|
Instrument Information |
|
| IDENTIFIER | urn:nasa:pds:context:instrument:pvo.oetp::2.0 |
| NAME |
ELECTRON TEMPERATURE PROBE (OETP) FOR PIONEER VENUS |
| TYPE | |
| DESCRIPTION |
Scientific Objectives
=====================
Krehbiel, J.P., L.H. Brace, R.F. Theis, J.R. Cutler, W.H.
Pinkus, and R.B. Kaplan, 'Pioneer Venus Orbiter Electron
Temperature Probe', IEEE Transactions on Geoscience and Remote
Sensing, January 1980. [KREHBIELETAL1980].
ABSTRACT - The Orbiter Electron Temperature Probe (OETP)
instrumentation and measurement technique has been designed to
perform in-situ measurements of electron temperature and
electron and ion density in the ionosphere of Venus. Adaptive
sweep voltage circuitry continuously tracks the changing
electron temperature and spacecraft potential while
auto-ranging electrometers adjust their gain in response to the
changing plasma density. Control signals used in the
instrument to achieve this automatic tracking provide a
continuous monitor of the ionospheric parameters without
telemetering each volt-ampere curve. Internal data storage
permits high data rate sampling of selected raw characteristic
curves for low rate transmission to Earth. These curves are
used to verify or correct the inflight processed data. Sample
in orbit measurements are presented to demonstrate instrument
performance.
I. INTRODUCTION
THE PIONEER VENUS Orbiter Electron Temperature Probe (OETP) is
one of several instruments used on the orbiter to perform
in-situ measurements of the ionospheric plasma of Venus. The
instrument employs cylindrical Langmuir probes to measure the
electron temperature, Te, the electron and ion densities, Ne
and Ni, and the spacecraft potential, Vs. To provide high
spatial resolution of these parameters the instrument takes
several hundred volt-ampere curves during each brief passage
through the ionosphere. Owing to the limited telemetry rate
available to each instrument, circuitry was included for
inflight processing of the volt- ampere curves. Onboard
storage of raw data from selected curves was provided to permit
ground confirmation of the inflight processing method. Prior
to launch, the instrument design was outlined briefly by Brace
in a paper edited by Colin and Hunten (1977) [BRACE1977]. The
purpose of this paper is to present a a more detailed account
of the instrument and some of the data acquired at Venus to
illustrate how the instrument performs and how the ground data
analysis is used to verify the flight measurements. Early
results from this experiment have been reported.
[BRACEETAL1979A] [BRACEETAL1979B] [BRACEETAL1979C]
II. THEORY OF THE METHOD
The OETP is the latest spaceflight version of cylindrical
electrostatic probe of Langmuir probe instrumentation. I.
Langmuir and H. Mott-Smith, Jr., first reported use of the
electrostatic probe in a laboratory plasma in 1924
[LANGMUIRETAL1924]. The cylindrical probe technique has been
used extensively to characterize the Earth's ionosphere
[BRACE1970], most recently on board the Atmosphere Explorer
(AE) satellites [BRACEETAL1973]. The three AE instruments with
their adaptive control circuitry and auto ranging electrometer
provided the basis for the OETP instrument.
Langmuir probe theory and application has been widely reported
in the literature [BRACE1977] [BRACEETAL1979A] [BRACEETAL1979B]
[BRACEETAL1979C] [LANGMUIRETAL1924] [BRACE1970] [BRACEETAL1973]
[HOEGYETAL1973]. Here it will suffice to provide a brief
description of the technique and to remind the reader of the
theoretical volt-ampere curve produced by a Langmuir probe in a
plasma as shown in Fig. 1 [KREHBIELETAL1980]. The curve
begins in the ion saturation region with the probe potential
sufficiently negative to prohibit plasma electrons from
reaching the probe. At this point, current to the probe is due
only to ions. In the retardation region where the probe
potential is less negative, the more energetic electrons
overcome the retarding potential, and produce an exponentially
increasing current. The electron temperature Te determines the
power of the exponential with lower Te yielding a narrower
retarding region. In the electron saturation region, the probe
is positive with respect to the plasma and therefore attracts
additional electrons from the plasma. Equations appropriate to
the three regions of the curve are given below.
