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A Random Code?
The Pseudo Random Code (PRC, shown above) 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 here: The signal is so complicated that it almost looks like random electrical noise. Hence the name "Pseudo-Random." 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. And it makes it more difficult for a hostile force to jam the system. In fact the Pseudo Random Code gives the DoD a way to control access to the system. 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 use "information theory" to "amplify" the GPS signal.


And that's why GPS receivers don't need big satellite dishes to receive the GPS signals. We glossed over one point in our goofy Star-Spangled Banner analogy. It assumes that we can guarantee that both the satellite and the receiver start generating their codes at exactly the same time. But how do we make sure everybody is perfectly synced? Stay tuned.

In Review:

    Measuring Distance Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite.
  • To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time.
  • By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us.
  • Multiply that travel time by the speed of light and you've got distance.
GPS technology has matured into a resource that goes far beyond its original design goals. These days scientists, sportsmen, farmers, soldiers, pilots, surveyors, hikers, delivery drivers, sailors, dispatchers, lumberjacks, fire-fighters, and people from many other walks of life are using GPS in ways that make their work more productive, safer, and sometimes even easier. In this section you will see a few examples of real-world applications of GPS. These applications fall into five broad categories.
    The applications are listed below:
  • Location - determining a basic position
  • Navigation - getting from one location to another
  • Tracking - monitoring the movement of people and things
  • Mapping - creating maps of the world
  • Timing - bringing precise timing to the world
    Here's how GPS works in five logical steps:
  • The basis of GPS is "triangulation" from satellites.
  • To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
  • To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
  • Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
  • Finally you must correct for any delays the signal experiences as it travels through the atmosphere.
We'll explain each of these points in the next five sections of the tutorial. We recommend you follow the tutorial in order. Remember, science is a step-by-step discipline! Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth. That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth. Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space.