Three coherent radars have been used to obtain simultaneous radial Doppler velocity measurements in three directions from a common volume in the E layer of the high-latitude ionosphere. The cosine relationship, which was derived in the fluid approximation of the plasma instabilities associated with radar aurora, provides a consistent framework in which to interpret radial Doppler velocity observations. However, a systematic discrepancy between the observed radial velocities and the cosine relationship is found and discussed. For observations in the eastward electrojet this discrepancy is qualitatively in line with expectations based on the kinetic theory of the instabilities. It appears that the possible phase velocities in the plasma are limited to lower values than previously thought. This implies that an estimate of the electron drift velocity obtained using the fluid approximation at times may represent an underestimate of the actual electron drift velocity.