2.3 - MEASURING DISTANCES

What's a "Pseudo Random Code" ? The Pseudo Random Code (PRC) is a fundamental part of GPS. Physically it's just a very complicated digital code, or in other words, a complicated sequence of "on" and "off" pulses as shown in the picture. The signal is so complicated that it almost looks like random electrical noise. Hence the name "Pseudo-Random". It's a Spread Spectrum radio trasmission. There are several good reasons for that complexity: First, the complex pattern helps make sure that the receiver doesn't accidentally sync up to some other signal. The patterns are so complex that it's highly unlikely that a stray signal will have exactly the same shape. Since each satellite has its own unique Pseudo-Random Code this complexity also guarantees that the receiver won't accidentally pick up another satellite's signal. So all the satellites can use the same frequency without jamming each other. But there's another reason for the complexity of the Pseudo Random Code, a reason that's crucial to making GPS economical: The codes make it possible to amplify the GPS signal. And that's why GPS receivers don't need big satellite dishes to receive the GPS signals.