The influences of horizontal wind gradients on ''apparent'' and ''true'' spaced antenna wind estimators are examined within the scope of a linear wind model. It is found that apparent velocity estimators are highly sensitive to horizontal gradients in horizontal wind components. Systematic errors in true velocity estimates increase with the probing radar beam width. For a beam width of ~6¿ and realistic values of horizontal wind gradients, systematic errors in true velocity estimates remain less than ~4 m/s at mesospheric altitudes in the absence of gradients in the vertical wind component. ¿ American Geophysical Union 1995