Introduction:
The ORPA processed data consist of 4 file types: high resolution thermal
electrons, high resolution superthermal electrons, high resolution ions, and
a key parameters file at 12 second sampling. High resolution data are provided
(for the entire orbit / only the portion of the orbit near periapsis) The
sample rate of the high resolution data is variable and dependent on the
telemetry rate and other operational considerations. All of these data are
derived from the reformatted EDR data (RDR) which contains the raw I-V curve
values. The moments of the distributions have been computed by a least squares
fitting algorithm. The low resolution data are resampled from the high
resolution files. All 5055 orbits of the mission, (Dec 5, 1978 - Oct 7, 1992)
are included in the data set. The PV RPA is described in considerable detail
by Knudsen et al. [1979-1980]. The principles of measurement are also described
therein together with some of the factors affecting the accuracy of the derived
quantities. Additional information on the theory of measurement by an RPA is
presented by Knudsen [1966].
Sampling:
The ORPA instrument sweeps through the ion, electron, and photoelectron
modes in an EIIIP sequence covering 5 spacecraft spin periods spending one
spin period in each mode. A single retarding scan can be completed
approximately 40 times in a spin period (0.3 sec). There are various algorithms
by which the instrument can decide which scan to place in the telemetry frame.
Please read the instrument description for more details.
Data Processing:
High resolution data were processing by using an automated least squares
fitting procedure. Values derived from these fits are provided in the archive.
Some of the quantities contained in this submittal of RPA data to
the PDS are erroneous because of bad least-squares fits to the I-V curves.
These bad fits were not detected by the data reduction algorithms
and have not been removed by a trained observer viewing the I-V curves and
making an educated judgment. A trained observer, looking at an I-V plot, can
rather quickly recognize data that will produce erroneous fit results
but it is difficult to write an algorithm that can recognize all the possible
situations and make the necessary adjustments.
High Resolution Electrons:
In the electron mode, the ion current to the collector is
negligible with the collector at 47 V, A potential
difference of 20 V between G-4 and C was found sufficient
to suppress most of the secondary electrons produced by ion
or electron impact at the collector. The three front grids
G-0, G-1 and G-2 are the energy analyzing grids. They are
stepped together from +6.8 to -4.2 V in the coarse scan.
The corresponding collector current is measured by an
electrometer and then digitized. The straight line portion
is the retarding region, and the logarithmic slope determines
the electron temperature by the relation:
e Delta(V)
Te = - - --------------------
k Delta( log (- Ie) )
where e is the electron charge; k, the Boltzmann constant;
and Ie, the electron current. The left side with larger
positive voltage is the attractive region. The voltage
Vp at which these two portions of the curve join is the
potential of plasma relative to spacecraft. Vp is expected
to vary from a few volts negative in interplanetary space
to 1 or 2 V positive in the Venusian ionosphere. For the
simulation it was set to zero. The lower portion of the
curve bends away from the straight-line portion because
the velocity distribution is not a true Maxwellian
distribution. An additional population of suprathermal
electrons exists with higher energies than the thermal
distribution.
High Resolution Ions:
The algorithm that scans the data in an ion I-V curve and computes the initial
estimates of the ion quantities must also make a decision as to what ion mass
is represented by a peak in DI [Knudsen et al., 1979; 1980]. The voltage at
which the DI peak occurs for a given mass may be substantially smaller or
larger than the nominal value because the Venus ionosphere is moving relative
the planet with a velocity that can approach that of the spacecraft.
Consequently, some peaks in DI have been assigned the wrong mass. The result is
not only an erroneous concentration for that mass but also an erroneous ion
velocity and total ion density. It is possible to recognize the incorrect
assignments when comparing several I-V curves which are adjacent to each other
in time, but the analysis algorithms are not this sophisticated. A few errors
in ion quantities are present in the PDS files resulting from this
difficulty. The uncertainties in the ion fitting process are included in
an ancillary file.
