The first time your wireless phone rings in the middle of nowhere, it hits you just how electronically enmeshed the world has become. A similar realisation dawns when you find your way out of the woods using directions from a satellite. Global Positioning System - a constellation of 24 satellites operated by the US Department of Defense - provides travellers with a constant fix on their locations. Until recently, inflated prices and Cold War politics conspired to keep GPS receivers out of the hands of the average civilian. But now, for the cost of a VCR, you can hold a palm-sized device that will calculate your exact latitude, longitude, and altitude. With a GPS receiver, you'll never be lost again.
Despite its high-tech trappings, the central concept behind GPS is as old as celestial navigation. But instead of using stars for triangulation, modern-day mariners use GPS satellites. If you know your exact distance from a satellite, you know your location lies somewhere on the sphere defined by that radius. If you know the distance from a second satellite, you know your location must lie along the circumference of the circle where the two spheres intersect. A third satellite results in two points where all three spheres intersect. One of these points is way out in space; the other is your precise location.
Simple enough. But how do you determine your exact distance from an orbiting transmitter? Each GPS satellite radios a data packet back to Earth every 30 seconds. This packet contains information on the satellite's location, its velocity, and the time of transmission. By calculating how long the signal took to travel from the satellite to your receiver, you can determine the exact distance between the two.
Of course, the signal delay can't be measured with just a stopwatch. Instead, an atomic clock aboard each satellite regulates a 1.023-Mbit pseudo-random bit pattern that is superimposed on top of the satellite's data signal. Each bit in the pattern corresponds to an exact moment in the 30-second transmission cycle. In order to calculate the time difference, a GPS receiver compares the phase shift between the bit pattern from the satellite and an identical pattern generated internally. Some margin of error results from the receiver's imperfect quartz clock, but by taking measurements from a fourth satellite to factor out time errors, you can get within 50 nanoseconds.
This straightforward technique of measuring various distances and then triangulating your position works well. In fact, it works a little too well, according to the US Department of Defense. Worried that GPS could be used by an enemy to guide missiles or smart bombs, the defense engineers built errors into the system. GPS satellites send out two signals: an encrypted signal for military use and an unencrypted, less accurate signal for civilians.
The military's official reports claim an accuracy of around 16 metres, but in practice, it's believed to be closer to 3 metres. The transmission is encrypted for two reasons: it keeps everyone from picking up the signal, and more importantly, it serves as an identification tag so that US planes and cruise missiles can't be tricked by enemy signals into thinking they're someplace they're not.
The civilian signal is almost identical to the military one, but it is corrupted by the department to provide an accuracy of only 76 metres. The method the government uses to degrade the signal is classified, but researchers believe that the satellite's clock is skewed in a pre-programmed way. While the military's signal is subject to the same distortion, it can easily correct it using information encrypted into the satellite's data message. Although 76-metre accuracy is adequate for maritime and most terrestrial navigation, it just doesn't make the grade when it comes to orchestrating the automated landing of a Boeing 747. That's why researchers have come up with two methods of working around the degraded signal to achieve even higher accuracy than is possible when using the straight military signal.
One method, known as Differential GPS or DGPS, combines GPS with signals from special land-based stations located around the world. Using its precise, known location, a DGPS station calculates the error present in each satellite's data and beams that correction out to any mobile GPS receiver within range. The receivers then use this information to come up with a revised position accurate to within about 1 metre.
For even higher precision - the kind needed to measure continental drift or changes in ocean levels - researchers use a system called Survey GPS. A receiver picks up the GPS transmission and strips out all of the data, leaving only the raw carrier signal. By measuring the Doppler shift in that signal as the satellite moves across the sky, the receiver can fine-tune its position down to mere millimetres.
While both DGPS and Survey GPS provide the accuracy needed to guide enemy missiles, they can't provide the necessary real-time performance. Both systems require lengthy computations to improve precision. This is probably why the department doesn't consider these systems a threat to national security. For GPS, it's now a matter of developing compelling applications. The first widespread commercial use will probably be automobile navigation systems that use GPS to provide real-time directions. These systems are already popular in Japan, and they'll soon become an option in US cars. Of course, just like cellular phones, GPS applications can be both liberating and enslaving. Sometimes you want to be out of reach, and sometimes you want to get lost.
Andrew Rozmiarek (andy@ wired.com) is a staff editor at Wired magazine.