How Does GPS Work?

Have you ever wondered, "How does GPS work"?What makes that little gadget able to find your exact position on the planet?

Understanding how GPS works will make you aware of its abilities and, more importantly, its limitations. Having this knowledge could help keep you out of trouble.

As the saying goes, "A little knowledge is a dangerous thing." This is never truer than when your personal safety is riding on that knowledge.

Illustration of the 24 GPS satellites orbiting the earth. Courtesy of Garmin

So, how does GPS work, anyway?

GPS stands for Global Positioning System. The backbone of this system is a network of 24 satellites orbiting the earth at a distance of 11,000 miles, twice a day.

NAVSTAR is the official U.S. Department of Defense name for GPS. NAVSTAR is short for "Navigation System using Timing And Ranging", and is managed by the U.S. Air Force, which put the first of those satellites in orbit beginning in 1978.

The satellites' paths are arranged in such a way that at least four satellites are visible at any time from any place on Earth. Well, they're not really "visible" to the human eye--but are able to send signals to your GPS unit.

Your GPS device receives radio signals from these orbiting satellites encoded with information such as each satellite's identification, location, and the time. If it can "see" at least three satellites, it can determine its location using the principle of triangulation.

It translates that location information into coordinates such as latitude and longitude and bingo!--you're on the map. Since at least four satellites should be "visible" at any one time, theoretically you should be able to know your location at any time from anyplace on the planet.

I say "theoretically" because the GPS signal can be blocked by solid objects. If you are indoors, your device is basically worthless, unless you are close to a large window. Anything that blocks the signal--trees, buildings, canyon walls, even your own body--will hinder the unit's ability to read its location. Thus, a clear view of the sky is essential to get the most accurate position fix.

The good thing is that there are usually more than four satellites scattered across the sky throwing signals your way. Up to twelve satellites may be available at any one time. So even if one or two are blocked, chances are you will still get a good reading. If you don't, moving to a more open view of the sky will usually do the trick.

So the bottom line is, if you can get a clear signal from at least three satellites, you will know your exact position. That should answer the basic question, 'How does GPS work'?, but there is more.

There is some error involved, though, due to a variety of factors. Your GPS device's internal clock may be slightly off. Sometimes the satellite signals don't travel straight to the receiver because they bounce off a flat, hard surface. This is called a multipath error.

Sometimes they are slowed down by changes in the thickness of the earth's atmosphere. And sometimes a satellite wanders a little from its orbit or malfunctions; this is known as an ephemeris error.

With all these sources of interference, you might wonder just how does GPS work - how accurate is it?

Would you be surprised to learn that your receiver is normally accurate to within 49 feet (15 meters)? Pretty amazing, isn't it? And a WAAS-enabled GPS is more accurate still, reducing error to within 16 feet (5 meters).

WAAS stands for Wide Area Augmentation System. It is a network of stations which constantly check GPS satellite signals for error and communicate a corrected signal to two WAAS satellites. These satellites then send that corrected signal out to WAAS-enabled GPS units.

The WAAS satellites are in a stationary orbit over the equator, one over the Pacific Ocean and one over the Atlantic Ocean. This makes their signal much more vulnerable to being blocked than the high-flying, fast-moving regular GPS satellites. Still, it's nice to get that extra level of accuracy when you can.

Now that you know how GPS works, you'll be a smarter and more capable user. The next time someone asks you, 'How does GPS work'?, you can tell them, "Well, I'm glad you asked. It's like this..."

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