The antenna designed for radiometer applications on the planned NASA Earth Science Geostationary Platforms is described. This antenna consists of two parts: a regular parabolic dish 5 m in diameter which converts the radiation from feeds into a collimated beam, and a movable mirror that redirects the beam to a prescribed scan direction. The mirror is composed of 28 segmented planar conducting plates of rectangular shape, mostly one square meter in size. For frequencies between 50 and 230 GHz and for a scan range of +or-8.0 degrees (270 beamwidths scan at 230 GHz), the beam efficiency calculated is above 95%. To cover such a wide frequency and scan range, each plate is individually controlled for a tilting of less than 4.0 degrees , and for a sliding of less than 0.5 cm. The sliding is done at discrete steps. The plate positions must be reset for each frequency and for each scan direction. It is shown that this antenna system is inherently bandlimited. >