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Data Sets and Information
• data set : P10 JUPITER HVM PA SUMM MERGED 1 HOUR
  This directory contains the Pioneer 10 plasma (PA) and magnetic field (HVM)data from the Jupiter close encounter period between 1973-11-26T00:00:00and 1974-01-01T00:00:00. The data include 1 hour averages.
• data set : P10-J-PA-3-RDR-1HR-V1.0
  Pioneer 10 plasma analyzer 1 hour data.
• data set : P10-J-PA-4-SUMM-1HR-V1.0
  Pioneer 10 plasma analyzer summary data.
• data set : P10-J-PA-6-TRAJ-1HR-V1.0
  Pioneer 10 plasma analyzer trajectory data.
• data set : P11-J-HVM-PA-4-SUMM-MERGED-1HR-V1.0
  This dataset contains data merged frommultiple instruments from the Jupiter close encounter. Pioneer 11 plasma(PA) and magnetic field (HVM) data from 1974-11-03T00:00:00 to1975-01-01T00:00:00. The data include 1 hour averages.
• data set : P11-J-SW-PA-4-SUMM-1HR-V1.0
  Pioneer 11 Plasma Analyzer (PA) datafrom the Jupiter close encounter period between 1974-11-03T00:00:00 and1975-01-01T00:00:00. The data include 1 hour averaged data.
• data set : P11-J-SW-PA-6-TRAJ-1HR-V1.0
  Pioneer 11 plasma analyzer trajectory data.
• data set : P11-S-HVM-PA-4-MERGED-ENC-1HR-V1.0
  This directory contains the Pioneer 11 plasma (PLS) and magnetic field (HVM)data from the Saturn close encounter period between 1979-07-31T00:00:00and 1979-10-04T00:00:00. The data include 1 hour averages.
• data set : P11-S-SW-PA-4-SUMM-1HR-V1.0
  Pioneer 11 plasma analyzer summary data.
• data set : P11-S-SW-PA-6-TRAJ-1HR-V1.0
  Pioneer 11 plasma analyzer trajectory data.
• data set : PVO VENUS RETARD. POTENT. ANLYR. EDITED I/V CURVE (RDR) V1.0
  The Pioneer Venus Orbiter Retarding Potential Analyzer RDR data set is data reformatted to facilitate analysis using a Least Squares Fit (LSF) technique for determining plasma parameters.
• data set : PVO RPA PROC THERM ELEC, ION, PHOTOELEC, LOW RES. V1.0
  Pioneer Venus Orbiter (PVO) processeddata from the retarding potential analyzer (ORPA), including thermal andsuperthermal electrons, ions, and a key parameters file for all 5055 orbits(Dec 5, 1978 - Oct 7, 1992).
• instrument : ORBITER RETARDING POTENTIAL ANALYZER for PVO
  The Pioneer Venus Orbiter Retarding Potential Analyzer (ORPA) isa plane gridded, retarding potential device. It is designed tomeasure both ion and electron quantities in the 0-50eV energy rangein the Venusian ionosphere and solar wind. The PVO spacecrafthad an eccentric orbit with approximately a 24 hour period. For theinitial phase of the mission, periapsis was maintained at about 200 kmand apoapsis was at about 66,00 km. The low periapsis allowedsounding of the Venusian ionosphere while the apoapsis altitudeprovided an opportunity for observing the solar wind interaction. The quantities measured by the ORPA were electrontemperature* (Te), total ion concentration (Ni), individualion temperature Ti(j) of the most abundant species, theirconcentrations Ni(j), thermal ion drift velocity(approximately D), and energy distribution of suprathermalelectron and ion fluxes f(E) up to 50 eV. * The following conventions are used to create ASCII versions of equations, subscripts, etc.: Xe or Xi is the electron or ion value, respectively, of parameter XX(j) is the jth value of a sweep of parameter X^ indicates exponentiation ----------------------------------------------------------- Measured Plasma Quantities----------------------------------------------------------- Closest Distance Sample Between Distance Samples Range Uncert. Symbol Quantity (km) (1) (km) (2) (3) (4)===========================================================Ni = Ne Total Ion 1x10^-3 20x10^-3 ~10-10^7 ~5% concentration (cm^-3) Ni(j) Concentration 1.6 120 ~10^2-10^7 ~5% of up to 4 (j) (cm^-3) abundant ions Ti(j) Temperature of 1.6 120 100-10,000K ~5% the jth ion ~10^2<N-i<10^7 (cm^-3) Mi(j) Mass of the 1.6 120 1 - 56 ~1 jth ion (amu) (amu) D Ion drift 1.6, 500 0.05 - 5 ~50 velocity 240 (km/s) (m/s) fe Low-energy 0.5 120 1-50ev ~25% electron ~10^6-10^12 distribution (cm^-2s^-1sr^-1eV^-1) function Te Ionospheric 0.4 0.4, 300-25000K ~10% electron 120 ~10^2<N-e<10^7 temperature (cm^-3) Te Solar Wind 0.4 120 25000-5x10^5K ~25% electron N-e >= 0.5 temperature (cm^-3) Ne Solar Wind 0.3 120 ~0.5-10^2 ~30% Electron (cm^-3) Concentration============================================================(1) This is the distance traveled by the spacecraft at a velocity of 10 km/s during which the plasma is sampled.(2) This distance is dictated by the assigned bit rate.(3) These ranges depend in some instances on the values of other parameters such as ion composition.