High Resolution Suprathermal Electrons:
The analysis of a suprathermal electron I-V curve is similarly difficult. The
interpretation of the electron distributions contributing to the I-V curve
depends on the potential of the spacecraft relative to the ambient plasma
which, in turn, depends on the location of the spacecraft and the properties of
the ambient plasma. The spacecraft is negative in the dense ionospheric plasma.
It is positive in the low density solar wind plasma provided the spacecraft is
not in the umbra of the planet. An additional complication arises in that the
sign of the current to the electrometer occasionally changes from negative to
positive during a sweep. This can occur because the background current, with
maximum retarding potential applied to the retarding grids, is compensated
close to the noise level of the electrometer just before the sweep begins
[Knudsen et al., 1979]. If the background current that has been compensated is
significant relative to the saturation current and changes in the right
direction during the ensuing sweep, the total current will go through zero and
the sign change. The background current can change because the orientation of
the rotating spacecraft relative to the sun changes, because a purely temporal
change occurs or because the location of the spacecraft changes. Switching of
the current from one sign to the other with the electrometer in its most
sensitive mode produces a noise spike in -the electrometer that is digitized
and becomes part of-the I-V curve. Writing an algorithm that recognizes the
noise spike and the change in current sign is difficult because the sign of
only the saturation current and background current of a sweep has been retained
in the I-V data for reasons of minimizing the telemetry requirements of the
RPA. A trained observer, looking at an I-V plot, can rather quickly recognize
in most cases when this condition has occurred, but it is difficult to write an
algorithm that can recognize all the possible situations and make the necessary
adjustments.
Low Resolution Data:
The PV PDS LFD SEDR tapes have time tags at 12 second intervals from 30
minutes prior to periapsis to 30 minutes after periapsis. These time tags are
the tags specified in the first four quantities of each of the EDR data
records. All PV instruments are to report their data at these common time tags
for the purpose of easy intercomparison of data. Principal Investigators (PIs)
with instruments with a sampling period much less that 12 seconds are to report
the average of measured quantities over a 12 second interval centered on the
time tags. The RPA, because of a low telemetry word assignment, records at most
one current-voltage (I-V) characteristic curve per spacecraft spin period.
Except for one set of 14 orbits, the spin period of the PV spacecraft has been
about 12 seconds. Thus, RPA physical quantities are derived at intervals of 12
seconds or more. Since the RPA operates in several modes, a particular quantity
such as thermal electron temperature may be typically measured at much longer
intervals. The thermal electron temperature is typically measured at either
approximately 48 or 60 second intervals. In a few orbits, it was measured at 12
second intervals.
The quantities TOTI,- H+, O+, M29+, CO2+, TI, VX, VY, VZ, N1, TL, N2, and T2
are derived by least-squares fitting a strongly non-linear numerical algorithm
to an I-V curve. It is necessary in performing such a fit to supply an initial
estimate of the quantities that are to be derived. If the estimates are not
sufficiently close to the true least-squares values, the algorithm may yield a
grossly erroneous value by converging to a relative minimum of the variance and
not to the absolute minimum. Also, it may not converge at all. Although some
such erroneous values have been eliminated from our basic tables tapes by
checking for the magnitude of the variance, some erroneous value are known to
be present. Such values can be way outside the nominal uncertainty quoted in
Table 1.
ASCII low resolution data are included as part of the high resolution data
archive to facilitate browsing of the key parameters of the dataset.
Missing values:
RPA quantities may be unavailable for assigning to a specific time tag for
several reasons as follows: The spacecraft data format in use at the time may
not have contained any words for the RPA. The RPA may have been turned off for
power conservation reasons. The spacecraft telemetry bit rate and/or data
format may have been such that an RPA I-V curve was recorded only at long time
intervals. RPA data for an interval of time, including the time tag, has not
been reduced. (RPA data at the time of this submission have been reduced for
only a small time interval about periapsis: plus and minus approximately 15
minutes for the first 800 orbits, plus and minus approximately 30 minutes for
orbits 800-1300, plus and minus 60 minutes for orbits 1300-2890.)"
