Getting the message out
Once the DoD has measured a satellite's exact position, they relay that information
back up to the satellite itself. The satellite then includes this new corrected position
information in the timing signals it's broadcasting.
So a GPS signal is more than just pseudo-random code for timing purposes. It also
contains a navigation message with ephemeris information as well.
With perfect timing and the satellite's exact position you'd think we'd be ready to
make perfect position calculations. But there's trouble afoot. Check out the next section
to see what's up.