If anyone's interested in the
really technical details of how GPS works, have a look
here .
A few years ago I used to browse the physics newsgroups occasionally. There was a long-running flame war based on whether or not the GPS satellites' timing signals were adjusted for the relativistic effects resulting from their high speeds... there was a conspiracy-theorist claiming that GPS was a fiction and that man never really went to the moon, etc and so forth...
Off topic but interesting... hmm...