Data:
Each of the four data file types are described in this section. The 3 high
resolution data files all contain 17 ephemeris data columns necessary for
the interpretation of the data. Rather than repeat the description 3 times,
we will described the ephemeris data columns once, and then place the
phrase ephemeris(17) in the column name field of the file description.
Ephemeris(17)
____________________________________________________________________
name type description
____________________________________________________________________
EPH1 float_4 Roll spin angle <degrees> (NRSC - RAMROLL)**
EPH2 float_4 Spin period <seconds>
EPH3 float_4 Attitude of spin axis X (NRSC - ATTX)**
EPH4 float_4 Attitude of spin axis Y (NRSC - ATTY)**
EPH5 float_4 Attitude of spin axis Z (NRSC - ATTZ)**
EPH6 float_4 Distance from Venus to center of spacecraft,
X component (ICC_ECLP - XP1SFF)*
EPH7 float_4 Distance from Venus to center of spacecraft,
Y component (ICC_ECLP - YP1SFF)*
EPH8 float_4 Distance from Venus to center of spacecraft,
Z component (ICC_ECLP - ZP1SFF)*
EPH9 float_4 Spacecraft velocity X component (ICC_ECLP - DXP1SF)*
EPH10 float_4 Spacecraft velocity Y component (ICC_ECLP - DYP1SF)*
EPH11 float_4 Spacecraft velocity Z component (ICC_ECLP - DZP1SF)*
EPH12 float_4 Distance from Sun to center of spacecraft,
X component (ICC_ECLP - XPHSFF)*
EPH13 float_4 Distance from Sun to center of spacecraft,
Y component (ICC_ECLP - YPHSFF)*
EPH14 float_4 Distance from Sun to center of spacecraft,
Z component (ICC_ECLP - ZPHSFF)*
EPH15 float_4 Venus center velocity, X component (ICC_ECLP)*
EPH16 float_4 Venus center velocity, Y component (ICC_ECLP)*
EPH17 float_4 Venus center velocity, Z component (ICC_ECLP)*
* Inertial Cartesian Coordinate System - Ecliptic
** Non-rotation spin coordinate system
Ions (binary table - IEEE UNIX byte order):
____________________________________________________________________
name type description
____________________________________________________________________
PDSTIME char_24 Output from Time fitting
IORBT integer_4 orbit number
TPER integer_4 Periapsis time (UT)
KDAY integer_4 Periapsis day of year
NDSI integer_4 Unique sweep number
TIMI float_4 Unique time of msrmnt for NDSI
ALTI float_4 Altitude of NDSI.
SZAI float_4 Solar Zenith Angle of NDSI
TOTI float_4 Total ion density
H+ float_4 Hydrogen ion density
HE+ float_4 Helium ion density
O+ float_4 Oxygen ion density
M29+ float_4 Sum density of CO+, NO+, N2+ ions
O2+ float_4 O2+ ion density
CO2+ float_4 Carbon dioxide ion density
VELI float_4 Ion bulk velocity component in direction of
RPA out
TI float_4 Ion temperature
PHII float_4 Spacecraft ground potential
CSQ float_4 Chi square, goodness of curve fit
FRSI float_4 Saturation ion current
BCKI float_4 Background ion current
ephemeris(17)
Thermal Electrons(binary table - IEEE UNIX byte order):
____________________________________________________________________
name type description
____________________________________________________________________
PDSTIME char_24 Output from Time fitting
IORBT integer_4 orbit number
TPER integer_4 Periapsis time (UT)
KDAY integer_4 Periapsis day of year
NDSE integer_4 Unique sweep number
TIME float_4 Unique time of measurement for NDSE
ALTE float_4 Altitude of NDSE
SZAE float_4 Solar Zenith Angle of NDSE
TOTE float_4 Electron density
TE float_4 Electron temperature
PHIE float_4 Spacecraft ground potential
FRSE float_4 Saturation electron current
BCKE float_4 Background electron current
ephemeris(17)
Photoelectrons(binary table - IEEE UNIX byte order):
____________________________________________________________________
name type description
____________________________________________________________________
PDSTIME char_24 Output from Time fitting
IORBT integer_4 orbit number
TPER integer_4 Periapsis time (UT)
KDAY integer_4 Periapsis day of year
NSPH integer_4 Unique sweep number
TIMPH float_4 Unique time of measurement for NSPH
ALTPH float_4 Altitude of NSPH.
