The closer you get to certain US Government facilities, it seems the less accurate they sometimes become. When I am working in the field, I have three different GPS devices on-board. All three are from differing manufacturers, and all three show me to be in a different place when near some USAF bases. The same day, I ran into a group of Air Controllers, and their GPS was dead on, while my three thought I was in three different places at the same time. I also had some surveyors with me, and their GPS was giving them fits also.