(4) These uncertainties apply to the higher concentration ranges. As the concentration drops toward the lower range value, the accuracies degrade. The assembled ORPA consists of the sensor head which is surrounded by alarge ground plane 30 cm in diameter and the necessary electronics. Theplanar sensor consists of a sequence of grids. The physical quantitiescan be derived for a plasma from the integral flux of ions or electrons,gathered by collector C, as a function of energy. Only particles whoseenergy is greater than the retarding voltage applied on the retardinggrid G-2 can strike the collector. Use of appropriate voltage programsallows analysis of the particle energy. The other grids and thecollector are biased with voltages, which separate positive and negativeparticles and minimize secondary electrons produced by particle impactand photons. The sensor is used to measure both electrons and ions by changing thesensor voltages. The voltage program can be subdivided into three basicmodes with different options selectable through ground command. Electron mode In the electron mode, the ion current to the collector is negligiblewith the collector at 47 V, A potential difference of 20 V between G-4and C was found sufficient to suppress most of the secondary electronsproduced by ion or electron impact at the collector. The three frontgrids G-0, G-1 and G-2 are the energy analyzing grids. They are steppedtogether from +6.8 to -4.2 V in the coarse scan. The correspondingcollector current is measured by an electrometer and then digitized. Thestraight line portion is the retarding region, and the logarithmic slopedetermines the electron temperature by the relation: e D VTe = - - ------------ (1) k D log (- Ie) where e is the electron charge; k, the Boltzmann constant; and Ie, theelectron current. The left side with larger positive voltage is theattractive region. The voltage Vp at which these two portions of thecurve join is the potential of plasma relative to spacecraft. Vp isexpected to vary from a few volts negative in interplanetary space to 1or 2 V positive in the Venusian ionosphere. For the simulation it wasset to zero. The lower portion of the curve bends away from thestraight-line portion because the velocity distribution is not a trueMaxwellian distribution. An additional population of suprathermalelectrons exists with higher energies than the thermal distribution. The retarding region of the characteristic curve is auto- maticallyrecognized by taking the measured differences of a logarithmicelectrometer. On-board logic selects the peak difference and 4 or 10adjacent values, depending on the commanded option. The retardingvoltage in the electron mode consists of a coarse scan over the entirevoltage range followed by one or three fine scans over the appropriatesubrange centered where the largest difference in the coarse mode hasoccurred. The retarding voltage at which the largest value of -deltalog (-Ie) occurs in the coarse electron scan is quite close to theplasma potential and is used as a reference potential for theretarding voltage in the ion mode. ------------------------------------------------------------ Control Voltage------------------------------------------------------------ Suprathermal Electron (a) Ion (a) Electron(a)Symbol Element Mode (V) Mode (V) Mode (V)============================================================ G0 Entrance Grid 6.8 - -4.2 0 or -4.6 0 G1 Ion Suppressor Grid 6.8 - -4.2 -0.1 to 36(b) 47 G2 Retarding Grid 6.8 - -4.2 -0.1 to 36(b) 0 - -50 G3 Displacement 47 -4.6 47 Current Shield G4 Electron Suppressor 27 -24.6 27 GridC, GR Collector, Guard 47 -4.6 47 Ring============================================================(a) Referenced to satellite ground except where otherwise indicated.(b) Referenced to plasma potential. ------------------------------------------------------------ Retarding Potential Programs------------------------------------------------------------ Number Step Size Voltage Range(a)Mode Scan of steps (V) (V)============================================================ E Electron, coarse 64 -0.176 6.8 - -4.6 Electron, fine 20 -0.044(c) 0.88(c) I Ion 80 J x 0.011(d) -0.1 to 36(b) S Suprathermal 48 J x 0.044(d) 0 - -50 Electron============================================================(a) Referenced to satellite ground except where otherwise indicated.(b) Referenced to plasma potential.(c) Subdivides 5 coarse steps.