To measure the distance from your GPS receiver to the satellite, the receiver must know how long it took for the signal to make the trip. That means it has to know both when the signal was sent and when it was received. The latter is easy. Figuring out when the signal was sent is a little trickier. To do that, a nifty code-matching scheme is used. Satellites send out something called “pseudo random code,” which is a very long pattern of 0s and 1s. Your GPS receiver — and everyone else’s on the planet — runs that same code in synchronization with the satellites. When a satellite’s signal reaches a receiver, it lags behind the portion of the pseudo random code currently running on the receiver. (Even at the speed of light, it takes somewhere in the neighborhood of .06 seconds for a radio signal to make the trip.) By looking back to find out when it played the portion of the code that it received from the satellite, the receiver can calculate the length of time it took for the transmission to arrive, and then calculate the distance.
A Network of Satellites Let’s say that the satellite in question is directly overhead, so your GPS receiver has calculated that it is 10,900 miles away. Where are you? Who knows? Strictly speaking, you could be anywhere on a sphere with a radius of 10,900 miles that has the satellite at its center. Even if you throw out all of the points on that sphere that are out in space, that still leaves a lot of places that you could be — and it's certainly not enough information to get you from the rental car place at the Los Angeles airport to Disney Land.
But suppose we added data from a second satellite. Now, suddenly, there are two spheres, and they overlap in a nice neat circle. Add a third, and the overlap consists of just two points. Add a fourth and suddenly that GPS device can tell you not only where you are, but at what altitude.
That’s why there are 24 satellites. The GPS satellite array, also called a constellation, was designed so that at least four satellites are “visible” to a receiver at any given time. In practice, there’s a good chance that your GPS receiver is monitoring signals from eight satellites. (See interactive at side for a graphical explainer.)
There is one other neat trick that is worth discussing. Remember those atomic clocks? What about the GPS receivers? If timing is so important — remember, we are talking about measuring radio signals traveling at the speed of light — does that mean there’s an atomic clock tucked away in every GPS receiver?
The answer, of course, is no. After all, a decent atomic clock is going to set you back somewhere in the neighborhood of $100,000. Instead, most GPS receivers use standard quartz clocks. But that introduces significant timing errors, and it means that the initial distance measurements aren’t entirely accurate, resulting in a set of spheres that don’t actually overlap at a single point. Fortunately, because the incoming signals are measured with the same clock, the errors are proportional to each other. The GPS receiver is designed to adjust the relative sizes of the four sphere until they do overlap at a single point.