SZAT float_4 Solar Zenith Angle of NSPH
VPPH float_4 Suprathermal electron spacecraft potential of NSPH
DENPH1 float_4 First suprathermal electron density of NSPH
TEMPH1 float_4 First suprathermal electron temp of NSPH
DENPH2 float_4 Second suprathermal electron density of NSPH
TEMPH2 float_4 Second suprathermal ele temp of NSPH
SDENPH1 float_4 Least-squares fit statistical uncertainty of DENPH1
STEMPH1 float_4 Least-squares fit statistical uncertainty of TEMPH1
SDENPH2 float_4 Least-squares fit statistical uncertainty of DENPH2
STEMPH2 float_4 Least-squares fit statistical uncertainty of TEMPH2
CSQPH float_4 Chi square, goodness of curve fit
FRSPH float_4 Saturation suprathermal electron current
BCKPH float_4 Background suprathermal electron current
ephemeris(17)
Key Parameters File (low resolution - ASCII table):
____________________________________________________________________
name type description
____________________________________________________________________
PDSTIME char_24 UT of UADS record time which is defined for all PVO inst
providing UADS data.
ORBIT integer_4 Orbit number
TPER integer_4 Time from periapsis
UT integer_4 Universal time
TOTI integer_4 Total ion density
H float_4 Hydrogen ion density
O float_4 Oxygen ion density
O2 float_4 O2 ion density
CO2 float_4 Carbon dioxide ion density
TI float_4 Ion temperature
VX float_4 Ion bulk velocity X component
VY float_4 Ion bulk velocity Y component
VZ float_4 Ion bulk velocity Z component
F1I float_4 First ion current, no retarding potential
BKGI float_4 Background current at start of ion sweep, +37V potential
VPI float_4 S/C potential at start of ion sweep
TOTE float_4 Total electron density
TE float_4 Electron temperature
F1E float_4 First thermal electron current, +6.8V from S/C ground
BKGE float_4 Background current at start of thermal electron sweep,
-4.6V potential
VPE float_4 S/C potential at start of thermal electron sweep
N1 float_4 Cold electron density
T1 float_4 Cold electron temperature
N2 float_4 Hot electron density
T2 float_4 Hot electron temperature
F1P float_4 First photoelectron current, no retarding potential
BKGP float_4 Background current at start of photoelectron sweep, -58V
potential
VPPE float_4 S/C potential at start of photoelectron sweep
Ancillary Files:
The ion fitting process involves both mass and velocity discrimination In many
cases, one or the other parameter can not be well determined. As an
ancillary component of the high resolution ion data files, we have included an
ion uncertainties file. Some of the reasons for the uncertainties in the
fits are described in the following section on measured quantities. The ion
uncertainty files have the following structure:
____________________________________________________________________
name type description
____________________________________________________________________
PDSTIME char_24 Output from Time fitting
IORBT integer_4 orbit number
TPER integer_4 Periapsis time (UT)
KDAY integer_4 Periapsis day of year
NDSI integer_4 Unique sweep number
TIMI float_4 Unique time of measurement for NDSI
ALTI float_4 Altitude of NDSI.