(d) The step size is proportional to J. ION MODE The basic principles of ion measurements with an RPA aredescribed by W.C. Knudsen (1966). The ion current given by I = A * Tr * e * v * cos(alpha) * SIGMA(j) (2) where -- exp(- K(j)^2)SIGMA(j) = > (Ni(j) [ 0.5 + 0.5 * erf(K(j)) + ------------------- -- 2 * sqrt(pi * Bi) j K(j) = B(j) - sqrt( e * s^2 / (k * Ti(j)) ) v * cos(alpha) B(j) = ----------------- sqrt(2 * k * Ti(j) / Mi(j)) s = V - Vi where e is the electron charge (1.6X10^-19 Coulomb),v is theion velocity relative to the satellite, alpha is the angleof attack, Tr is the grid transparency, A is the grid areaand V is the retarding voltage. The array index j denotesthe ion species and the subscript i denotes that thequantities refer to ions. The expression for dI/ds is dI -- Ni(j) s -e -- = -c > -------------- * --- * exp[-------- * (s - s(j))^2] (3) ds -- sqrt(k * Ti(j)) si k * Ti(j) j where Mi(j) * v^2 * cos^2(alpha) s(j) = sqrt[ -------------------------- ] 2 * e and A * Tr * e^1.5 * v * cos(alpha) c = ----------------------------------- sqrt(pi) The current derivative with respect to the square root ofthe effective retarding potential s is the sum of individualion terms. Each term contains a Gaussian factor centeredon the value s(j), which is determined by the ion mass.The half width of the Gaussian, sqrt( kTi(j)/e), isproportional to the square root of the ion temperatureand the peak value is proportional to Ni(j)/sqrt(Ti(j)). The 80 retarding potential step height increments in theORPA ion mode vary according to the relation delta V = V(j-1) - V(j) = J * 0.011V J = 1, 79, and the beginning value Vi(j) is started at Vref. Thus, J * (J - 1) 0.011 Vi - Vref = ---------- * 0.011 ~ J^2 * ------, J > 10 (4) 2 2 The difference in collector current between adjacentretarding potential steps becomes delta I(j) = I * [ V(j-1) - I * V(j)] 0.011 ~ - sqrt( -----) * c * SIGMA(j) (5) J(j) where -- Ni(j) J -0.011 * e SIGMA(j) = > ------------ * --- * exp[ ---------- * (J-J(j))^2] -- sqrt(k * Ti(j)) J(j) 2k * Ti(j) j and Mi(j) * v^2 * cos^2(alpha) J(j) = sqrt( --------------------------- ) 0.011 * e Depending on the commanded mode of operations, either I(j) values or -delta I(j) values are selected and stored in temporary storage registersfor subsequent readout to the spacecraft. The -delta I(j) values areseen to consist of well separated Gaussian peaks and, as expected fromEquation (5) the values of -delta Ii(j) are approximately proportionalto the concentrations Ni(j). The computer simulation is described insomewhat more detail hereafter. In what we anticipate to be the mostcommon mode of operations, the ORPA program logic recognizes and selectsa maximum of two sets of -delta I values corresponding to the ionmasses. Each set consists of a peak value and the five adjacent valueson either side. In an optimal peak mode of operation, the ORPA selects up to four setsof -delta I values. Each set consists of the peak value and twoalternate values of -delta I on either side. If more that four (two)peaks are sensed, the logic retains the largest peak and three (one)additional peaks. Ion temperatures, masses, concentrations, v cos alpha,and plasma potential are deduced from the sets of -delta I values byfitting the theoretical values for -delta I(j) formed from Expression(2) to the measured values. The approximation, Equation (5), is notused. In addition to the peak values of -delta I, the first currentvalue I(1) is stored in all modes and permits computation of the totalion concentration given by I(1) Ni = --------------------------- (6) A * Tr * e * v * cos(alpha) The technique of taking differences in log Ie or Ii in the electron andion mode, respectively, and selecting values only around the sensedpeaks reduces the number of telemetry words needed to define a scanwithout losing much scan information. The finest possible spatialresolution of plasma quantities is obtained with a limited bit rate.For special investigations, 48 consecutive or alternate values of theelectron or ion characteristic curve (log Ie or Ii) may be measured andtransmitted with consequent reduction in spatial resolution. PHOTOELECTRON MODE The retarding potential varies quadratically through the range 0 to -50V. The photoelectron energy distribution function fe is derived from aset of 25 or 49 values of Ie(j) depending on commands. MEASUREMENT SEQUENCE The basic measurement sequence of the ORPA is EIIIP where E, I and Prepresent electron, ion, and photoelectron modes, respectively. Theinstrument remains in each mode for one complete spacecraft spin cyclebefore executing the next mode. The complete sequence takes five spincycles. An individual retarding potential scan is completed as rapidlyas possible