The FAA has issued a warning that GPS may be unreliable or unavailable in certain areas in the Southeastern US during certain times. On January 20th and February 22nd the US Department of Defense will be running tests in the areas indicated in the source. They say they are going to be running GPS tests for periods of 45 minutes with 15 minutes of down time between testing. Now they do specify altitudes of around 10,000 ft and 4,000 ft so we are not sure if that will only affect airplanes or if it will affect every GPS device in the area. However, the FAA will be issuing pilots in the area a 24 hour warning before the test to make sure they have a backup plan in-case GPS is unavailable. Nobody is sure exactly what they are testing, if it’s defense related it may be necessary Check out the source for more details.