In the ion saturation region at 90 degree angle of attack
[HOEGYETAL1973]
Ii = ANiew/pi(1 + kTi/miw2 + 2eV/miw2)1/2 (1)
In the electron retardation region [BRACEETAL1979A]
Ie = ANee(kTe/2pime)1/2 exp(eV/kTe0. (2)
In the electron saturation region [MOTTSMITHETAL1926]
[SPENCERETAL1965]
Ie = (ANee/pi)(2eV/me)1/2 (3) where
Ne electron density Ni ion density Te electron temperature Ti
ion temperature A collector surface area w collector speed with
respect to plasma k Boltzmann constant mi mean ion mass me
electron mass V collector voltage with respect to the plasma e
electron charge.
OETP measurements are made with respect to spacecraft ground.
This causes the voltage applied to the probe VA to be
translated by the spacecraft potential Vs, as illustrated in
Fig. 1 [KREHBIELETAL1980]. Special steps must be taken to
account for this and to assure a stable spacecraft potential.
This will be described later.
Instrument Overview
===================
III. THE EXPERIMENTAL ARRANGEMENT
The OETP instrumentation system consists of two cylindrical
sensors and a central electronics unit. Fig. 2
[KREHBIELETAL1980] illustrates the relative position of the
sensors and the larger appendages of the spacecraft. The
radial sensor is mounted at the end of a 1-m boom which was
folded against the solar array and deployed after Venus orbit
insertion so as to be perpendicular to the spacecraft spin
axis. The axial sensor is mounted on a fixed boom which places
it 0.4 m away from the spacecraft forward surface. Because the
axis of the axial sensor is parallel to the spin axis it
maintains a relatively constant angle of attack to the incident
plasma and is therefore not subject to spin modulation.
To provide an adequate path for return current to the plasma,
the spacecraft provides 1.73 m2 of exposed conducting area
which is spacecraft ground. This area consists of a metal band
around the solar array, a metal mesh over the outer kapton
surface of the forward thermal blanket, and the outer surface
of the magnetometer boom. Silicone rubber was applied to all
solar cell edges and exposed electrical conductors making up
the solar array to insulate these positive potential areas from
the plasma and thus minimize the electron current they would
otherwise collect producing a concomitant change in spacecraft
potential.
A. The Central Electronics
The OETP central electronics unit contains independent
electrometer amplifiers and adaptive sweep voltage circuitry to
service each probe. Fig. 3 [KREHBIELETAL1980] is a simplified
functional block diagram of the system. Each amplifier feeds
its output into the common A/D converter and data handling
circuitry. The autoranging electrometer and adaptive sweep
voltage circuits are identical to that employed in the three AE
missions [BRACEETAL1973]. The signal multiplexing (Mux), A/D
conversion, data formatting, and First-In/First-Out memory
(FIFO) are new features to permit the inflight processing and
recovery of data, a capability required by the lower data rate
available from the orbiter. The radial probe electrometer has
a sensitivity range of 1 x 10 to the neg. 10 power to 1 x 10
to the neg. 6 power A/V, while the axial probe electrometer
has a sensitivity range of 1 x 10 to the neg. nine power to 1
x 10 to the neg. 5 power A/V. Electrometer sensitivity is
automatically adjusted to one of 1024 possible values.
Electrical power is provided by a common dc/dc converter having
separate floating outputs for each of the two electrometers.
Adaptive circuitry is provided to adjust the sweep voltage to
resolve that region of the volt-ampere curve needed to derive
Ni, Ne, Te, and Vs and to track changes in Te and Vs
encountered along the orbit. The adaptive process provides a
more continuous monitor of these plasma parameters than could
be obtained if we were to rely solely on ground-based analysis
of volt-ampere curves produced by the instrument, given the
available data rate. The VA generator converges on the proper
starting value VA Start, and the proper rate of change VA Slope
through an iterative process involving the auto-ranging
electrometer and the adaptive VA circuitry. When the adaptive
process is completed, usually within two sweeps, the curves are
properly framed to maximize the resolution of the measured
parameters. Once proper framing had been achieved, it is
maintained by slight adjustments in VA Start, VA Slope, and
electrometer gain which track the changes in electron
temperature, density, and spacecraft potential.