consistent with required measurement accuracy so that thescan will closely represent a 'point' measurement. The scan togetherwith other activities, including zero adjustment, background currentadjustment, and background current measurement, typically requires only0.3s. Thus, approximately 40 scans can be completed in one spacecraftspin cycle. The telemetry rate assigned to the ORPA is sufficient foronly one of these scans to be telemetered to earth for each spin cycle.In one command option, the ORPA selects that scan which has the largestfirst current value. This scan will have been recorded when the angleof attack was smallest (Equation (2)) when the ORPA is in the I mode,and close to optimum orientation in the E and P modes. Alternatively,in a second command option, the ORPA selects the first scan that occursafter receipt of a ram pulse from the spacecraft. The space- velocityvector lies in or close to the plane defined by the spacecraft spin axisand the centerline to the ORPA at the time of receipt of this pulse.Three ion scans at three different directions are required to determinethe ion drift velocity. To provide the second and third directions, theion scans in the second and third ion spin cycles are selected 45 degreein roll angle before and after the first scan. By command, the basicmeasurement sequence described above may be altered by skipping over oneor more of the modes in the sequence. SENSOR SUBASSEMBLY The sensor has seven grids of large diameter (6 cm diameter) coveringnearly the whole sensor front (8 cm diameter). The large entrance gridand the small collector plate (1.9 cm diameter) surrounded by a guardring provide for a radially uniform particle flux around the sensoraxis. The collector samples either electrons or ions from the uniformcentral region. Disturbing field effects at the grid edges are thusavoided. The first two grids are connected together to reduce theelectric field produced outside the sensor by the stepping retardingpotential in the ion mode. Any change in spacecraft potential producedby the ORPA is thus minimized. The retarding grid G-2 is also a doublegrid which provides a reasonably uniform retarding voltage in thephotoelectron mode. To satisfy the need for a more uniform potential inthe ion and electron modes additional grids are biased with the sameretarding voltage during these modes. The grids G-0, G-1, G-2 are coatedwith Aquadag, a graphite emulsion, to achieve a uniform surfacepotential. All other conduction sensor parts are gold plated. The gridmaterial of G-0, G-1, G-2 consists of electro-formed copper- berylliumwith about 30 lines/cm and 82 percent transparency. The distance betweenindividual grids is approximately 0.3 cm. Electrical connection to thegrids is provided by two redundant connectors. This design provides foreasy grid exchange without soldering from the front side of the sensor.To provide a plane plasma sheath, the ORPA is surrounded by an Aquadagcoated aluminum ground plane 20 cm in diameter. It is thermallyinsulated from the experiment to reduce the heat flux to both theexperiment and spacecraft but is electrically connected to spacecraftground. With the sensor axis at 25 degrees from the spacecraft spinaxis, the angle the sensor axis makes with the spacecraft velocityvector will be zero or small during a portion of each roll period whenthe spacecraft is within the ionosphere. For ion measurements, it isimportant that the angle of attack be small, otherwise the assumptionsused in deriving Equation (2) are progressively violated. The angle ofattack must not change too much during one scan to avoid ambiguousresults. With our 160-ms scan time, the angle of attack changes onlyapproximately 5 degrees. To estimate and correct for systematic errorsarising from nonuniform retarding potential in the plane of theretarding grids, the average potential must be known. The difference inpotential between the applied potential and the mean potential has beenarrived at through capacity measurements. For the ion mode, the meanpotential differs from that applied by less than one part in a thousand: delta Vr = (-2 - 0.8Vr) * 10^-3 * V (7) The highest accuracy is required in this mode. For the photoelectron mode: delta Vr = (380 - 8 * Vr) * 10^-3 * V (8) The deviation in the electron mode is still smaller thanthat given for the ion mode because of the five grids. Thegrids are coated with Aquadag to minimize work functionvariation in the plane of the grids. The one sigmavariation from the mean is approximately 5 mV.Equations (7) and (8) do not include the work functionvariation. CURRENT