How GPS Works

How navigation systems work

A navigation system uses an antenna to receive information from Global Positioning System (GPS) satellites to determine a car's location. (External antennas are an option if the integrated antenna can't get an adequate signal.) There are 24 orbiting GPS satellites that serve as navigational aids for military, civilian, and commercial purposes. To get a two-dimensional fix on your location and track movement, a navigation system triangulates signals from at least three satellites at once. With data from a fourth satellite, it is also possible to determine altitude. While built-in systems use an external antenna to receive GPS signals, portable units have an integrated antenna.

Once a navigation system has determined your longitude and latitude, it indicates your location on a map retrieved from its database. Typically, the car's position and direction of travel is indicated by a large arrowhead-shaped icon.

Manufacturers of navigation systems buy their map data from specialty map-making companies, such as NavTeQ and Tele Atlas. These outfits create the map database through their own real-world surveying, as well as by aggregating roadway and point-of-interest data from government sources, municipal agencies, and satellite imagery.

No map databases are perfectly accurate or completely up to date, because on any given day, new roads are opened and old roads or parts of roads are closed. Likewise, gas stations and ATMs come and go. Sometimes the map itself isn't accurate, so you appear to be driving across a river rather than across the bridge nearby. And it is possible that a perfectly serviceable road just isn't drawn in on the map you're using. But map-making companies strive to regularly update the maps they produce. The map flaws we've found are quite minor and easy to tolerate, but the problems can vary by region. For example, developing cities can quickly render maps outdated. Generally, updated maps are made available for future purchase and installation.


Working with navigation systems

Programming a navigation system is quite involved. The better systems make the process intuitive, whereas the worst ones suffer from an excess of onscreen menus and forms.

Most touch-screen systems automatically display an onscreen keyboard that lets you enter the destination with a fingertip. Others require using a stylus. Better designs have "dynamic" address searching: As you enter letters, the system whittles down the possibilities so that you can find the correct address with the fewest keystrokes. On models without a touch screen, you must use buttons, scroll knobs, or keys to select your option at each programming step.

A built-in speaker calls out audible directions as you drive. "Turn right onto Bedford Road in two-tenths of a mile," for instance. Most let you hear a repeat of an instruction. If you find the voice unnecessary or annoying, you can turn it off.

Most systems also provide a close-up diagram of an upcoming intersection, with a large route arrow that shows your recommended path through it. This is easier to view while driving than a map of the surrounding area.
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