urn:nasa:pds:context:instrument:pvo.orpa 1.0 1.7.0.0 Product_Context 2020-08-26 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.pvo::1.0 instrument_to_instrument_host Knudsen W.C. , Evaluation and demonstation of the use of retarding potential analyzers for measuring several ionospheric quantities, J. Geophys. Res., vol. 71, pp. 4669-4678, 1966. reference.KNUDSENETAL1966 Knudsen, W.C., J. Bakke, K. Spenner, and V. Novak, Retarding Potential Analyzer for the Pioneer Venus Orbiter, Space Sci. Inst., 4, 351, 1979. reference.KNUDSENETAL1979 Knudsen, W.C., K. Spenner, J. Bakke, and V. Novak, Pioneer Venus Orbiter Planar Retarding Potential Analyzer Plasma Experiment, IEEE Trans. on Geosci. Remote Sens., 18, 1, 60, 1980. reference.KNUDSENETAL1980 ORBITER RETARDING POTENTIAL ANALYZER Particle Detector not applicable not applicable The Pioneer Venus Orbiter Retarding Potential Analyzer (ORPA) is a plane gridded, retarding potential device. It is designed to measure both ion and electron quantities in the 0-50eV energy range in the Venusian ionosphere and solar wind. The PVO spacecraft had an eccentric orbit with approximately a 24 hour period. For the initial phase of the mission, periapsis was maintained at about 200 km and apoapsis was at about 66,00 km. The low periapsis allowed sounding of the Venusian ionosphere while the apoapsis altitude provided an opportunity for observing the solar wind interaction. The quantities measured by the ORPA were electron temperature* (Te), total ion concentration (Ni), individual ion temperature Ti(j) of the most abundant species, their concentrations Ni(j), thermal ion drift velocity (approximately D), and energy distribution of suprathermal electron 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 X X(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 a large ground plane 30 cm in diameter and the necessary electronics. The planar sensor consists of a sequence of grids. The physical quantities can be derived for a plasma from the integral flux of ions or electrons, gathered by collector C, as a function of energy. Only particles whose energy is greater than the retarding voltage applied on the retarding grid G-2 can strike the collector. Use of appropriate voltage programs allows analysis of the particle energy. The other grids and the collector are biased with voltages, which separate positive and negative particles and minimize secondary electrons produced by particle impact and photons. The sensor is used to measure both electrons and ions by changing the sensor voltages. The voltage program can be subdivided into three basic modes with different options selectable through ground command. Electron mode 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 D V Te = - - ------------ (1) k D 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. The retarding region of the characteristic curve is auto- matically recognized by taking the measured differences of a logarithmic electrometer. On-board logic selects the peak difference and 4 or 10 adjacent values, depending on the commanded option. The retarding voltage in the electron mode consists of a coarse scan over the entire voltage range followed by one or three fine scans over the appropriate subrange centered where the largest difference in the coarse mode has occurred. The retarding voltage at which the largest value of -delta log (-Ie) occurs in the coarse electron scan is quite close to the plasma potential and is used as a reference potential for the retarding 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 Grid C, 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 are described 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 the ion velocity relative to the satellite, alpha is the angle of attack, Tr is the grid transparency, A is the grid area and V is the retarding voltage. The array index j denotes the ion species and the subscript i denotes that the quantities 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 of the effective retarding potential s is the sum of individual ion terms. Each term contains a Gaussian factor centered on the value s(j), which is determined by the ion mass. The half width of the Gaussian, sqrt( kTi(j)/e), is proportional to the square root of the ion temperature and the peak value is proportional to Ni(j)/sqrt(Ti(j)). The 80 retarding potential step height increments in the ORPA 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 adjacent retarding 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 registers for subsequent readout to the spacecraft. The -delta I(j) values are seen to consist of well separated Gaussian peaks and, as expected from Equation (5) the values of -delta Ii(j) are approximately proportional to the concentrations Ni(j). The computer simulation is described in somewhat more detail hereafter. In what we anticipate