Abstract: Several recent and upcoming sounding rocket missions have utilized retarding potential analyzers (RPAs) for measurement of ionospheric plasma. While RPAs have been used extensively from orbital vehicles (with supersonic velocity relative to the thermal plasma), their use on suborbital vehicles (moving at subsonic velocity) has been limited. Approximations made for high-mach-number measurements become invalid when the sensor velocity is close to the thermal velocity of the plasma (subsonic), as is the case in many suborbital missions. Additional complications, including 3D sheath effects, can also affect the measurement. This talk overviews the derivation of the familiar high-mach-number expression, then explores the modifications required for interpretation with reduced relative velocity. This form is applied to measurements from a recent test sounding rocket flight where subsonic RPA measurements were made at mid-latitudes. Analysis techniques for auroral plasmas relevant to the February 2017 auroral ISINGLASS mission will also be discussed.