SA, IDI, and DBS horizontal wind estimators are examined and demonstrated to share common biases under inhomogeneous flow field conditions. The biases can reach magnitudes of several tens of meters per second at mesospheric heights, contaminate gravity wave flow field measurements, cause distortions in measured flow field spectra, and possibly account for large wind measurement errors evidenced in recent experimental studies including the AIDA '89 effort. However, being a linear response to gravity wave flow field fluctuations, the biases are expected to cancel out in long-term wind and tidal statistics compiled with radar wind data. ¿ American Geophysical Union 1993