to be the most common mode of operations, the ORPA program logic recognizes and selects a maximum of two sets of -delta I values corresponding to the ion masses. Each set consists of a peak value and the five adjacent values on either side. In an optimal peak mode of operation, the ORPA selects up to four sets of -delta I values. Each set consists of the peak value and two alternate 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 by fitting the theoretical values for -delta I(j) formed from Expression (2) to the measured values. The approximation, Equation (5), is not used. In addition to the peak values of -delta I, the first current value I(1) is stored in all modes and permits computation of the total ion 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 and ion mode, respectively, and selecting values only around the sensed peaks reduces the number of telemetry words needed to define a scan without losing much scan information. The finest possible spatial resolution of plasma quantities is obtained with a limited bit rate. For special investigations, 48 consecutive or alternate values of the electron or ion characteristic curve (log Ie or Ii) may be measured and transmitted with consequent reduction in spatial resolution. PHOTOELECTRON MODE The retarding potential varies quadratically through the range 0 to -50 V. The photoelectron energy distribution function fe is derived from a set 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 P represent electron, ion, and photoelectron modes, respectively. The instrument remains in each mode for one complete spacecraft spin cycle before executing the next mode. The complete sequence takes five spin cycles. An individual retarding potential scan is completed as rapidly as possible consistent with required measurement accuracy so that the scan will closely represent a 'point' measurement. The scan together with other activities, including zero adjustment, background current adjustment, and background current measurement, typically requires only 0.3s. Thus, approximately 40 scans can be completed in one spacecraft spin cycle. The telemetry rate assigned to the ORPA is sufficient for only 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 largest first current value. This scan will have been recorded when the angle of 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 occurs after receipt of a ram pulse from the spacecraft. The space- velocity vector lies in or close to the plane defined by the spacecraft spin axis and the centerline to the ORPA at the time of receipt of this pulse. Three ion scans at three different directions are required to determine the ion drift velocity. To provide the second and third directions, the ion scans in the second and third ion spin cycles are selected 45 degree in roll angle before and after the first scan. By command, the basic measurement sequence described above may be altered by skipping over one or more of the modes in the sequence. SENSOR SUBASSEMBLY The sensor has seven grids of large diameter (6 cm diameter) covering nearly the whole sensor front (8 cm diameter). The large entrance grid and the small collector plate (1.9 cm diameter) surrounded by a guard ring provide for a radially uniform particle flux around the sensor axis. The collector samples either electrons or ions from the uniform central region. Disturbing field effects at the grid edges are thus avoided. The first two grids are connected together to reduce the electric field produced outside the sensor by the stepping retarding potential in the ion mode. Any change in spacecraft potential produced by the ORPA is thus minimized. The retarding grid G-2 is also a double grid which provides a reasonably uniform retarding voltage in the photoelectron mode. To satisfy the need for a more uniform potential in the ion and electron modes additional grids are biased with the same retarding voltage during these modes. The grids G-0, G-1, G-2 are coated with Aquadag, a graphite emulsion, to achieve a uniform surface potential. All other conduction sensor parts are gold plated. The grid material of G-0, G-1, G-2 consists of electro-formed copper- beryllium with about 30 lines/cm and 82 percent transparency. The distance between individual grids is approximately 0.3 cm. Electrical connection to the grids is provided by two redundant connectors. This design provides for easy grid exchange without soldering from the front side of the sensor. To provide a plane plasma sheath, the ORPA is surrounded by an Aquadag coated aluminum ground plane 20 cm in diameter. It is thermally insulated from the experiment to reduce the heat flux to both the experiment and spacecraft but is electrically connected to spacecraft ground. With the sensor axis at 25 degrees from the spacecraft spin axis, the angle the sensor axis makes with the spacecraft