An idealized 1/2-s instrument measurement cycle of applied
voltage, and the resulting electrometer output for a properly
framed curve are shown in Fig. 4 [KREHBIELETAL1980]. Also
shown are the parameters used to carry out the adaptive process
and to achieve the inflight analysis. The cycle begins at time
T0 by setting VA equal to VA Start as determined from
parameters measured during the previous curve. After a
settling time the electrometer auto-ranging algorithm is
initiated to adjust the electrometer gain as needed to drive
its output to the -3.30-Vthreshold. Current to the sensor at
this time is entirely ion current and is used to determine Ni.
At time T1 the VA is increased linearly at a rate also
determined from the previous curve. The VA and electrometer
outputs are monitored by level detectors. When the
electrometer output reaches +1.41 V a level detector starts a
counter and the value of VA at T2 is measured. When the
electrometer output reaches +9.50 V, a second level detector
stops the counter and the value of VA at T3 is measured. At T3
the electrometer downranges by one decade and displays the rest
of the volt-ampere curve until the linear sweep is stopped at
T4. At T4 a fixed 2-V step is added to drive the sensor into
the electron saturation region. The electrometer then
downranges until its output is on scale and a reading of output
voltage and gain are taken and used to determine Ne.
In each 1/2-s instrument cycle we are thus able to determine,
from each electrometer, Ni from the gain and electrometer
output just prior to T1, Te from the change in VA which
produced a factor of e change in electrometer output, and Ne
from the gain and electrometer output during the Ne sample
time. The curve framing process continues by automatically
computing new values of VA Slope and VA Start. VA Slope is
computed from our definition of a properly framed curve which
requires that the full amplitude of VA be ten times the value
of the change in VA from T2 to T3 when the 1.41- and 9.50-V
thresholds are reached. VA Start is computed using (4) which
places the exponential electron retardation region such that
the 9.50-V threshold is reached at 86 percent of the sweep
interval.
VA Start = 8.6 VA at T2 - 7.6 VA at T3. (4)
If the VA at T1 does not provide a net ion current to the
collector or if the electrometer fails to reach the +9.50-V
output, called the T3 threshold, the VA generator will preset
to the 'fault' condition for the next sweep causing VA to start
at -7 V and sweep up to +5V. The fault sweep amplitude is
sufficient to locate the operating region of the volt-ampere
curve and to permit the convergence algorithm to repeat at the
beginning of the following sweep.
As a safeguard against unforeseen difficulties which might
cause our adaptive approach to fail, a fixed amplitude sweep
mode can be selected by ground command in which a sequence of
high and low voltage sweeps are applied to either or both
sensors. The high VA was -7 to +5 V and the low VA was -2 to
+1 V. In this mode a bias voltage of +/- 1 V can be added to
account for Vs uncertainties. The fixed sweep mode is not
normally used because the adaptive mode yields better resolved
stored curves.
B. The Sensors and Booms
A sensor with its guard electrode and a portion of the boom are
schematically shown in Fig. 5 [KREHBIELETAL1980]. The guard
electrode, driven at VA, is the exposed inner shield of a rigid
triaxial boom fabricated with titanium and teflon. The outer
triaxial shield is held at spacecraft ground potential. A
special white conductive paint, GSFC Code No. NS43C
[SHAI1978], was applied to the outer surface of the booms to
provide thermal control. Flight sensors were screwed onto the
boom center conductor and held in place with a high temperature
silicone (Dow Corning X 12561, silver filled for electrical
conductivity) to assure that the sensors remained in place
during the launch vibration.
The accuracy of the temperature measurements is affected by the
characteristics of the sensor surface [SMITH1972]. In
particular, the work function of different crystal surfaces can
vary by as much as several tenths of a volt and thus introduce
an uncertainty in the value of V in (1) - (3). To reduce this
error the collectors were fabricated using a chemical vapor
deposition (CVD) process. Studies of the CVD Rhenium show that
Rhenium deposited by the pyrolyctic decomposition of Rhenium
pentachloride (ReCl5) show a very high degree of crystal
orientation. Rhenium deposited in this manner exhibits a
(0001) preferred orientation [YANG1972] perpendicular to the
plane of growth. Cylindrical tubes of this type have yielded
uniform vacuum work functions of 5.1 eV. Molybdenum deposited
from MoCl6 also shows a very high degree of crystal
orientation. Thus both materials become candidates for sensor
materials.
