From 911Encyclopedia:

GPS-Signal Hack January 2002 Dorothy E. Denning and Peter F. MacDoran wrote a article on the subject which was published in Computer Fraud & Security in February of 1996. GPS-Hacks in stealing TV signals are possible, but it seems pretty impossible to hijack military devices http://www.cs.georgetown.edu/~denning/infosec/Grounding.txt

Also see... 911review.org/Wiki/Gps.shtml GPS testing september 11th 9/11 at 911review Wiki

911review.org/Wiki/GPStesting.shtml GPS testing and september 11th 2001 GPS test signal jamming air traffic