How GPS devices pinpoint the location of moving objects

By Curt Suplee
Tuesday, November 3, 2009

America has seen its last Lost Generation.

Thanks to an invisible armada of incessantly broadcasting satellites, collectively called the Global Positioning System, and to the explosive proliferation of GPS receivers in gadgets from dashboard map units to cellphones to dog collars, even the cartographically clueless are now good to go.

The same technology that allows the military to drop precision-targeted bombs on terrorists has become a $30 billion worldwide market, spawning devices that lead hikers through the trackless wild, recover itinerant tykes with GPS units sewn into their backpacks, let golfers see the distance to the next hole, stamp the location on digital photos and show the nearest pizza joint on a PDA screen. Very soon it may be possible to find your lost keys as receivers shrink to the size of a dime and smaller.

It has all happened deliriously fast. Modern GPS has been fully operational only since 1995. Today it consists of 30 one-ton satellites, each the size of a pickup truck, orbiting the Earth about 12,000 miles high. At least 24 have to be working at any given time; that way, almost everywhere you go on the planet, six of them are above the horizon.

The U.S. Air Force runs the whole show. But it has been a dual-use system since 1983, when the Soviet Union downed a Korean Air Lines passenger jet that strayed into Soviet airspace because of navigational error, prompting President Ronald Reagan to open GPS to civilian and commercial users. For the next 17 years, nonmilitary users got a deliberately degraded signal until President Bill Clinton ended the practice, instantly improving precision fivefold.

Although GPS is seen as a navigational system, it's actually a choreographed array of flying clocks. Every few seconds, each satellite transmits the exact time (derived from multiple on-board atomic clocks), its specific location at that time and the whereabouts of its siblings. When your GPS unit receives the signal, it determines how long it took to arrive, divides by the speed of light, and comes up with the distance to the satellite.

That demands near-perfect timekeeping. Light travels about 186,000 miles per second. Thus an error of only a thousandth of a second would produce an uncertainty in your position of 186 miles. Since that could add half a day to the average drive to the grocery store, vastly better accuracy is essential. So the satellite clocks are updated and corrected every few minutes by signals from ground stations that in turn get their orders from the U.S Naval Observatory Master Clock (an average of 90 super-clocks), which might be off by maybe a tenth of a nanosecond per day.

To determine your position, the GPS receiver has to "acquire" at least three, and preferably four or more, satellites. That can entail an infuriating wait for drivers raring to hit the road. But, hey: The signals are pretty faint to begin with, you're sitting inside two tons of metal, and the unit has to find four of them just to fire up.

Why do you need four satellites if space as we know it has only three dimensions? It's easy to understand but hard to picture. Think of a satellite as a point source that radiates an electromagnetic beep outward simultaneously in all directions, forming a sphere. If you can get the signal, you're location is somewhere on that sphere. Add a second satellite, and it's like two spheres intersecting, with the intersection area forming a flat circle--not very helpful, unless you never leave the Beltway. Keep adding spheres and when you hit four, you're down to a single spot.

Civilian users typically know their location only within about 15 to 100 feet, depending on noise, weather, errors, etc. (Military applications, which use higher-speed encoded satellite signals, are about 10 times better.) As a result, many systems have been developed to augment accuracy. The Coast Guard, the Federal Aviation Administration and the Defense and Transportation departments provide services that can locate a boat, an airplane, a train, etc. within a foot or two; high-tech surveying systems are accurate within a couple of millimeters.

Of course, sometimes your car unit's maps are buggy or out-of-date, steering you to a landfill instead of to lunch. In that case, you might want your location monitored through a GPS-equipped cellphone -- an increasingly frequent occurrence. Federal regulations mandate that emergency services be able to trace the location of any cellphone calling 911. This requirement seems to have encouraged a trend, already underway, to place GPS chips in mobile phones. The result has made possible voluntary buddy-tracker systems such as Google's Latitude, which allows consenting friends with suitably tricked-out phones to reveal their locations to one another via the Web, further adding to the national cyber-orgy of social networking.

Happily, however, these innovations are arriving just in time for America's impending Geezer Boom, in which a growing number of senior citizens spend a growing fraction of their lives looking for misplaced items -- or, indeed, misplaced geezers themselves. As our demographics get grayer, GPS will be there to help, confirming the noted philosopher who succinctly said it all: "No matter where you go . . . there you are."


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