A conventional radio interferometer consists of two or more radio telescopes, connected via wire or radio link to an unique larger antenna system.
Baselines are the distances between the telescopes.
An interferometer gives much more detailed images of simultanous observed radio sources than a single telescope of an antenna array. Conventional two-element Interferometer VLBI achieves high angular resulution by using two or more widely separated radio telescopes and recording video signals on magnetic tapes. With regard to the angular resolution, such an interferometer can be seen as a huge single radio telescope that can achieve the diameter of continents or the whole earth. With very long baselines, smallest details with an angular extension of 0.0001 seconds of arc can be separated. This system requires very high accuracy in time.