"Sat Nav" (GPS satellite navigation)

Last updated: June 2, 2009.

Have you ever been lost? Unless you're a polar explorer, a fan of tropical rainforests, or the sort of person who drives across the Sahara desert in a rally car just for fun, chances are that you always have a fairly good idea of where in the world you are. Centuries ago, before the invention of cars, airplanes, and fast ocean ships, Earth must have seemed a huge, dangerous, and scary place. Not any more. With satellite navigation devices (also known as "sat nav" and GPS), you can pinpoint your location to within a few feet, wherever you happen to be. Let's take a closer look at GPS and find out how it works!

Photo: A US Air Force officer uses a handheld GPS (global positioning system) on exercise in Virginia. Portable GPS devices like this are made by such companies as Garmin, Tom Tom, and Magellan. Picture by Bryan Stevens courtesy of US Department of Defense.

Using landmarks to find where you are

Suppose you're in the centre of a strange town, where you've arranged to meet a friend. You call them up on your cellphone and try to explain where you are—but how do you do it, exactly? Most people would look around them for landmarks and say something like: "I'm in the square, next to the bank, just across from the statue of George Washington." In other words, we define our location relative to known landmarks. The more landmarks we use, the more precisely we can locate ourselves. If we just say "I'm in the square", that could still be quite vague if the square is a large area. But adding in the extra details about the bank and the statue helps our friend locate us precisely.

Photo: Explorers like Ferdinard Magellan (1480-1521) sailed the globe with great skill and ingenuity, but imagine how much easier their lives would have been with satellite navigation! Public domain engraving courtesy of US Library of Congress.

How satellites work as landmarks in the sky

Ancient navigators, sailing in the open ocean, had no landmarks they could use to locate themselves—so they used the fixed positions of the stars to guide them instead. Modern navigators use a hi-tech version of the same idea called satellite navigation. Instead of looking at lights from the stars, they use radio signals emitted by networks of satellites orbiting around Earth. The satellites are effectively "sky landmarks" that tell you where you are.

GPS and other satellite navigation systems

There are three different satellite navigation systems used around the world, the best known of which is the US Global Positioning System (GPS), which uses 24 satellites named NAVSTAR orbiting 18,000 km (11,000 miles) above Earth. Originally developed by the US military, GPS is now widely used for civilian purposes too; most car-based satellite navigation devices use GPS, for example. In Europe, a rival system called Galileo was launched in 2005 and the Russians have their own system called GLONASS (Global Navigation Satellite System).

Photo: A NAVSTAR satellite pictured during construction on Earth in 1981. You can get an idea how big the satellite is from the engineer pictured some distance beneath it. Picture courtesy of US Department of Defense.

What can we use satellite navihation for?

Satellite navigation systems are incredibly accurate. The NAVSTAR satellites have atomic clocks on board that make their time signals accurate to one second in 300,000 years. That means the military versions of GPS receivers can pinpoint things to within just 5 cm (2 inches). This incredible accuracy makes satellite navigation amazingly useful. It can show ships, aircraft, and cars where they are. It can help farmers to monitor their crop yields. And, combined with an audio system that speaks out directions, it can even help blind people to navigate their way around unfamiliar places.