GPS Brings Military Precision to Your Car

By Marshall Brain

In the Brain household, there are four small children, and lately we have been doing something called geocaching as a family. Here’s how it works. At thousands of points around the United States (and the world), people have hidden a wide variety of “caches.” A cache can be as small as a pill bottle or as big as a 5-gallon bucket. A cache can contain anything from a paper log sheet where you sign your name, to a collection of paperback books to read.

To find these caches, you go to a Web site like Geocaching.com. You get the GPS coordinates of a cache you would like to visit. You plug the coordinates into your GPS receiver and then drive or hike to the cache. Usually the caches are hidden pretty well, but they are fairly easy to find. With kids, it can be a lot of fun — sort of like a treasure hunt.

The key to geocaching is the GPS receiver. These are the same devices that now make it easy to map your way around in a big city. GPS receivers also keep pilots, boaters and hikers on track. But how do these receivers work? What is the Global Positioning System? How can a little box that you hold in your hand tell you exactly where you are anywhere on the planet?

The GPS system was created originally by the Pentagon to help soldiers find their way on the battlefield, and it is definitely a high-tech tool. It involves orbiting satellites, atomic clocks and some fascinating calculations.

The basic idea behind the Global Positioning System is pretty simple. It uses triangulation. If you know how far away you are from three landmarks, you can use triangulation to find out exactly where you are on the planet. For example, if I tell you that you are 625 miles from Boise, Idaho, 690 miles from Minneapolis, and 615 miles from Tucson, Ariz., you can plot three circles on a map and discover that you are in Denver.

The Pentagon wanted to use this kind of triangulation system, but it wanted it to work all the time and from any place on Earth. So for the Global Positioning System, the Pentagon uses satellites in space as the landmarks. There are at least 24 working GPS satellites, along with a few spares, orbiting earth right now. They are flying at an altitude of about 11,000 miles. This means that at any given time, at any point on earth, there are normally eight or so GPS satellites visible overhead. The job of your GPS receiver is to listen to those satellites and calculate exactly how far away they are. Then, by triangulating, your GPS receiver pinpoints your location on Earth.

But, how can your GPS receiver triangulate from moving satellites? This is where pure genius comes in. First, your GPS receiver needs to know where the satellites are located overhead. It can figure this out fairly easily because the satellites fly in very precise orbits. The receiver contains a computerized almanac that tells it where each satellite is located in space. Second, the receiver needs to be able to calculate the distance to each satellite. This is a little trickier.

To calculate the distance to a satellite, your GPS receiver listens to radio signals coming from the satellite. Each satellite contains an incredibly precise atomic clock. Let’s say that, at exactly midnight, the satellite sends a signal.

Your GPS receiver listens for the signal and calculates, down to the nanosecond level, how much of a delay there is between the time the satellite sent the signal and the time the receiver hears it. That delay tells your receiver how far away the satellite is.

By doing these calculations with several satellites at once, your GPS receiver triangulates and finds its exact position on Earth.

You might have one other question. Your GPS receiver only cost $200 or so, and it is small. It can’t possibly contain its own atomic clock. So how does the receiver know what time it is with any accuracy? The Global Positioning System has a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary, cheap quartz clock, which it constantly resets. The receiver looks at incoming signals from four or more satellites and measures its own inaccuracy. In other words, there is only one value for the “current time” that the receiver can use. The correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the synchronized satellites. So the receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic-clock accuracy “for free.”

The next time you use your GPS receiver, you can now marvel at the multibillion-dollar collection of atomic clocks and satellites in the sky that make it all possible. It truly is an amazing system.

For more on this or the scoop on other fascinating topics, go to HowStuffWorks.com. Contact Marshall Brain, founder of HowStuffWorks, at marshall.brain@howstuffworks.com.

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