A three‐element radio interferometer can measure the complex ratio of spatial‐frequency components in the presence of atmospheric phase fluctuations, even when the interference fringes are badly distorted by the fluctuations. The measurement is based on the principle that the phase‐modulated fringes of two interferometers will have essentially identical envelopes if the interferometer baselines are not greatly different. A least‐squares fitting process determines the amplitude ratio and phase difference with variances only slightly higher than would be obtained in the absence of phase fluctuations. Three applications of the basic technique to measuring a spatial‐frequency spectrum are discussed.