OK, so my NXT GPS Receiver was a fake,... an April Fool's joke. :-)
But can you figure out how it works? See the Fake GPS Receiver challenge. The first three people who can explain how it works get a free copy of nxtprograms.com on CD! Email the link shown in the challenge instead of commenting here if you think you know how and want to explain it. I may post some hints in the comments for this post here if necessary.