The Blue Cube, as it is sometimes called, is Onizuka Air Force Station, a military and intelligence satellite control station. Some of the Cube's functions have been relocated to Schriever Air Force Base in Colorado, though it remains active, with some still calling it a "spy satellite control center," which is what it was built for (to operate the Lockheed/CIA Corona program).
http://en.wikipedia.org/wiki/Onizuka_Air_Force_Station