SZAI float_4 Solar Zenith Angle of NDSI
STOTI float_4 Estimated uncertainty in total ion density
SH+ float_4 Least-squares fit statistical uncertainty of H+ ion
density
SHE+ float_4 Least-squares fit statistical uncertainty of HE+ ion
density
SO+ float_4 Least-squares fit statistical uncertainty of O+ ion
density
SM29+ float_4 No significance
SO2+ float_4 Least-squares fit statistical uncertainty of O2+ ion
density
SCO2+ float_4 Least-squares fit statistical uncertainty C02+ ion
density
SVELI float_4 Ion velocity
STI float_4 Ion temperature
SPHII float_4 Spacecraft ground potential
CSQ float_4 Chi square, goodness of curve fit
ephemeris(17)
RPA Measured Quantities
The PV RPA is described together with some of the principles of measurement in
some detail by Knudsen et al. [1979,1980]. Many of the factors affecting
accuracy are also described therein. We present in this section the quantities
recorded on the PDS EDR data files following the four time tag quantities,
their nominal uncertainty and measurement noise level, and additional
limitations of the quantities.
Table 1 lists the symbol, quantity, measurement range with units in which the
quantities are quoted, noise level of measurement, and uncertainty of the
measurement for the quantities reported by the RPA. We have included in the
list of quantities the vector components of the ion bulk velocity even though
we do not supply values in this Oct 1988 submission to PDS.
TABLE 1
SYMBOL QUANTITY RANGE NOISE LEVEL
UNCERTAINTY
UT UNIVERSAL TIME OF MEASUREMENT 0 - 8.7x10**7ms - 0.1s
TOTI TOTAL ION DENSITY 10 - 1x10**7cm 10 cm-3 10%
H+ HYDROGEN ION DENSITY 300 - 10**7cm-3 300 cm-3 10%
O+ OXYGEN ION DENSITY 300 - 10**7cm-3 300 cm-3 10%
M29+ SUM DENSITY OF CO+,N2+,NO+,O2+ 300 - 10**7cm-3 300 cm-3 10%
CO2+ CARBON DIOXIDE ION 300 - 10**7cm-3 300 cm-3 10%
TI ION TEMPERATURE 150 - 10,000 K - 10%
VX ION BULK VELOCITY, X COMPONENT 0 - 7 km/s 0.4 km/s 0.4 km/s
VY ION BULK VELOCITY, Y COMPONENT 0 - 7 km/s 0.4 km/s 0.4 km/s
VZ ION BULK VELOCITY, Z COMPONENT 0 - 7 km/s 0.4 km/s 0.4 km/s
F1I SATURATION ION CURRENT 0 - 1.3x10-4 A 1x10-12 A 1%
BKGI ION BACKGROUND CURRENT 0 - 1.3x10-4 A 1x1O-12 A 1%
VPI SPACECRAFT GROUND POTENTIAL -5 - +3 V - 0.1V
TOTE ELECTRON DENSITY 102 - 107 cm 3 - -
TE ELECTRON TEMPERATURE 300 - 20,000 K - 10%
F1E SATURATION ELECTRON CURRENT 0 - 1.3x10-4 A 1x10-12 A 1%
BKGE ELECTRON BACKGROUND CURRENT 0 - 1.3x10-4 A 1x10-12 A 1%
VPTE SPACECRAFT GROUND POTENTIAL -5 - +3V - 0.1V
N1 FIRST SUPRATHERMAL 0 - 107 cm-3 1 cm-3 20%
ELECTRON DENSITY
T1 FIRST SUPRATHERMAL 0 - 100 eV 0.2 eV 20%
ELECTRON TEMPERATURE
N2 SECOND SUPRATHERMAL 0 - 105 cm-3 1cm -3 20%
ELECTRON DENSITY
T2 SECOND SUPRATHERMAL 0 - 100 eV 0.2 eV 20%
ELECTRON TEMPERATURE
F1P SATURATION SUPRATHERMAL 0 - 1.3x10-4A 1x10-12A 1%
ELECTRON CURRENT
BKGP BACKGROUND SUPRATHERMAL 0 - 1.3x10-4A 1x10-12A 1%
ELECTRON CURRENT
VPPE SUPRATHERMAL ELECTRON 0 - +20V - 0.1 - 5V
SPACECRAFT POTENTIAL
UT: UT is the universal time in milliseconds assigned to the physical
quantities recorded in this record. UT will typically, but not always, lie
within plus or minus 6 seconds of the time of day assigned to the time tag of
this record. UT should be accurate to within plus or minus 0.1 second.