AMPLIFIER The collector current of either polarity isamplified by a linear electrometer. The electrometer hasa sensitivity of approximately 6 X 10-13 to 1.3 X 10-4 Ain eight overlapping ranges. The absolute accuracy within 1 year of aging was about 1percent. A zero adjustment in the input of the electro-meter after every scan provides for long-term stability.The collector is disconnected from the electrometer duringthis adjustment and the differential input is controlledby the output signal. Additionally, the gain of theelectrometer can be checked internally by a calibrationcommand. Five different current values are produced byhigh impedance resistors and a well-regulatedvoltage supply. The largest feedback resistor is a 309megaohm metal oxide resistor with 100 ppm temperaturevariation. This means that a temperature correction inthe expected temperature range from 0 to 60 degree C is notnecessary even in the most sensitive range. The amplifiertime constant is 0.4 ms in all ranges. The linear outputvoltage is rectified to get only positive signalsbetween 0 and 10 V for currents of positive and negativepolarity. The sign of the current is provided by anadditional logic signal. The linear output voltage V-lin isrelated to the input current by the formula V-lin * 8^(L - 3) I = --------------------- * sign (9) R where R is the feedback resistor (309 megaohm) and Lis one of the automatically adjusted ranges between0 and 7. In the ion mode, the linear output voltage Vhn isdigitized. In the electron mode, a logarithmicscale has to be used and therefore a logarithmic post-amplifier converts the linear signal to a logarithmvoltage where 10 V and 1.25 V linear are related to 7.5 Vand 5 V, respectively, in a logarithmic scale. Theelectrometer and directly related circuits such ascalibration generator and background compensation are builtin a shielding box. The electrometer potential depends onthe mode and therefore all circuits connected with theelectrometer and the shielding case are insulated from thenormal signal ground and biased with the collector voltage. A background compensation circuit is incorporated in thesensor electronics to provide the full dynamic range andaccuracy of the instrument available in the mostsensitive range even when a constant background currentis superposed on the signal current. The retardingpotential at its maximum retarding value before everyretarding scan, the background current produced by highlyenergetic particles or by secondary electrons in the sensoris compensated to a very low value. The compensationcircuits add current of the opposite polarity to the inputof the electrometer until the output voltage is less than0.5 V in the most sensitive range. The compensationcurrent is held fixed for the duration of the followingscan. Currents from approximately 10^-8 to 3 X 10^-12 Acan be compensated with a stability of about 3 X 10^-13 A,which is in the overall electrometer noise. The outputnoise (standard deviation) of the complete systemcorresponds to a current below 10^-12 A or 0.02 %,whichever is higher. Some attention was given to minimizing the microphonicsensitivity of the sensor since the spacecraftcontains a despun antenna and several instrumentshave stepping mechanisms. To this end the G-4 grid wasgiven a very high tension to reduce the amplitude ofany possible oscillation. Further, all connections tothe electrometer input were quite rigid and short andthe electronic board was fastened with some dampingmaterial. The microphonic noise was below 5 X 10^-12 Afor a sinusoidal 0.1-g acceleration at all frequencies. CONTROL VOLTAGES The voltages are supplied by low impedance amplifiers which arecontrolled by a 12 bit digital-to-analog (D/A) converter. The retardingpotential steps are generated by appropriate digital values transferredto the D/A converter and appropriately amplified. The time intervalbetween successive steps is 2 ms when no range change occurs. Everyrange change increases the step length by 2 ms. The noise ripple on theretarding potentials in the pass- band of the electrometer has beenmeasured at 0.8 mV peak to peak (=4 sigma). In the electron mode, theretarding potential uncertainty is 1 mV or 0.1 percent, whichever islarger. In the ion mode the uncertainty is 5 mV or 0.1 percent,whichever is higher. DATA PROCESSING Depending on mode, the linear or logarithmic amplifier output isdigitized during the last 100 microseconds before the next retardingpotential is applied. A 12-bit analog-to-digital (A/D) converter is usedbut only the 10 most significant bits are retained. The 10-bit mantissaand