Fig. 6 [KREHBIELETAL1980] shows part of a cross section of a
collector having a CVD Rhenium surface deposited on a
polycrystalline substrate. Flight collectors were selected on
the basis of examining similar cross sections cut from one end
of the collector and from volt-ampere curves taken in
laboratory plasmas. The radial collector surface is Rhemium
and the axial is Molybdenum.
C. Measurements Format
The orbiter is able to operate in several data formats and at
spacecraft data rates from 16 to 2048 b/s. The OETP is
designed for optimum operation when its output data rates are
80, 128, or 160 b/s, one of which is normally used during a
periapsis pass. Various instrument data formats can be
selected by command to provide for more or less dense coverage
depending on such factors as spacecraft bit rate and whether
the OETP data system is dedicated to one sensor or shared by
both of them.
IV. Verification of Inflight Analysis
To permit ground calibration of the values of Ni, Ne, and Te
determined by the inflight processing, the instrument
periodically samples volt-ampere curves from either or both
electrometers. The electrometer output is measured at 50
equally spaced points between T1 and T4. The FIFO memory is
used to store the fifty measurements , made at 1132 b/s, for
later readout at the lower spacecraft telemetry rate. Fig. 7
[KREHBIELETAL1980] illustrates the ground analysis of such a
stored curve taken from orbit 112. The solid line represents a
least squares fit of a straight line (ion current) and
exponential (electron current) to the actual data points within
the electron retardation region.
V. Illustration of Operation at Venus
Fig. 8 [KREHBIELETAL1980] is a computer plot of the values of
Te and Ne taken from a single pass of the orbiter through the
nightside ionosphere. The line segments connect the inflight
values and reveal the small-scale spatial structure which is
often present. The asterisks represent the values of Te and Ne
derived later by computer fitting stored volt-ampere curves of
the type illustrated in Fig. 7 [KREHBIELETAL1980]. Values
derived from the stored curves are used to normalize the
inflight processed data. Thus through the use of on-board
processing of curves, high spatial resolution is achieved
without sacrificing the accuracy provided by ground computer
fitting of raw volt-ampere curves. As of this writing more
than 250 passes through the ionosphere of Venus have been
completed and the instrument continues to operate well.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the aid of L.R.O. Storey in
the testing of candidate collectors in the low temperature
plasma chamber at the Centre de Recherches en Physique de
L'Environment Terrestre en Planetaire in Orleans, France.
REFERENCES
L.H. Brace, 'Orbiter electron temperature Probe', L. Colin
and D.M. Hunten, Eds. Space Sci. Rev., vol. 20, no. 4,
p.454, June 1977. [BRACE1977]
L.H. Brace, R.F. Theis, J.P. Krehbiel, A.F. Nagy, T.M.
Donahue, M.E. McElroy, and A. Pedersen, 'Electron
temperatures and densities in the Venus ionosphere', Science,
vol. 203, p. 763, Feb. 23, 1979. [BRACEETAL1979A]
L.H. Brace, H.A. Taylor Jr., P.A. Cloutier, R.E. Daniell
Jr., and A.F. Nagy, 'On the configuration of the nightside
Venus ionopause', Geophys. Res. Lett., vol. 6, p. 345, 1979.
[BRACEETAL1979B]
L.H. Brace, R.F. Theis, H.B. Niemann, H.G. Mccayr, W.R.
Hoegy, and A.F. Nagy, 'Empirical models of the electron
temperature and density in the nightside Venus ionosphere',
Science, vol. 205, p. 102, 1979. [BRACEETAL1979C]
I. Langmuir, and H. Mott-Smith, Jr., 'Studies of the electric
discharges in gases at low pressures', Gen. Elec. Rev., p.
