Spring 2002 Forward to a Friend


A constellation of orbiting satellites is the heart of the Global Positioning System.

Originally developed by Rockwell International for the Department of Defense, the Navigation Signal Timing and Ranging Global Positioning System (Navstar GPS) was built in 1978. The Navstar system features a constellation of 24 satellites orbiting approximately 12,000 miles above the Earth.

Each 2,000-pound, solar-powered satellite circles the earth twice a day in a unique pattern, emitting radio signals on two different frequencies. The signals provide the satellite’s position and velocity within the constellation, as well as a time stamp and its identity.

How It Works

GPS functions by triangulation. Basically, the GPS unit compares the time a radio signal was sent from a satellite to the time it was intercepted by the GPS receiver. The travel time indicates the GPS unit’s distance from the satellite. Distance is calculated by multiplying velocity (the speed of light - 186,000 miles per second) by time (the travel time of radio signal).

GPS can be used to find a location quickly and accurately anywhere in the world. The system relies on a network of radio signals delivered by a constellation of 24 satellites circling the Earth.

With distance measurements from additional satellites, the receiving unit can pinpoint a location’s latitude and longitude, and the location can be displayed on the unit’s electronic map. While three satellite ranges can provide a “rough” position, the system requires four satellite ranges to determine an exact position. This fourth measurement helps the GPS receiver’s computer compensate for timing inaccuracies and delays as the radio signals pass through the Earth’s atmosphere.

The GPS receiver intercepts radio signals from four rapidly moving satellites and calculates how long it took for each signal to travel to the receiver. Because no two signals are identical and the satellites are in different parts of the sky, the resulting calculations can be used to produce a latitude and longitude display.

For around $200, GPS can be enjoyed by just about anyone. Mariners, pilots and outdoor enthusiasts rely on GPS to locate their position and guide them to even the most remote destinations. Surveyors rely on GPS to plot land masses. Even our computer clocks are calibrated using GPS radio signals, which are accurate to within a few fractions of a second.

The use of GPS in automotive navigation is perhaps its most exciting application. Imagine a dash-mounted device that provides step-by-step driving directions, right down to the estimated travel time. Low on gas? You can use GPS to find the nearest gas station. While you’re at it, you can locate your favorite fast-food restaurant for a bite to eat along the way. Automotive GPS is capable of performing these tasks, as well as driver assurance services like roadside assistance, vehicle tracking, remote diagnostics and more.

Orbiting 12,000 miles above the Earth, satellites work together to determine the position of a GPS receiver.

Recent innovations in GPS technology include smaller, lighter designs, better reception and longer battery life. Basic models provide reliable position and navigation information, while more advanced receivers offer PC-uploading capabilities and an integrated database of cities, highways, roads, restaurants, lodging, lakes, parks, and other points of interest. It’s easy to see why GPS has created such excitement in the world of navigation.

Jennifer Fischer is the former editor of Drive and is a frequent contributor to the magazine.


KNOWING WHERE WE ARE and where we are going has always fascinated us. Centuries-old stories, as well as myths and legends, describe remarkable quests to unfamiliar lands. How did the travelers of old find their way?

Early explorers marked trails with piles of stones. Voyagers who sailed the seas guided their ships by the movement of the sun and stars. Often, navigators compensated for their lack of technology with creativity. Floki Vilgjerdarsson, the Viking explorer who is believed to have discovered Iceland, carried a cage of ravens aboard his ship. When he believed land was nearby, a bird was released. If it flew aimlessly around the boat, land was not near, but if it took wing in one direction, the sailors followed it, anticipating that the bird was headed toward land.

Fortunately, following ravens in flight is ancient history. Since primeval times, we’ve developed better, easier-to-use navigational tools. Maps, compasses and protractors enhanced navigational capabilities, but sometimes fell short in their ability to pinpoint a traveler’s exact position.

The creation of the GPS changed all that. For the first time in history, we can fulfill our unexplained, inborn need to know where in the world we are, all the time.

Drivers of select 2003 Outback models can now drive and enjoy the great outdoors with confidence, knowing they have a live OnStar advisor available to help if they’re in a major accident, experience an emergency, or have vehicle problems. The advisor can help with less-serious matters, too, such as finding a restaurant for dinner.

OnStar is the innovative in-vehicle safety, security and information service that uses the Global Positioning System (GPS) satellite network and cellular technology to link vehicle and driver to the OnStar Center, where advisors are available 24 hours a day, 365 days a year to offer real-time personalized help.

“Having access to OnStar adds greatly to the confidence that comes from driving a Subaru,” says T.K. Saito, Chairman and CEO of Subaru of America, Inc.

OnStar-equipped Outback vehicles will include a one-year subscription to OnStar’s Safe & Sound services, which offers automatic notification of air bag deployment, emergency services, remote door unlocking, roadside assistance and more. Subaru plans to expand OnStar availability to other Subaru models. Call 1-800-OnStar-7 or visit www.onstar.com for more details.