Where In The World Is Ernie (Not Bert)?

I got a GPS unit at work today, and I thought I'd connect it to my desktop PC, just to see how it works. Once I got it all installed and stuck the receiver to my office window, I discovered a strange thing. My office seems to move around a lot! As you can see by the track above, I actually move across the street from time to time at speeds of up to 3 miles per hour. Who'd a thunk it? The scale on the screen shot above tells me that I roam around a radius of about 100 yards. This track is from only 20 minutes or so of leaving the unit on. I let it run for a couple of hours earlier, and during that time my office crossed the street and floated across the pond behind the shop more than once. And my altitude is showing as about 100 feet below sea level. I suspect there's something going on here, like reflections from metal structures, or interference from machinery or something, but I'm not entirely sure. Maybe some of this is from the built-in signal degradation that the government adds in there to confuse the enemy. Maybe.