INSTRUMENT_HOST_ID = "PVO"
INSTRUMENT_ID = "ONMS"
INSTRUMENT_NAME = "ORBITER NEUTRAL MASS SPECTROMETER"
INSTRUMENT_TYPE = "QUADRUPOLE MASS SPECTROMETER"
Instrument Overview
===================
The ONMS instrument is a quadrupole mass spectrometer with an electron impact ion source for
measurement of neutral gas composition in the mass range 1 to 46 amu. The sensor consists of
an ion source, a quadrupole mass filter and secondary electron multiplier as an ion detector.
The ion source is partially enclosed and exposed to the ambient atmosphere of neutral and ion
particles through an entrance aperture. Just inside the aperture is the ion repeller grid at
36 V above spacecraft ground designed to reject ions of this energy or less. On either side of
the ion repeller grid is a grid at -4.3 V designed to reject low energy electrons. Neutral gas
particles are not influenced by these grid potentials. They pass through to the ionization
region where a small fraction are ionized by electron impact from an electrostatically focused
electron beam generated by a hot filament. The ions are then focused into the quadrupole
analyzer for separation according to mass to charge ratio. The ion source grid assembly can
also function as a retarding potential analyzer (RPA) for analysis of direct streaming
particles that have not had any surface collisions. With the filament off and the ion repeller
set to 0 V, the sensor can detect thermal ions. With the ion repeller set to 36 V,
superthermal ions exceeding this energy can be detected.
There are two selectable electron energies which can be used to identify neutral species based
on the mass spectral cracking patterns. The ionization and dissociation cross sections are a
function of electron energy. Mass spectra are simplified at lower electron energy but with a
considerable loss in ionization efficiency.
The ion source enclosure is such that it can function as a closed or an open source. The
neutral particle density in the ionization region consists of direct streaming particles and
particles which have been reflected from the surfaces. As a closed source, the neutral
particle flux into the enclosure is balanced by the thermalized flux exiting the enclosure.
The thermalized flux results from particles colliding many times with the ion source surfaces.
This increased ram pressure results in a significant increase (about 60 for N2) over the
ambient pressure of the species and lowers the detection threshold. Species such as O and N
chemically react on the surfaces forming O2 and NO. The open source measures direct streaming
particles that have not had surface collisions. The view cone for the open source in the
ionization region has a half angle of about 38 degrees. The detection threshold for this mode
is higher than that for the closed source since the ambient density is directly detected. A
retarding potential field is used to discriminate between direct streaming particles at
spacecraft energy and surface reflected particles at a much lower energy.
A radio frequency electromagnetic field is used to select a given mass to charge ratio ion.
Mass peak (mass/charge ratio) selection is accomplished through a proper combination of the ac
and dc voltages applied to the hyperbolic rods. Resolution is determined by the ratio of the
ac to dc amplitude. The peaks produced are flat topped and stepping from one mass peak to the
other can be accomplished without requiring peak searching.
A secondary electron multiplier is used as a detector operating either in a pulse counting mode
or a current measuring mode. Either value can be output to the telemetry stream. Normally the
pulse counter values are output as long as the electrometer current does not exceed a given
value.
The sensor itself is constructed of stainless steel and was sealed under vacuum prior to
delivery for launch. A small ion getter pump was used to maintain the vacuum below 1.E-4
pascal. The ion source was covered by a metal-ceramic break-off cap which was removed by a
pyrotechnic actuator after orbit insertion.
The basic instrument data consists of a mass number being sampled, pulse counter or
electrometer data, RPA mode type, and housekeeping data. Proper operation of the instrument is
verified through its housekeeping data.
Data accumulation is always done at equally spaced time intervals. Each 16 bit telemetry
represents one reading obtained over an integration time determined by the spacecraft bit rate
and format. The individual 8 bit telemetry words assigned to the ONMS in a minor frame are not
equally spaced in time. Therefore, the data has to be stored internally for a period of time
and then sent to the telemetry. The storage and readout are accomplished through the use of
two memories. One memory is reading out unequally spaced data to the spacecraft telemetry
system while the other is accumulating equally spaced data from the instrument. Both memories
are first in, first out (FIFO) type and are switched simultaneously. The time interval over
which the ONMS data is accumulated in memory depends on the spacecraft bit rate, format, and
spin segment status. With no spin segment, the memories are switched on at a minor frame
beginning, data is accumulated for one minor frame and then read out to the telemetry system
during the next minor frame. With spin segment mode commanded, the data is accumulated at four
times the rate of the no spin segment mode and at equally spaced time intervals from -45
degrees to +45 degrees roll angle relative to the ONMS velocity ram direction. The memory
switch time and data accumulation start time are determined from the spacecraft RAM pulse,
which is assumed to represent the maximum velocity ram for the spacecraft +X axis (0 degrees
spacecraft azimuth angle), and the ONMS azimuth angle.
