What is GPS and How Does it Work?

Once exclusive to the military, GPS is now found everywhere from satellite dishes to running shoes. But how does it work, and what does it really do? Read on to learn more about this new technology.

Still bringing a map on your next trip? You may be a bit behind on the times. Today’s travelers are using a new technology called GPS, or Global Positioning System. For less than $100, you can get a handy little device that tells you exactly where you are on Earth any day.

What it is

GPS is a system of 27 satellites orbiting the earth some 22,000 miles above the ground. Only 24 of them are in operation; the other three are ‘spares’ in case one of them falls. The satellites, each weighing between 3,000 and 4,000 pounds, orbit the earth at 12,000 miles per hour and make about two revolutions a day. They are arranged in orbit so that at any given time, you can see at least four of the satellites in the sky.

How it works

Each GPS satellite transmits to earth a constant stream of information indicating its location, speed, altitude, and other geographic attributes. An atomic clock is built into each satellite also indicates the time at which the signals are sent. The signals travel at a constant speed—the speed of light—so people on earth are able to calculate exact arrival times from this information.

What people call ‘a GPS’ is actually a receiver, which decodes these signals into readable information. A GPS receiver obtains signals from four of the closest satellites and calculates the data to determine your exact location. This process is called trilateration.

How trilateration works

So why do you need four reference points? Think of it this way: if a satellite says that you’re 100 miles from New York, you can be at any point within a 100-mile radius from the Big Apple. If it says you’re also 120 miles from Montreal, you know you’re somewhere up north. Another receiver may say you’re 310 miles from Milwaukee, so you deduce further that you are on the western end of the northern arc.

So far, these three make up what is called 2D trilateration. If the world were a giant map, three data points would be all you need to find your way. But we’re working with an uneven planet. The curves, peaks and falls on the earth’s surface would make the data slightly inaccurate. That’s why you need a fourth reference point for altitude. By determining the vertical distance between you and a satellite, your receiver can take this factor into account and give you a precise location. Ships at sea have a fixed altitude of zero, so they don’t need this fourth factor.

What it’s for

GPS was originally invented for military use and is still widely used in the field today. Over the years, however, it has evolved into a dual-use technology, which means it has significant applications for both the government and civilians. In the military, GPS receivers are used for search and rescue, navigation, target tracking, reconnaissance, missile guidance, and nuclear detonation detection.

Consumer-level GPS is mostly used for navigation assistance. Pocket receivers are often used by hikers, mountaineers, and other outdoor enthusiasts to find their way in remote areas and keep track of each other in the wild. A lot of new cars are also built with GPS devices, which basically work as live world maps. Other civilian applications include speed detection, radio and cellular communications, timekeeping, and location tracking and hunting.

Most GPS receivers are built into other devices such as cars, PDAs, laptops, and even wristwatches. If you have any GPS-capable device, you can usually use it on its own unless you want to save data or upload maps and other supplementary information. For advanced applications, you can get a separate GPS device with removable storage, so that you can save and transfer data to your computer for later viewing.