Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:pvo.orpa::2.2
NAME Orbiter Retarding Potential Analyzer (ORPA) for Pioneer Venus
TYPE Langmuir Probe
DESCRIPTION The Retarding Potential Analyzer (ORPA) is a Langmuir-probe retarding-potential analyzer designed to measure various characteristics of the ionosphere, including electron concentration, temperature, major ion concentrations, and ion drift velocities. An adaptation from an instrument flown on the German Aeros satellite in 1972, it has two sensor heads and operates in electron, ion, or photoelectron modes. The investigation aims to enhance understanding of ionic reactions in the Venusian ionosphere, plasma transport processes, and the interaction between the ionosphere and solar wind.The ORPA weighs 2.8 kg and operates on 2.4 W of power. It features an offset axis for alignment with the spacecraft's velocity vector and utilizes multiple collector grids to selectively funnel ionospheric particles. It operates in three modes: Langmuir probe, ion, and photoelectron, with varying voltage settings for different measurements. Scans are performed every 12 seconds while the spacecraft is in the ionosphere, allowing for numerous measurements and sampling data on concentrations, temperatures, and drift velocities of ions and electrons at specified intervals. A specific operational mode can measure total ion concentration over a 1-meter sampling distance along the spacecraft's trajectory.
MODEL IDENTIFIER
NAIF INSTRUMENT IDENTIFIER
SERIAL NUMBER not applicable
REFERENCES 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.

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 Geosci. Remote Sens., 18, 1, 60, 1980.