616, Sept. 1924. [LANGMUIRETAL1924]
L.H. Brace, 'Global structure of ionosphere temperature', in
Space Research X. Amsterdam, The Netherlands: North-Holland,
p. 633, 1970. [BRACE1970]
L.H. Brace, R.F. Theis, and A. Dalgarno, 'The cylindrical
electrostatic probes for atmosphere explorer -C, -D, -E',
Science, vol. 8, no. 4, p. 341, Apr. 1973. [BRACEETAL1973]
W.R. Hoegy and L.E. Wharton, 'Current to moving spherical and
cylindrical electrostatic probes', J. Appl. Phys., vol. 44,
no. 12, p. 5365-5371, 1973. [HOEGYETAL1973]
H. Mott-Smith and I. Langmuir, 'The theory of collectors in
gaseous discharges', Phys. Rev., vol. 28, pp. 727-763, 1926.
[MOTTSMITHETAL1926]
N.W. Spencer, L.H. Brace, G.R. Carignan, D.R. Taeusch, and
H.B. Niemann, 'Electron and molecular nitrogen temperature and
density in the thermosphere', J. Geophys. Res., vol. 70, pp.
2665-2698, 1965. [SPENCERETAL1965]
M.C. Shai, 'Formulation of electrically conductive thermal-
control coatings ', NASA Tech. Paper 1218, Apr. 1978.
[SHAI1978]
D. Smith, 'The application of Langmuir probes to the
measurement of very low electron temperatures', Planet. Space
Sci., vol. 20, p. 1721, 1972. [SMITH1972]
L. Yang, 'Preparation & evaluation of CVD rhenium thermionic
emitters', in The Third Annual Conference on Chemical Vapor
Deposition, F.A. Glaski, Ed., American Nuclear Society, 1972.
[YANG1972]
Calibration
===========
To permit ground calibration of the values of Ni, Ne, and Te
determined by the inflight processing, the instrument
periodically samples volt-ampere curves from either or both
electrometers. The electrometer output is measured at 50
equally spaced points between T1 and T4. The FIFO memory is
used to store the fifty measurements , made at 1132 b/s, for
later readout at the lower spacecraft telemetry rate. Fig. 7
[KREHBIELETAL1980] illustrates the ground analysis of such a
stored curve taken from orbit 112. The solid line represents a
least squares fit of a straight line (ion current) and
exponential (electron current) to the actual data points within
the electron retardation region.
The Sensors and Booms
A sensor with its guard electrode and a portion of the boom are
schematically shown in Fig. 5. [KREHBIELETAL1980] The guard
electrode, driven at VA, is the exposed inner shield of a rigid
triaxial boom fabricated with titanium and teflon. The outer
triaxial shield is held at spacecraft ground potential. A
special white conductive paint, GSFC Code No. NS43C
[SHAI1978], was applied to the outer surface of the booms to
provide thermal control. Flight sensors were screwed onto the
boom center conductor and held in place with a high temperature
silicone (Dow Corning X 12561, silver filled for electrical
conductivity) to assure that the sensors remained in place
during the launch vibration.
The accuracy of the temperature measurements is affected by the
characteristics of the sensor surface [SMITH1972]. In
particular, the work function of different crystal surfaces can
vary by as much as several tenths of a volt and thus introduce
an uncertainty in the value of V in (1) - (3). To reduce this
error the collectors were fabricated using a chemical vapor
deposition (CVD) process. Studies of the CVD Rhenium show that
Rhenium deposited by the pyrolytic decomposition of Rhenium
pentachloride (ReCl5) show a very high degree of crystal
orientation. Rhenium deposited in this manner exhibits a
(0001) preferred orientation [YANG1972] perpendicular to the
plane of growth. Cylindrical tubes of this type have yielded
uniform vacuum work functions of 5.1 eV. Molybdenum deposited
from MoCl6 also shows a very high degree of crystal
orientation. Thus both materials become candidates for sensor
materials.
Fig. 6 [KREHBIELETAL1980] shows part of a cross section of a
collector having a CVD Rhenium surface deposited on a
polycrystalline subtrate. Flight collectors were selected on
the basis of examining similar cross sections cut from one end
of the collector and from volt-ampere curves taken in
laboratory plasmas. The radial collector surface is Rhemium
and the axial is Molybdenum.