velocity vector will be zero or small during a portion of each roll period when the spacecraft is within the ionosphere. For ion measurements, it is important that the angle of attack be small, otherwise the assumptions used in deriving Equation (2) are progressively violated. The angle of attack must not change too much during one scan to avoid ambiguous results. With our 160-ms scan time, the angle of attack changes only approximately 5 degrees. To estimate and correct for systematic errors arising from nonuniform retarding potential in the plane of the retarding grids, the average potential must be known. The difference in potential between the applied potential and the mean potential has been arrived at through capacity measurements. For the ion mode, the mean potential 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 than that given for the ion mode because of the five grids. The grids are coated with Aquadag to minimize work function variation in the plane of the grids. The one sigma variation from the mean is approximately 5 mV. Equations (7) and (8) do not include the work function variation. CURRENT AMPLIFIER The collector current of either polarity is amplified by a linear electrometer. The electrometer has a sensitivity of approximately 6 X 10-13 to 1.3 X 10-4 A in eight overlapping ranges. The absolute accuracy within 1 year of aging was about 1 percent. 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 during this adjustment and the differential input is controlled by the output signal. Additionally, the gain of the electrometer can be checked internally by a calibration command. Five different current values are produced by high impedance resistors and a well-regulated voltage supply. The largest feedback resistor is a 309 megaohm metal oxide resistor with 100 ppm temperature variation. This means that a temperature correction in the expected temperature range from 0 to 60 degree C is not necessary even in the most sensitive range. The amplifier time constant is 0.4 ms in all ranges. The linear output voltage is rectified to get only positive signals between 0 and 10 V for currents of positive and negative polarity. The sign of the current is provided by an additional logic signal. The linear output voltage V-lin is related 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 L is one of the automatically adjusted ranges between 0 and 7. In the ion mode, the linear output voltage Vhn is digitized. In the electron mode, a logarithmic scale has to be used and therefore a logarithmic post- amplifier converts the linear signal to a logarithm voltage where 10 V and 1.25 V linear are related to 7.5 V and 5 V, respectively, in a logarithmic scale. The electrometer and directly related circuits such as calibration generator and background compensation are built in a shielding box. The electrometer potential depends on the mode and therefore all circuits connected with the electrometer and the shielding case are insulated from the normal signal ground and biased with the collector voltage. A background compensation circuit is incorporated in the sensor electronics to provide the full dynamic range and accuracy of the instrument available in the most sensitive range even when a constant background current is superposed on the signal current. The retarding potential at its maximum retarding value before every retarding scan, the background current produced by highly energetic particles or by secondary electrons in the sensor is compensated to a very low value. The compensation circuits add current of the opposite polarity to the input of the electrometer until the output voltage is less than 0.5 V in the most sensitive range. The compensation current is held fixed for the duration of the following scan. Currents from approximately 10^-8 to 3 X 10^-12 A can be compensated with a stability of about 3 X 10^-13 A, which is in the overall electrometer noise. The output noise (standard deviation) of the complete system corresponds to a current below 10^-12 A or 0.02 %, whichever is higher. Some attention was given to minimizing the microphonic sensitivity of the sensor since the spacecraft contains a despun antenna and several instruments have stepping mechanisms. To this end the G-4 grid was given a very high tension to reduce the amplitude of any possible oscillation. Further, all connections to the electrometer input were quite rigid and short and the electronic board was fastened with some damping material. The microphonic noise was below 5 X 10^-12 A for a sinusoidal 0.1-g acceleration at all frequencies. CONTROL VOLTAGES The voltages are supplied by low impedance amplifiers which are controlled by a 12 bit digital-to-analog (D/A) converter. The retarding potential steps are generated by appropriate digital values transferred to the D/A converter and appropriately amplified. The time interval between successive steps is 2 ms when no range change occurs. Every range change increases the step length by 2 ms. The noise ripple on the retarding potentials in the pass- band of the electrometer has been measured at 0.8 mV peak to peak (=4 sigma). In the electron mode, the retarding potential uncertainty is 1 mV or 0.1 percent, whichever is larger. 