The FAS Panel is aware that some U.S. military satellites are being hardened adequately and recommends that hardening of individual military and commercial satellites, especially commercial satellites used by the military, be taken into account before the government puts them to any critical use. It is important to note that the GPS satellites, which are at 20,000-km altitude, are designed to survive a million-rad dose of total radiation over a 10-year lifetime. Moreover, the cost of shielding GPS satellites is reported to be 1% of the program cost. The GPS constellation consists of 24 satellites, which are spread over different orbital planes at an altitude of 20,000 kilometers. To substantially degrade the GPS, the satellites have to be attacked individually, which is difficult to do. The satellites are also hardened against nuclear effects and have on-orbit spares. The robustness of the GPS constellation has been analyzed by Geoffrey Forden and is reported in Appendix D. The analysis shows that the GPS constellation is robust to the extent that it can lose up to four satellites and yet only suffer from periodic loss of function at any place. As stated earlier, this robustness makes the vulnerability of the GPS constellation to ASAT-type attacks rather small.

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Lewis, Leo and Phillip E. Coyle. Ensuring America's Space Security. Washington, D.C.: Federation of American Scientists, September 2004. [ 7 quotes ] [ page 28 ]