The ONMS has telemetry words in the formats PERC (6 words), PERB (14 words), and APOB (2
words). Each word consists of 8 bits. The length of the data cycle for housekeeping data is
128 words. The unit of 64 16-bit words contains all of the information to determine the
current command status, monitor values, etc., and is the basic unit of processing for the data.
In addition to the internal monitors read out by the instrument electronics, other monitors are
available from the spacecraft telemetry which do not require the ONMS to be operating:
instrument temperature, bus voltage and instrument on/off status.
A summary of the instrument parameters are as follows:
Ion source: closed/open with particle retarding deployment by metal-ceramic break off cap,
pyrotechnically activated electron impact ionization dual filaments, 20 ua
emission electron energy 27 eV or 70 eV
Analyzer: quadrupole mass filter, hyperbolic rods 7.5 cm long, field radius 0.2 cm rf
frequency 5.6 mhz
Detector: secondary electron multiplier, copper-beryllium, box and grid design, pulse
counting up to about 852000 counts/sec (0.160 ua current) current measurement from
0.160 ua to 15 ua minimum detectable signal is 1 count per integration period
multiplier noise signal < 1 count / minute
Total detector dynamic range: 1.3E7
Resolution/crosstalk: < 1.E-4
Mass range: 1 to 46 amu
Mass measurement modes: programmed mass, 8 individual mass numbers, 1 to 46 amu unit sweep,
1 to 46 in steps of 1 amu 1/8 unit sweep, 1 to 46 in steps of 0.125 amu
Sample rates:
a) Normal mode: equally time spaced samples for all spacecraft spin angles 6
samples/sec with nominal bit rate 1024 bits/sec and PERC format actual
sample rate dependent on spacecraft telemetry format and bit rate used
b) Spin segment mode: equally time spaced samples for 45 degrees with respect to occurrence
of velocity ram effective data rate 4 times the normal rate
Number of words in telemetry format per minor frame:
PERB: 14 8-bit words
PERC: 6 8-bit words
APOB: 2 8-bit words
1 minor frame = 64*8 bits
Integration time: 0.171875 s maximum, 0.006 s minimum
The spacecraft orbit is nearly polar (105.6 degrees inclination) with periapsis near the
equator (17 degrees north celestial latitude) and has an average period of 24.03 hours. The
local time of periapsis increases 1.6 degrees/day (or orbit) so that it takes 224.7 days to
sample one complete diurnal cycle (dayside, evening terminator, nightside, and morning
terminator). For the first 600 orbits, the altitude of periapsis varied from 142 km to 250 km.
After this period, the periapsis altitude was no longer controlled and increased in altitude as
a result of solar gravitational perturbations. The spacecraft spins with a nominal period of
12 seconds about an axis which points approximately toward the south ecliptic pole.
Operational Modes
=================
Currently there are three distinct types of data that can be acquired by the ONMS instrument:
Neutral density, superthermal ion flux and thermal ion density. Independent of these three
data types, there are a number of modes in which the data can be taken as well as a number of
modes in which the instrument can be configured and tuned. Generally speaking these of the
instrument.
Data measurement types:
Neutral gas composition: closed source
open source with particle retarding
Ion composition: superthermal ions ❯ 36 eV relative to
spacecraft ground (ion repeller set to 36 V)
normal ions ❯ 0 V relative to spacecraft
ground (ion repeller set to 0 V)
Detection ranges{(1)}:
Neutral composition:
N2 sensitivity: 5.E4 (particles/cm**3) / (count/sec)
Open source density range: 3.E5 to 4.E12 particles/cm**3
for N2
Closed source density range: 5.E3 to 7.E10 particles/cm**3
for N2
Superthermal (❯36 eV) ions:
O+ sensitivity: 4.E3 (particles/cm**2/sec)/(count/sec)
O+ flux range for 40 eV ions: 2.E4 to 3.E11
particles/cm**2/sec
O+ density range for 40 eV ions: 0.01 to 1.E5
particles/cm**3
Thermal ions:
O+ sensitivity: 0.02 (particles/cm**3)/(counts/sec),
normalized to OIMS
O+ density range: 0.02 to 2.6E5 particles/cm**3
for a spacecraft speed of 9.6 km/s
(1) Based on minimum count rate, detector dynamic
range and instrument sensitivity for mode used
The instrument configuration modes:
a) Ion source:
Two different electron energies can be selected by command. Ions can also be measured
with the filament off. An ion repeller grid just inside the entrance aperture at 36 V or
0 V rejects positive ions of this energy or less. Neutral gas particles are not affected
by the 36 V grid potential and pass through into the ionization region.