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 is digitized during the last 100 microseconds before the next retarding potential is applied. A 12-bit analog-to-digital (A/D) converter is used but only the 10 most significant bits are retained. The 10-bit mantissa and a 3-bit range value formed for each measurement are shifted into a memory during the photoelectron mode and also during the electron and ion modes when the I-V option is exercised. At the completion of the roll period, the memory is normally enabled for read- out within the next spin period. In the 'peak select' mode of operation, each current value is subtracted from the preceding value, and the difference -delta I, together with the range information, is shifted into a buffer register. Second differences delta(delta I) are also formed and used to recognize 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 genuine peak. When the criteria are satisfied, either 4 alternate or 10 adjacent values around a peak together with the peak value and retarding potential step information are transferred into memory for later transfer 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 recognize and return the scan with the smallest angle of attack measured during a spin revolution. COMMAND SYSTEM The ORPA can be commanded from earth into several mode sequences. The normal sequence starts with an electron mode followed by three ion modes and one photoelectron mode. The photoelectron mode can be removed to increase the spatial resolution of the other measured quantities. Alternately, a photoelectron-only mode can be commanded with measurements spaced at 90 degrees in roll angle to investigate the angular distribution. An electron-only mode may also be commanded for high 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 in which I values are stored. The spatial resolution is typically reduced by one-third in the latter option. With the peak-select option, it is possible to choose 4 peaks with 5 -delta I values each or 2 peaks with 11 -delta I values, as previously explained. In a special mode of operation, with the control voltages set to measure ions and the retarding potential fixed at its lowest step, the ion current is sampled at 2-ms intervals until the memory is filled. The saturation ion current thus measured with high time resolution. Irregularities in total ion density are measured at distance intervals of 20 m. Beginning with the next roll period the total ion concentration is sampled at approx- imately 4-km intervals until the memory is filled. The voltage of the entrance grid may be commanded to 0 or -4.6 V relative to spacecraft ground when the ORPA is in the ion mode. This option will permit measurement of low-mass, low-energy ions should the spacecraft become slightly positive relative to the plasma. THE CALIBRARTION SEQUENCE When the ORPA is commanded into calibration the experiment cycles through two calibration sequences of EIIIP and then returns to the normal measuring sequence. In the first calibration sequence the electrometer is disconnected from the collector. In the electron mode a sequence of internal calibration currents satisfying the peak criteria are produced to permit an evaluation of electrometer sensi- tivity and peak selection logic. In the ion mode the internal noise is measured in the most sensitive range. In the second sequence the retarding voltage is sampled to verify amplifier gain and proper logic operation. ------------------------------------------------------------ Instrument Parameter Summary ------------------------------------------------------------ Parameter Value ============================================================ Current Range <A> 10^-4 - 10^-12 Current Accuracy <percent> 98 (10 bit A/D) Sampling Interval <s> 0.002 Retarding Voltage Uncert. <percent> 0.1 Bit Rate <bps> 40 or less Commands 6 Power <W> 2.4 Weight <kg> 2.9 Volume <cm^3> 4440 ============================================================ References Knudsen, W. C., Evaluation and Demonstration of the use of Retarding Potential Analyzers for Measuring Several Ionospheric Quantities, J. Geophys. Res., 71, 4669, 1966. Knudsen, W. C., J. Bakke, K. Spenner, and V. Novak, Retarding Potential Analyzer for the Pioneer Venus Orbiter, Space Sci. Inst., 4, 351, 1979. Knudsen, W. C., K. Spenner, J. Bakke, and V. Novak, Pioneer Venus Orbiter Planar Retarding Potential Analyzer Plasma Experiment, IEEE Trans. on Geoscience and Remote Sensing, 18, 1, 60-65, 1980.