High resolution earth observation from geostationary orbit offers several advantages compared to traditional low earth orbit systems. Among the advantages are decreased time to scan the visible geo-disk and the ability to stare at a particular event. The following work is concerned with the design of a reflector antenna configuration for passive remote sensing and suitable for use on a geostationary platform; however, the resultant configuration is not limited to this application. The specific goal is the design of a reflector antenna configuration capable of precision beam scanning over a range of several degrees in all directions while minimizing vibration and inertial torque such as to have minimal effect on the other instruments sharing the platform. Desirable characteristics of such a reflector configuration are: a stationary feed consisting of a single element or a small array; simple reflector motions; and high primary aperture utilization for all scan directions (high illumination efficiency). This dissertation documents the development of a novel tri-reflector antenna configuration which addresses the design goals outlined above. The reflector configuration has been named the conjugate tri-reflector. The conjugate tri-reflector consists of a parabolic primary reflector an elliptical secondary reflector and a shaped tertiary reflector. Beam scanning is performed entirely by motion of the relatively small tertiary reflector. The proposed reflector configuration offers substantial improvement in scan performance compared to that achieved by feed displacement with a prime focus parabolic reflector and has a much higher aperture efficiency than comparable spherical reflector configurations. / Ph. D.