The timing problem is tricky. First,
the times are going to be awfully short. If a satellite were right overhead the travel
time would be something like 0.06 seconds. So we're going to need some really precise
clocks. We'll talk about those soon.
But assuming we have precise clocks, how do we measure travel time? To explain it let's
use a goofy analogy:
Suppose there was a way to get both the satellite and the receiver to start playing
"The Star Spangled Banner" at precisely 12 noon. If sound could reach us from
space (which, of course, is ridiculous) then standing at the receiver we'd hear two
versions of the Star Spangled Banner, one from our receiver and one from the satellite.
These two versions would be out of sync. The version coming from the satellite would be
a little delayed because it had to travel more than 11,000 miles.
If we wanted to see just how delayed the satellite's version was, we could start
delaying the receiver's version until they fell into perfect sync.
The amount we have to shift back the receiver's version is equal to the travel time of
the satellite's version. So we just multiply that time times the speed of light and BINGO!
we've got our distance to the satellite.
That's basically how GPS works.
Only instead of the Star Spangled Banner the satellites and receivers use something
called a "Pseudo Random Code" - which is probably easier to sing than the Star