Measurements from the BARS radars have been used to derive the dependence of the relative backscatter cross section on flow angle, aspect angle and irregularity drift velocity. Aspect angles between 87¿ and 81¿ and drift velocity up to 700 m/s could be covered. The variation of the backscatter cross section shows general agreement with the growth rate for two-stream instability. The flow angle dependence is approximately a cos2 variation, whose amplitude increases with velocity and decreases with aspect angle. The aspect angle dependence shows almost no variation with velocity; the cross section decreases steeply between 87¿ and 86¿ and stays constant between 86¿ and 81¿. ¿ American Geophysical Union 1989