage electron multiplier, where they were turned 90 deg to strike the first dynode. For each impacting
ion, the multiplier output was a pulse of 2.E6 electrons. These output pulses constituted the measurement,
and the count rate was proportional to the chamber density of the selected species. These density values
were then converted to ambient concentrations. The analyzer normally operated at a resolution of 1 u over
the mass range. Pulses occurring during 0.015-s integration intervals were accumulated in a 16-bit counter.
Multiple integration periods (up to 16) were assigned to each measurement for less dense atmospheric species.
Automatically selected ranges of ionizing electron currents were used. The overall dynamic range of the
measurements was greater than 1.E7. More experiment details can be found in
D. T. Pelz et al., Radio Sci., v. 8, n. 4, p. 277, 1973. NSSDC has all the useful data that exist
from this investigation.