M.I.T. Lincoln Laboratory is engaged in supporting the Electronic Systems Division (ESD) of the Air Force Systems Command (AFSC) in developing the Ground-based Electro-Optical Deep Space Surveillance (GEODSS) system. As a part of this program, low-light-level, large-format, silicon-diode-array vidicon sensors capable of photon-noise-limited operation under typical dark-sky conditions have been developed and tested both in the Laboratory and at the Experimental Test Site (ETS), White Sands Missile Range, New Mexico. This paper begins with descriptions of the GEODSS system, and ETS's role within that system. This is followed by a brief history of the design and development of the sensors, a general discussion of the theory of operation, a description of point-source experiments, and the presentation of an empirical model which describes the detection capability achieved in the field with the ETS electro-optical sensors. In the final sections of the paper, the problem of the detection of satellites by reflected sunlight is described and the highlights of data collected at the ETS are presented.