From CNN's Department Of The Obvious:
http://www.cnn.com/2001/TECH/09/11/s....ap/index.html
"WASHINGTON (AP) -- A Transportation Department report cautions that the satellite system the Federal Aviation Administration plans to use to help track airliners in flight is vulnerable to natural obstructions and electronic jamming.
The report by the Volpe National Transportation Systems Center said the signal from the satellites, known as GPS or Global Positioning System, could be blocked by buildings or interference from other transmissions.
In addition, the signals could be jammed or the satellites could even be attacked in a "highly unlikely" scenario.
"As GPS further penetrates into the civil infrastructure, it becomes a tempting target that could be exploited by individuals, groups or countries hostile to the United States," the report said.
Use of the existing satellite system is part of the FAA's $11.5 billion plan to allow more planes to land, take off and fly from place to place." (there's more at the site)
R