Aperture synthesis is the method used by astronomers to determine the accurate brightness distribution of the radio sky with a resolution much better than that possible with a single large antenna. The technique, now over a decade old, utilizes a large number of connected radio antennas, some of them physically moveable, to follow a region of sky for many hours or days in order to sample the spatial coherence function of the radiation field over a sufficiently large area and with a reasonable filling factor. Landmark references for aperture synthesis are McCready et al. (1947), Stanier (1950), Christiansen and Warburton (1955), Lequeux et al. (1962), Read (1961) and Ryle and Hewish (1960).