The Global Positioning System (GPS) is a constellation of twenty-one primary satellites, and three spares, in near-circular orbits 11,000 miles above the earth at an inclination of 55 degrees to its equator. While the satellites are best known for their role in allowing the precise location of individuals and objects, by 1995 they had been performing an important secondary mission for over a decade. Every GPS satellite since GPS-8, launched in 1983, had carried a Nuclear Detonation (NUDET) Detection System (NDS) package on board. The NDS includes x-ray and optical sensors, bhangmeters, electromagnetic pulse sensors, and a data-processing capability that can locate a nuclear explosion to within one hundred meters. Data is reported on a real-time basis directly to either AFTAC or ground stations at Diego Garcia, Kwajelein Atoll, Ascension Island, or Keana Point, Hawaii. See Jeffrey T. Richelson, The U.S. Intelligence Community, 4th Ed. (Boulder, Colo.: Westview, 1999), pp. 218-219.
Richelson, Jeffrey T. Spying on the Bomb: American Nuclear Intelligence from Nazi Germany to Iran and North Korea. W.W. Norton and Company; New York. 2007. p. 403-4 (paperback)