Instrument Electronics
======================
A. The Central Electronics
The OETP central electronics unit contains independent
electrometer amplifiers and adaptive sweep voltage circuitry to
service each probe. Fig. 3 [KREHBIELETAL1980] is a simplified
functional block diagram of the system. Each amplifier feeds
its output into the common A/D converter and data handling
circuitry. The autoranging electrometer and adaptive sweep
voltage circuits are identical to that employed in the three AE
missions [BRACEETAL1973]. The signal multiplexing (Mux), A/D
conversion, data formatting, and First-In/First-Out memory
(FIFO) are new features to permit the inflight processing and
recovery of data, a capability required by the lower data rate
available from the orbiter. The radial probe electrometer has
a sensitivity range of 1 x 10 to the neg. 10 power to 1 x 10
to the neg. 6 power A/V, while the axial probe electrometer
has a sensitivity range of 1 x 10 to the neg. nine power to 1
x 10 to the neg. 5 power A/V. Electrometer sensitivity is
automatically adjusted to one of 1024 possible values.
Electrical power is provided by a common dc/dc converter having
separate floating outputs for each of the two electrometers.
Adaptive circuitry is provided to adjust the sweep voltage to
resolve that region of the volt-ampere curve needed to derive
Ni, Ne, Te, and Vs and to track changes in Te and Vs
encountered along the orbit. The adaptive process provides a
more continuous monitor of these plasma parameters than could
be obtained if we were to rely solely on ground-based analysis
of volt-ampere curves produced by the instrument, given the
available data rate. The VA generator converges on the proper
starting value VA Start, and the proper rate of change VA Slope
through an iterative process involving the auto-ranging
electrometer and the adaptive VA circuitry. When the adaptive
process is completed, usually within two sweeps, the curves are
properly framed to maximize the resolution of the measured
parameters. Once proper framing had been achieved, it is
maintained by slight adjustments in VA Start, VA Slope, and
electrometer gain which track the changes in electron
temperature, density, and spacecraft potential.
An idealized 1/2-s instrument measurement cycle of applied
voltage, and the resulting electrometer output for a properly
framed curve are shown in Fig. 4 [KREHBIELETAL1980]. Also
shown are the parameters used to carry out the adaptive process
and to achieve the inflight analysis. The cycle begins at time
T0 by setting VA equal to VA Start as determined from
parameters measured during the previous curve. After a
settling time the electrometer auto-ranging algorithm is
initiated to adjust the electrometer gain as needed to drive
its output to the -3.30-V threshold. Current to the sensor at
this time is entirely ion current and is used to determine Ni.
At time T1 the VA is increased linearly at a rate also
determined from the previous curve. The VA and electrometer
outputs are monitored by level detectors. When the
electrometer output reaches +1.41 V a level detector starts a
counter and the value of VA at T2 is measured. When the
electrometer output reaches +9.50 V, a second level detector
stops the counter and the value of VA at T3 is measured. At T3
the electrometer downranges by one decade and displays the rest
of the volt-ampere curve until the linear sweep is stopped at
T4. At T4 a fixed 2-V step is added to drive the sensor into
the electron saturation region. The electrometer then
downranges until its output is on scale and a reading of output
voltage and gain are taken and used to determine Ne.
In each 1/2-s instrument cycle we are thus able to determine,
from each electrometer, Ni from the gain and electrometer
output just prior to T1, Te from the change in VA which
produced a factor of e change in electrometer output, and Ne
from the gain and electrometer output during the Ne sample
time. The curve framing process continues by automatically
computing new values of VA Slope and VA Start. VA Slope is
computed from our definition of a properly framed curve which
requires that the full amplitude of VA be ten times the value
of the change in VA from T2 to T3 when the 1.41- and 9.50-V
thresholds are reached. VA Start is computed using (4) which
places the exponential electron retardation region such that
the 9.50-V threshold is reached at 86 percent of the sweep
interval.
VA Start = 8.6 VA at T2 - 7.6 VA at T3. (4)
If the VA at T1 does not provide a net ion current to the
collector or if the electrometer fails to reach the +9.50-V
output, called the T3 threshold, the VA generator will preset
to the 'fault' condition for the next sweep causing VA to start
at -7 V and sweep up to +5V. The fault sweep amplitude is
sufficient to locate the operating region of the volt-ampere
curve and to permit the convergence algorithm to repeat at the
beginning of the following sweep.