b) Retarding potential analyzer:
Retarding and non-retarding modes can be commanded separately along with a mode in which
they alternate. A retarding potential sweep through a range of retarding voltages is also
commandable and is considered an engineering diagnostic tool. The retarding voltage is
VR=O+G*M where the offset O and and the gain G each have 4 separate commandable levels,
and M is the mass number in amu. The retarding sweep voltage is VR=O+0.0103*(64-S) where D
is a fixed value and S is number 1...64. For reference a spacecraft speed of 9.8 km/s
corresponds to an energy of 8 eV.
c) Quadrupole analyzer:
Tuning on the peak and mass resolution are separately commandable with 4 different values.
Mass peak desired can be programmed as 8 selectable mass numbers, a unit amu sweep, and
1/8 amu sweep. The latter is primarily an engineering tool to check tuning and resolution.
The unit sweep is a species survey mode, and programmed mass mode is used to concentrate
on high time resolution measurements of particular species.
d) Secondary electron multiplier detector:
There are 4 selectable gain values determined from four commandable high voltage levels.
e) Pulse counter discriminator:
There are 4 commandable discriminator levels used in counting the individual multiplier
pulses. A commandable mode is available to alternately read out pulse counter and
electrometer values for multiplier gain measurements.
Unsupported Keywords
====================
INSTRUMENT_ID = ONMS
INSTRUMENT_NAME = 'ORBITER NEUTRAL MASS SPECTROMETER'
INSTRUMENT_TYPE = 'QUADRUPOLE MASS SPECTROMETER'
PI_PDS_USER_ID = HNIEMANN
NAIF_INSTRUMENT_ID = 'N/A'
BUILD_DATE = 1977
INSTRUMENT_MASS = 3.8 <kg❯
INSTRUMENT_HEIGHT = 28 <cm❯
INSTRUMENT_LENGTH = 35 <cm❯
INSTRUMENT_WIDTH = 15 <cm❯
INSTRUMENT_MANUFACTURER_NAME = 'Goddard Space Flight Center'
INSTRUMENT_SERIAL_NUMBER = 'Flight Unit'
REFERENCES
----------
Niemann, H.B., J.R. Booth, J.E. Cooley, R.E. Hartle, W.T. Kasprzak, N.W. Spencer, S.H. Way,
D.M. Hunten, G.R. Carignan, Pioneer Venus Orbiter Neutral Mass Spectrometer Experiment,
IEEE Trans. Geo. Rem. Sens., _GE-18_, 60-65, 1980. (Niemann et al., 1980)
Scientific Objectives
=====================
The prime mission of the Orbiter Neutral Mass Spectrometer (ONMS) is to perform in-situ
measurements of the neutral gas composition and its variation with altitude and local solar
time in the thermosphere and exosphere of Venus. Measurements of these variations are
important in defining the dynamical, chemical and thermal state of the upper atmosphere. When
the periapsis altitude is below about 250 km, the neutral densities of helium, atomic oxygen,
atomic nitrogen, molecular nitrogen, carbon monoxide and carbon dioxide are measured. Gas
kinetic temperatures can be derived from an analysis of the density scale heights. Wave-like
perturbations consistent with gravity waves can be observed after removal of the altitude
variation in the data. Neutral density data were taken during orbit 1-645 and during orbits
4954-5055.
Superthermal ions with energy ❯ 36 eV in the spacecraft reference have also been detected.
