FAA warns of ongoing GPS issues in southeastern US due to Defense Department 'tests'

Courtesy of engadget. This freaks me out a little, taken with other recent events.. Anyway; Excerpts:

..A flight advisory just issued by the FAA, which warns pilots that their GPS signal "may be unreliable or unavailable" due to "GPS tests" that the Department of Defense will be conducting between January 20th and February 22nd. Details are fairly light beyond that, but the FAA does note that when the "tests" occur they will be active for 45 minutes, and be followed by 15 minutes of off time -- additional notices to pilots will apparently be issued at least 24 hours prior to any test.

Interesting. I wonder what they're testing?