If the signal pattern in the vicinity of three spaced receivers is assumed to be represented by a linear variation in distance and time for a short time interval τ, the apparent motion of the pattern must be normal to the lines of constant signal intensity. The instantaneous velocity of the ground pattern may be represented as the sum of the component of the average drift velocity in the direction of the gradient and a random velocity in the direction of the gradient. After the drift velocity has been determined by correlation analysis, the random velocity for each time interval may be obtained, and a spectral analysis of the random speed may be performed. This information may be used as an aid in describing the detailed motion of the signal pattern which is necessary in relating the observations to ionospheric motions