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- The Retarding Potential Analyzer (RPA) which was developed at the University of
Texas at Dallas, were flown aboard NASA's Dynamics Explorer-2 spacecraft. It
consists of two planar sensors that view approximately along the spacecraft
velocity vector. One sensor provides a measure of the total ion flux entering
the sensor aperture from which structure in this parameter can be measured with
extreme sensitivity. The other sensor has internal grids that can be stepped
through positive voltage waveforms to modulate the incoming ion flux that
reaches the collector. The resulting ion-flux versus retarding potential
measurements are used to derive the ion temperature, the ion drift velocity
along the sensor look direction ,and the ion composition.
[Source: University of Texas, Dallas.]
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