The Department of Defense (DoD) and the intelligence community rely on various systems to provide imagery to tactical commanders and intelligence analysts. In addition DoD uses manned and unmanned aircraft equipped with ground moving-target indication (GMTI) radars to detect military units, vehicles, and other moving targets on the ground and inform commanders of their disposition. This book examines the costs and potential performance of four possible designs for a Space Radar System. Those four notional alternatives include three constellations of various sizes with 40-square meter radar arrays and one constellation with 100-square-meter radar arrays, all in low earth orbit. The book also highlights some of the technological challenges associated with developing and operating Space Radar satellites. This is an edited, excerpted and augmented edition combining CBO and GAO publications.