This directory contains the document

  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, 1980

which describes the instrument in detail.

The document is available in both ASCII and HTML. The ASCII contains only
the text portions, and the HTML contains full graphics, as well.