Ions of this energy have sufficient energy to escape the planet and represent an atmospheric
loss. They have been observed in the near tail region and near the dayside ionopause. The
ions were first detected in orbits associated with measurements of the neutral density and are
evidenced as erratic signals above the usual gas background signal at high altitudes. The ions
must have an energy exceeding the voltage on the ion repeller grid (36 V) in order to be
detected. The ion composition can be determined and consists of mainly O+ with traces of He+,
N+, CO+ and/or N2+, NO+ and O2+. CO2+ occurs very rarely. H+ is not measurable with the
current instrument configuration. After the first 935 orbits, when the altitude of periapsis
was above 300 km and the neutral atmosphere could no longer be measured, the instrument was
configured to perform measurements of the superthermal ions with the filament off and ion
repeller at 36 V. The direction of the ion flow in the ecliptic plane can also be determined
from the spin modulation of the data.
Thermal ions can also be measured with the filament off and the ion repeller set at 0 V.
Species observed include He+, N+, O+, CO+ and/or N2+, NO+, O2+ and CO2+. H+ is not measurable
with the current instrument configuration. One component of the ion drift in the ecliptic
plane can also be determined. Thermal ion measurements have been taken sporadically at the end
of neutral density passes and on alternate orbits when superthermal ions are not being
measured.
The ONMS instrument was not operated on all orbits and some orbits are devoted to engineering
studies. Typically neutral density passes occupied -40 min. to +30 min. relative to the time
of periapsis. Ion and superthermal ion mode passes typically are 15 to 20 minutes in duration
on either side of periapsis. Neutral density passes during entry also took about this same
amount of time.
Operational Considerations
==========================
Instrument commanding: cannot directly command filament A to B;
must turn-off filament between A and B
cannot use highest multiplier voltage;
current levels too high
cannot use spin segment mode in PERB format and
bit rates exceeding 1024 bps;
4K memory overflow;
cannot use it when less than one housekeeping cycle is
stored in memory
Maximum signal limitations: filament is shut off if detector
multiplier current exceeds 15 ua
Calibration
===========
Initial testing of the retarding and ion modes was done using a low energy, 0-25 eV, ion beam.
Gas calibrations to establish the closed source neutral density were performed over the
pressure ranges expected in flight. This calibration established the overall relation
between the thermalized particle density in the ionizing region and the electronics telemetry
output. Gases used for calibration were CO2, N2, O2, Ar, He, CO and NO. The calibrations
established mass spectral cracking patterns, low and high electron energy sensitivities, pulse
counter dead time correction, ratio of electron multiplier count rate and current output,
tuning and mass resolution characteristics.
Comparison of the ONMS neutral densities with mass density measurements near 100 km deduced
from the Pioneer Venus lower atmosphere probes, the bus mass spectrometer measurements and
orbiter drag indicate that the overall sensitivity is about a factor of 1.6 low (Hedin et al.,
1983). Estimates of the gas flow into the ion source at satellite speeds indicate that the
sensitivity may deviate from that assumed in the data reduction. Laboratory tests of the
prototype instrument in a molecular beam system produced results consistent with this
hypothesis but not conclusive because of the limited range of speeds that could be obtained.
The relationship between the superthermal ion flux and instrument output was established in a
post-flight calibration of the flight backup unit. In the energy range 40-200 V, the maximum
transmission occurs 10 V above the ion repeller potential and drops to 15% of the maximum
transmission value above 100 eV. The transmission decreases with increasing mass number and
for Ar+ ions (m/e=40) it is about a factor of 2.5 below that for O+ at the maximum
transmission point.
The relationship between thermal ion density and instrument output was established by direct
comparison of the O+ signal with the O+ density determined from the Orbiter Ion Mass
Spectrometer (OIMS) instrument using O+ data from orbit number 530 at 300 seconds from
periapsis. Other species are assumed to have the same sensitivity as that of O+. In this mode
superthermal ions cannot be distinguished from thermal ions.
Stability of the electronics and sensor combination as a function of temperature was
established in tests using a vacuum chamber with temperature control.
References
==========
Hedin, A.E., H.B. Niemann, W.T. Kasprzak and A. Seiff, Global Empirical Model of the Venus
Thermosphere, Journal of Geophysical Research, vol. 88, 73-83, 1983.
Niemann, H.B., J.R.Booth, J.E. Cooley, R.E. Hartle, W.T. Kasprzak, N.W.Spencer, S.H. Way,
D.M. Hunten and G.R. Carignan, Pioneer Venus Orbiter Neutral Gas Mass Spectrometer,
IEEE Trans. on Geoscience and Remote Sensing, vol. GE-18 (1), 60-65, 1980.
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