Abstract: This paper introduced a new method about the speed correction with higher precision in measuring external trajectory──radiometer method.The error of missile speed was deduced by the distance error. The distance error was obtained by a microwave radiometer directly detecting the atmospheric radiation brightness temperature on the path of radio wave propagation. The standard deviation of the distance error obtained by using radiometer detecting method is lower by one-third to two-thirds than that obtained by using the normal method.Therefore, the precision of the distance variation rate is higher and can make real time correction on azimuth and elevation by using this method.