2.2 - MEASURING DISTANCES

The timing problem is tricky, because the times are going to be awfully short. If a satellite were right overhead the travel time would be something like 0.06 seconds. Assuming we have precise clocks, how do we measure travel time? Suppose there was a way to get both the satellite and the receiver to start playing a song at the same time. If sound could reach us from space then standing at the receiver we'd hear two versions of the song, 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 20,000 Km. 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 (we can hear when this happen). 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 we've got our distance to the satellite. That's basically how GPS works, only instead of the song the satellites and receivers use something called a "Pseudo Random Code" that's a Spread Spectrum digital radio trasmission.