The Scandinavian twin auroral radar experiment (STARE) has a demonstrated capability of providing estimates of the ionospheric electron drift velocities, which are useful for a wide range of geophysical studies. The accuracy of such estimates has for the first time been tested by comparison with simultaneous velocity measurements made with the European incoherent scatter facility (EISCAT). The magnitudes of the estimated drift velocities are in agreement with the EISCAT measurements for small velocities (<700 ms-1), but we find the estimates to be increasingly too low as the velocities become larger. The directions of the estimated vectors are in agreement with the EISCAT measurements for all drift magnitudes. The measured phase velocities are even lower than those predicted by the kinetic theory of the two stream instability. Agreement can only be obtained for relatively large neutral densities. The data are not inconsistent with an assumption that the possible phase velocities in the plasma are limited upward by the ion-acoustic velocity (which is an increasing function of the ionospheric electric field). Power spectral observations and an improved modelling with the kinetic theory suggest that a backscatter region, extended in altitude and allowing several values of k∥ to influence observations, leads to an improved theoretical fit to the measurements.