Measurements of the phase velocity of ionospheric plasma waves using the STARE radar system are compared with the electron drift velocity and the ion acoustic velocity calculated from data measured by EISCAT. This comparison shows that for those flow angles that are smaller than the largest flow angle (&thgr;s) for which the component of the electron drift velocity along the radar line of sight equals the ion acoustic velocity, the phase velocity observed by STARE tends to be limited by, or lower than, the ion acoustic velocity. Using observations with flow angles larger than &thgr;s, one can infer the electron drift from STARE measurements by the well-known cosine relation. ¿American Geophysical Union 1990