I just heard this on local radio so I googled it. This is what I found:

Posted 6/11/2004 5:02 PM

Military testing system designed to jam GPS signals off coast

WILMINGTON, N.C. (AP) - Military officials are testing a system for jamming
signals from navigational satellites - an experiment that could disable
boaters' GPS devices and force them to rely on dead reckoning.
The test began Friday and will run through June 20, as part of a NATO
training exercise along the Atlantic Coast.

The Coast Guard said GPS signals will be unreliable or unavailable in waters
up to 60 miles off North Carolina and along the coast of northern and
central Florida. Officials said the testing may affect cell phones as well
as GPS navigation devices in boats, airplanes or cars.

Coast Guard stations are trying to get word out about the interference.

"What we kind of foresee happening is some confusion on the mariners' end,"
said Petty Officer 1st Class David Christensen.

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Ken Heaton & Anne Tobin
Cape Breton Island, Canada
kenheaton AT ess wye dee DOT eastlink DOT ca