Interferometry is a technique which is used to overcome the poor resolution of a single dish and produce high resolution radio images.
Astronomers electronically simulate the effect of a very large dish by combining the signals from many smaller single-dish radio telescopes. This technique is called radio interferometry, demonstrated by Prof. Martin Ryle, Prof. Christiansen and others. Radio interferometry exploits an advantage, the much larger wavelength of the radio waves, and hence, detectors can be used to measure the phase as well as the intensity of the incoming radio signal.
If a pair of single-dish radio telescopes, physically separated on the ground, observe the same radio signal from a compact radio source, and the two signals are combined, the combined output of the two signals oscillates with time. This is because, as the Earth rotates, the relative phase of the signals received by the two dishes also varies as the projected distances, as seen by the source, varies. Or, in other words, periodically the radio wave from the object received by the two different dishes will add up in phase and produce a large signal and at a later time period the two different dishes will add up out of phase and produce a small signal, which depends on the slight variation in the distances from the object to each radio telescope dish. Interferometry uses this constructive and destructive addition of the radiation, also called as the `fringe', and thereby, determines the information of the intensity and the structure of the object that is observed by an interferometer.