urn:nasa:pds:context:instrument:pvo.orpa 2.0 1.20.0.0 Product_Context 2020-08-26 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model 2024-04-02 2.0 Updated to IM 1.20.0.0 urn:nasa:pds:context:instrument_host:spacecraft.pvo 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 (ORPA) for PIONEER VENUS Langmuir Probe not applicable not applicable INSTRUMENT OVERVIEW =================== The Retarding Potential Analyzer (ORPA) investigation used a Langmuir-probe retarding-potential analyzer designed to measure electron concentration and temperature, major ion concentrations, temperatures, and masses, ion drift velocities, and the energy distribution function of ambient photoelectrons in the ionosphere. It was an adaptation of the instrument flown on the German Aeros satellite in 1972. Either one of two sensor heads could be used, each consisting of a multigrid cup and electrometer, which could operate in electron, ion, or photoelectron modes, initiated by spacecraft roll pulses. The aims of the investigation were to improve knowledge of the important ionic reactions in the Venusian ionosphere, to study the plasma transport processes to determine if Venus has a polar wind, to study the processes at the solar wind-ionosphere boundary, and to study similar aims concerning the ambient electron population. Although the instrument was designed to detect low-energy plasma particles in the ionosphere, it could also make measurements of the interaction between the ionosphere and solar wind at altitudes up to 500 km. The instrument had a mass of 2.8 kg and used 2.4 W power. The ORPA axis is offset from the spacecraft spin axis so the ORPA axis could be close in alignment to the spacecraft velocity vector near periapsis. Large entrance grids and a collector guard ring provide a uniform flux radially from the instrument axis, the uniform central region of the flux is sampled by the collectors. A 30 cm diameter ground plane surrounds the entrance grid. The instrument used multiple collector grids coated with colloidal graphite to selectively funnel various ionospheric particles to strike a detector. The grids were 6 cm in diameter spaced over 1.6 cm, and included an entrance grid, ion suppressor grid, ion retarding grid, displacement current shield, electron suppressor grid, and a collector connected to an electrometer which amplified the current induced in the detector. The grids could be set with varying electric potentials, or control voltages, depending on the mode of operation and the quantity being measured. The instrument could operate in three modes: Langmuir probe mode, ion mode, or photoelectron mode. Langmuir probe mode had a linear electron coarse scan of 64 steps of roughly 0.2 volts/step, covering -4.8 to 7.8 V. It also had a linear electron fine scan of 20 steps of 0.05 V subdividing 5 of the coarse steps. Ion mode used 80 quadratic steps over -0.5 to 39 V referenced to the plasma potential. The photoelectron mode had 25 quadratic steps covering 0 to -60 V. Scans were made at roughly 120 km (~12 sec) intervals while the orbital path was within the ionosphere, allowing for approximately 50 measurements below the ionopause on the dayside. Each scan took a small fraction of the spin period, (0.04 to 0.16 seconds), with on-board computing determining which scan to transmit based on being taken at the optimal point in the spacecraft's rotation when the sensor axis was closest to the plasma flow velocity vector (peak select). It also had an I-V mode were every other current value in the electron mode and in the ion mode were stored for transmission every third spacecraft rotation. Concentrations (from 100 - 10,000,000 per cubic cm), temperatures (300 - 10,000 K), and masses (1-56 AMU) of the four most abundant ions were measured every 12 seconds for 0.16 seconds per measurement. Ion drift velocity (from 0.05 to 5 km/s) were measured for 0.16 seconds or 24 seconds every 50 seconds. The low-energy electron distribution function (0-60 eV) was measured for 0.05 seconds every 12 seconds, and electron temperature (300-20,000 K) was measured for 0.04 seconds every 0.04 or 12 seconds. Vector ion velocity was measured by recording three scans with the instrument pointing to three different directions in three successive spin cycles. A special mode of operation allowed total ion concentration to be measured over a 1 meter sampling distance at a spacecraft velocity of 10 km/sec, at 20 m intervals.