A $12-billion investment whose exact mission remains classified by the Pentagon - though it was probably a component in the Star Wars defense programme - Navigation Satellite Timing and Ranging system (Navstar) is a halo of satellites that circle the world on exceedingly specific orbits every 12 hours. A GPS receiver listens to four or five satellites then divines its own location relative to each satellite.
How well does GPS work?
Consider the surgical precision of Tomahawk cruise missiles (which are GPS-equipped) during the Gulf War. The missiles, fired from American warships in the Persian Gulf 60 miles from Baghdad, flew over the sea at 1,000 feet - high enough to clear oil platforms - then ducked low, sometimes to less than 70 feet, after making landfall. Streaking overland at 550 mph, 116 Tomahawks struck their targets exactly during the war's first 24 hours.
Although GPS technology is available to the civilian world, the Pentagon, in an attempt to foil access by enemies, has added Selective Availability (SA) to Navstar, which slightly degrades the precision of non-military GPS units. But even when SA is on, science has found ways to refine GPS readings. Scientists use multiple- receiver systems to gauge ocean and lake levels, to follow glacial movement, to map hurricanes and ocean currents, and, ironically, to track satellites carrying astronomical and geophysical instruments as they hurtle through space.
Nowadays, GPS is also used by surveyors and cartographers, as well as by private and commercial aircraft and - in growing numbers - by transport companies as a means of keeping track of their fleets. The business of selling GPS technology - which a decade ago barely existed - will generate an estimated $8 billion this year, and more than $30 billion per annum by the end of the century. Most commercially available GPS units cost between pounds 150 and pounds 1,250, the price difference being how many coordinates they can retain: between 200 and 1,000 (the Pyxis has 400).
But is it any use to you or me?
Perhaps one day GPS will be able to suggest what to do with all the time we save by not getting lost, or taking the wrong turns. But haven't we lost something along the way? The joy of earth-bound exploration, or finding our own landmarks, has been permanently changed. Has technology finally shattered the romantic notion that, somewhere on earth, there will always be unknown places to visit; places in which we could still disappear if we wanted to?