type of radio telescope based on the interferometer, first demonstrated in the 1950s by the Australian astronomer Bernard Yarnton Mills. It consists of two interferometers erected in two straight rows intersecting at right angles. Up to a mile long, the rows may be composed of hundreds of antennas of several possible types. Electronic comparison of differences in the way the two perpendicular rows receive a signal allows exact determination of the position in the sky of the signal's source. For this purpose, the cross works practically as well as a single huge antenna with the same overall diameter; such a single antenna would be vastly more difficult, if not impossible, to build.