As a safeguard against unforeseen difficulties which might
cause our adaptive approach to fail, a fixed amplitude sweep
mode can be selected by ground command in which a sequence of
high and low voltage sweeps are applied to either or both
sensors. The high VA was -7 to +5 V and the low VA was -2 to
+1 V. In this mode a bias voltage of +/- 1 V can be added to
account for Vs uncertainties. The fixed sweep mode is not
normally used because the adaptive mode yields better resolved
stored curves.
Detectors
=========
The OETP instrumentation system consists of two cylindrical
sensors and a central electronics unit. Fig. 2
[KREHBIELETAL1980] illustrates the relative position of the
sensors and the larger appendages of the spacecraft. The
radial sensor is mounted at the end of a 1-m boom which was
folded against the solar array and deployed after Venus orbit
insertion so as to be perpendicular to the spacecraft spin
axis. The axial sensor is mounted on a fixed boom which places
it 0.4 m away from the spacecraft forward surface. Because the
axis of the axial sensor is parallel to the spin axis it
maintains a relatively constant angle of attack to the incident
plasma and is therefore not subject to spin modulation.
|
| MODEL IDENTIFIER | |
| NAIF INSTRUMENT IDENTIFIER |
not applicable |
| SERIAL NUMBER |
not applicable |
| REFERENCES |
Brace, L.H., 'Global Structure of Ionosphere Temperature', in Space
Research X. Amsterdam, The Netherlands: North-Holland, p.633, 1970. Brace, L.H., 'Orbiter Electron Temperature Probe', L. Colin and D.M. Hunten, Eds. Space Sci. Rev., Vol.20, No.4, p.454, June 1977. Brace, L.H., R.F. Theis, and A. Dalgarno, 'The Cylindrical Electrostatic Probes for Atmosphere Explorer -C, -D, -E', Science, Vol.8, No.4, p.341, Apr.1973. Brace, L.H., R.F. Theis, J.P. Krehbiel, A.F. Nagy, T.M. Donahue, M.E. McElroy, and A. Pedersen, 'Electron Temperatures and Densities in the Venus Ionosphere', Science, Vol.203, p.763, Feb.23, 1979. Brace, L.H., H.A. Taylor Jr., P.A. Cloutier, R.E. Daniell Jr., and A.F. Nagy, 'On the Configuration of the Nightside Venus Ionopause', Geophys. Res. Lett., Vol.6, p.345, 1979. Brace, L.H., R.F. Theis, H.B. Niemann, H.G. Mayr, W.R. Hoegy, and A.F. Nagy, 'Empirical Models of the Electron Temperature and Density in the Nightside Venus Ionosphere', Science, Vol.205, p.102, 1979. Hoegy, W.R., and L.E. Wharton, 'Current to Moving Spherical and Cylindrical Electrostatic Probes', J. Appl. Phys., Vol.44, No.12, p.5365-5371, 1973. Krehbiel, J.P., L.H. Brace, J.R. Cutler, W.H. Pinkus, and R.B. Kaplan, 'Pioneer Venus Orbiter Electron Temperature Probe', IEEE Transactions on Geoscience and Remote Sensing, GE-18, 49, 1980. Langmuir, I. and H. Mott-Smith, Jr., 'Studies of the Electric Discharges in Gases at Low Pressures', Gen. Elec. Rev., p.616, Sept.1924. Mott-Smith, H. and I. Langmuir, 'The Theory of Collectors in Gaseous Discharges', Phys. Rev., Vol.28, pp.727-763, 1926. Shai, M.C., 'Formulation of Electrically Conductive Thermal-Control Coatings ', NASA Tech. Paper 1218, Apr.1978. Smith, D., 'The Application of Langmuir Probes to the Measurement of Very Low Electron Temperatures', Planet. Space Sci., Vol.20, p.1721, 1972. Spencer, N.W., L.H. Brace, G.R. Carignan, D.R. Taeusch, and H.B. Niemann, 'Electron and Molecular Nitrogen Temperature and Density in the Thermosphere', J. Geophys. Res., Vol.70, pp.2665-2698, 1965. Yang, L., 'Preparation and Evaluation of CVD Rhenium Thermionic Emitters', in The Third Annual Conference on Chemical Vapor Deposition, F.A. Glaski, Ed., American Nuclear Society, 1972. |
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