TOTI: TOTI is the total ion density of the plasma in cm-3 and is derived from
the FORTRAN expression
TOTI=FlI/(VN*e*Area)
where F1I is the first ion current measured with zero retarding potential, VN
is the component of ion bulk velocity parallel to the RPA axis derived from the
lst-squares analysis when an analysis was possible, e is the electronic charge,
and AREA is the effective area of the RPA collector (= 0.81 cm2). When a lst-
squares analysis is not possible, VN is the component of the spacecraft
velocity in ecliptic coordinates parallel to the RPA axis.
H+: H+ is the hydrogen ion density. When the RPA is operating in one of its
peaks mode, H+ will be detected and recorded only when its density is greater
than approximately 10% of the sum of more massive ion densities. H+ can be the
second most abundant ion and still not be recorded when the RPA is operating in
its two peaks mode. The uncertainty of the H+ density also depends on its
density relative to that of more massive ions. For an H+ density comparable to
that of more massive ions, the accuracy should be of the order of 10%. The
detection noise level for H+ is estimated at 300 cm-3. Additional discussion of
the RPA ion peak detection capability and limitation is given by Miller et al.
[1984].
O+: O+ is the oxygen ion density. It will be detected in the presence of more
massive ions only when its density is greater than approximately 10% of the sum
of more massive ions. The RPA does not resolve C+, N+, or 0+. We have assumed
in our least-squares fitting that (CC+] + CN+])/CO+] is constant at 0.07, a
value derived from PV ion mass spectrometer results.
M29+: M29+ is the symbol assigned to the sum density of ions with mass near 32
atomic mass units, CO+, NO+, N2+, O2+. The RPA does not resolve these masses.
In performing a least squares analysis, I have permitted the algorithm to
adjust the density of a mass 32 ion and a fictitious mass 29 ion in fitting the
measured DI peak corresponding to this mass range [Miller et al., 1984].
Measurements by the PV IMS have revealed that the density of NO+ can approach
been that although the median density of each of the two masses varies with
altitude in the expected way, on successive sweeps the least squares analysis
can assign all the density to mass 29 for one sweep and to mass 32 in the next.
For the PDS files, I have added the densities of the mass 29 and 32 ions and
entered them under the symbol M29+. RPA results as well as IMS results show
that the predominant ion mass in the group is 32 in most regions of the
ionosphere. In future submissions, the sum density of this mass group will be
submitted under the symbol m32+.
C02+: C02+ is the density of the carbon dioxide ion.
TI: TI is the ion temperature and is assumed to be the same for all ion masses.
It is one of the adjustable variables in the least-squares analysis of ion
sweeps.
VX: VX is the x component of ion bulk velocity. The vector ion bulk velocity is
derived from three component velocities parallel to the RPA axis measured in
three successive spin periods of the PV spacecraft [Knudsen et al., 1980]. In
deriving the vector, it is necessary to assume that the ion bulk velocity is
uniform over the region of space traversed by the spacecraft in two spin
revolutions of the spacecraft, a distance of about 250 km. The coordinate
system in which VX, VY, and VZ are given will be specified when data are
submitted.
VY: VY is the y component of the bulk ion velocity.
VZ: VZ is the Z component of the ion bulk velocity.
F1I: F1I is the saturation (first) current measured in an ion I-V sweep. The
retarding potential is programmed to be slightly negative of plasma potential
during this measurement. F1I is measured relative to the ion current measured
with the retarding potential equal to 37V positive [Knudsen et al. 1979].
BKGI: BKGI is the current to the RPA collector measured just before the
beginning of an ion sweep with the retarding potential set at approximately
+37V relative to plasma potential.
VPI: VPI is the value of the spacecraft potential relative to plasma potential
that is assumed to exist at the time of the ion sweep. The value is derived by
interpolating between values of the spacecraft potential measured in the
thermal
electron mode.
