Extensive studies conducted by several researchers using truck-mounted active microwave sensors have shown the sensitivity of these instruments to soil-moisture variations for agricultural land covers. The logical extension of these results is the evaluation of similar systems at lower resolutions typical of operational systems and for other types of land cover. Data collected during a series of aircraft flights in 1978 and 1980 over four rangeland watersheds located near Chickasha, Oklahoma, were analysed in this study. These data included scatterometer measurements made at 1·6 and 4·75  GHz using a NASA aircraft and ground observations of soil moisture for a wide range of moisture conditions. Data were compared with previous truck and aircraft results. Results indicate that if the sensor system is calibrated it is capable of providing estimates of surface soil moisture for the rangeland conditions tested.