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 other onboard experiments:
namely, open-source spectrometer (75-096A-07), solar EUV spectrophotometer (75-096A-06), and
density accelerometer (75-096A-02). The mass-spectrometer sensor included 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 selection were available and, expressed in
atomic mass units (u), were as follows: (1) geophysical -1, 2, 4, total, 16, 28, 32, selected,
40, (2) analytical -12, 14, 18, 20, 22, 30, 44, calibrate, zero, (3) individual -selected,
selected, selected, ... (any mass 1 to 44), (4) sweep digital -1, 2, 3, 4, 5, ... 45 (in 3/16-u steps),
(5) sweep analog 2, 3, 4, 5, ... 45 (continuous). Five operational formats were available and
selected by ground command. 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 converted to ambient concentrations. The analyzer normally operated at
a resolution of 1 u over the mass range, so that a mass peak one-thousandth the amplitude of an
adjacent peak could be measured. For the dynamic range required, 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 range of the measurements was planned to be
greater than 1.E7. There was a provision for the instrument orifice to be covered during spacecraft
thruster operations. 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.