You may know that GPS is built around pseudo-random sequences. By correlating the known sequence with what you receive, you can extract the signal from a lot of

You may know that GPS is built around pseudo-random sequences. By correlating the known sequence with what you receive, you can extract the signal from a lot of noise. By determining the time offset in the correlation you get the distance from the satellite. The interesting thing is that Apollo used a very similar system for determining the distance of the spacecraft. They sent a pseudorandom signal from the ground and the spacecraft returned it. By correlating the sent and received signals, they determined the distance of the spacecraft.