Aperture synthesis Is; an emerging technology for passive microwave remote sensing from space, It is an interferometric technique similar to earth rotation synthesis employed in radio astronomy in which pail's of small antennas and signal processing are used to obtain the resolution of a single large antenna. The technique has the potential to overcome the barriers that antenna size has placed on passive microwave remote sensing From space, The technique has been demonstrated successfully for remote sensing at L-band with the aircraft prototype ESTAR. New aircraft instruments ape under development and propos- als have been submitted For instruments to demonstrate this technology in space.