a 3-bit range value formed for each measurement are shifted into amemory during the photoelectron mode and also during the electron andion modes when the I-V option is exercised. At the completion of theroll period, the memory is normally enabled for read- out within thenext spin period. In the 'peak select' mode of operation, each currentvalue is subtracted from the preceding value, and the difference -deltaI, together with the range information, is shifted into a bufferregister. Second differences delta(delta I) are also formed and used torecognize when a peak in -delta I has occurred. The criteria used are: delta I(j-2) , delta I(j-1) , delta I(j) , delta I(j+1) < 0 delta delta I(j-2) , delta delta I(j-1) , delta delta I(j) < 0 delta delta I(j+1) >= 0. These criteria reject noise peaks but permit detection of a genuinepeak. When the criteria are satisfied, either 4 alternate or 10adjacent values around a peak together with the peak value and retardingpotential step information are transferred into memory for latertransfer to the spacecraft. In addition to the differences -delta I, the first measured current I-1 of a scan is stored. It is used by instrument logic to recognizeand return the scan with the smallest angle of attack measured duringa spin revolution. COMMAND SYSTEM The ORPA can be commanded from earth into several mode sequences. Thenormal sequence starts with an electron mode followed by three ion modesand one photoelectron mode. The photoelectron mode can be removed toincrease the spatial resolution of the other measured quantities.Alternately, a photoelectron-only mode can be commanded withmeasurements spaced at 90 degrees in roll angle to investigate theangular distribution. An electron-only mode may also be commanded forhigh resolution of thermal electron parameters. In both the ion and electron modes, one may choose either the peak-select option in which -delta I values are stored or the I-V option inwhich I values are stored. The spatial resolution is typically reducedby one-third in the latter option. With the peak-select option, it ispossible to choose 4 peaks with 5 -delta I values each or 2 peaks with11 -delta I values, as previously explained. In a special mode of operation, with the control voltages set to measureions and the retarding potential fixed at its lowest step, the ioncurrent is sampled at 2-ms intervals until the memory is filled. Thesaturation ion current thus measured with high time resolution.Irregularities in total ion density are measured at distance intervalsof 20 m. Beginning with the next roll period the total ionconcentration is sampled at approx- imately 4-km intervals until thememory is filled. The voltage of the entrance grid may be commanded to 0 or -4.6 Vrelative to spacecraft ground when the ORPA is in the ion mode. Thisoption will permit measurement of low-mass, low-energy ions should thespacecraft become slightly positive relative to the plasma. THE CALIBRARTION SEQUENCE When the ORPA is commanded into calibration the experiment cyclesthrough two calibration sequences of EIIIP and then returns to thenormal measuring sequence. In the first calibration sequence theelectrometer is disconnected from the collector. In the electron mode asequence of internal calibration currents satisfying the peak criteriaare produced to permit an evaluation of electrometer sensi- tivity andpeak selection logic. In the ion mode the internal noise is measured inthe most sensitive range. In the second sequence the retarding voltageis sampled to verify amplifier gain and proper logic operation. ------------------------------------------------------------ Instrument Parameter Summary------------------------------------------------------------Parameter Value============================================================Current Range <A> 10^-4 - 10^-12Current Accuracy <percent> 98 (10 bit A/D)Sampling Interval <s> 0.002Retarding Voltage Uncert. <percent> 0.1Bit Rate <bps> 40 or lessCommands 6Power <W> 2.4Weight <kg> 2.9Volume <cm^3> 4440============================================================ References Knudsen, W. C., Evaluation and Demonstration of the useof Retarding Potential Analyzers for Measuring SeveralIonospheric Quantities, J. Geophys. Res., 71, 4669, 1966. Knudsen, W. C., J. Bakke, K. Spenner, and V. Novak, RetardingPotential Analyzer for the Pioneer Venus Orbiter, Space Sci. Inst.,4, 351, 1979. Knudsen, W. C., K. Spenner, J. Bakke, and V. Novak, Pioneer VenusOrbiter Planar Retarding Potential Analyzer Plasma Experiment, IEEETrans. on Geoscience and Remote Sensing, 18, 1, 60-65, 1980.