TOTE: TOTE is the total electron density derived from the thermal electron mode
saturation current F1E. The formula used for this present PDS submission, in
FORTRAN language, is:
TOTE = 6.15E9*MAX(0, -3.5E-9 -F1E)~0.847
We consider this measure of the total electron density to be approximate and
valid only while the PV spacecraft is within the ionosphere.
TE: TE is the thermal electron temperature derived using equation (1) in
Knudsen et al. [1980]. When the spacecraft is positive relative to plasma
potential r a condition existing with the spacecraft in the sun and in a low
density plasma the value of TE is representative of the secondary electrons
trapped in the positive spacecraft potential well.
F1E: F1E is the saturation electron current measured at the beginning of a
thermal electron mode sweep. The front (retarding) grids are at a potential of
+6.8 V relative to the spacecraft ground.
BKGE: BKGE is the current measured by the RPA electrometer at the beginning of
the thermal electron mode. The front (retarding) grids of the RPA are held at a
potential of -4.6 V during the measurement.
VPTE: VPTE is the spacecraft potential relative to the ambient plasma
potential. It is derived from the thermal electron sweep data as described BY
Knudsen et al. [1980]. When the spacecraft is in the solar wind and exposed to
the sun, its potential is typically a few volts positive with respect to the
solar wind plasma potential. VPTE loses its meaning in this situation.
N1: N1 is the density of the low temperature Maxwellian electron distribution
used to fit the suprathermal electron I-V curve [Knudsen et al., 1985].
T1: T1 is the temperature of the low temperature Maxwellian electron
distribution.
N2: N2 is the density of the high temperature Maxwellian electron distribution
used to fit the suprathermal electron I-V curve [Knudsen et al., 1985]
T2: T2 is the temperature of the high temperature Maxwellian electron
distribution.
F1P: F1P is the electron current measured by the RPA with zero retarding
potential on the retarding grid.
BKGP: BKGP is the electron current to the RPA with the retarding potential on
the retarding grid equal to -58V.
VPPE: VPPE is the spacecraft potential relative to the ambient plasma
potential. When the spacecraft is in the solar wind and not in the Venus umbra,
the spacecraft is positive, and the potential is inferred from the suprathermal
electron I-V curve. When the spacecraft is within the ionosphere or in the
Venus umbra, the potential is either estimated or taken from the potential
measured in the thermal electron mode.
Coordinate systems:
Non-rotating spin coordinate system (NRSC):
The roll angle of the roll reference object will be calculated in this
coordinate system as well as the roll angles of the Fs, RIP, RAM, and
NADIR signals. The non-rotating coordinate system (Wx, Wy, Wz) is
centered at the spacecraft center of mass. The Wz-axis is parallel to the
spacecraft spin axis. The Wx-Wy plane is perpendicular to the spacecraft
spin axis. The Wx-Wz plane includes the Vernal Equinox of reference.
Thus the Wx-axis is at the intersection of the plane perpendicular to
the spacecraft spin axis and the plane containing the spin axis and the
Vernal Equinox. Roll angles in this coordinate system are measured in the
Wx-Wy plane from the roll reference direction.
Inertial Cartesian Coordinate System - Ecliptic (ICC-ECLP)
The Ecliptic Inertial Cartesian Coordinate System is defined for
the reference epoch of 1950.0 The X-direction lies in the Ecliptic Plane
and is positive away from the reference body towards the Vernal Equinox
which is determined by the line of intersection between the mean Earth
equatorial plane and the ecliptic plane of reference. The Y direction is
measured outward from the center of the reference body, perpendicular to
and east of the the X-axis, and lying in the ecliptic plane of reference.
The Z direction is positive toward the north ecliptic pole of reference,
from the center of the reference body.
Some of the quantities contained in this submittal of RPA data to the PDS are
erroneous because of bad least-squares fits to the I-V curves. These bad fits
have not been detected by our current algorithms for reduction of the data and
have not been removed by a trained observer viewing the I-V curves and making
an educated judgment.