A radio telescope in which two or more separate antennas observe the same object simultaneously. The signals received by a pair of antennas are fed into a receiver that multiplies the voltages. The amplitude and phase of the correlated output depend on the distribution of radio emission from the source being studied. Little can be learned from a single measurement of this kind but, if the spacing and orientation of the interferometer are changed, the correlated voltage varies and can be analysed by computer to generate maps showing the distribution of radio brightness on the sky. This technique is fully exploited in Earth rotation synthesis. See also: aperture synthesis, very-long-baseline interferometry.