A global positioning system is a means by which an observer can determine his or her position on Earth’s surface by reference to objects whose positions are known at the time of measurement. Ancient travelers used the positions of stars to achieve this goal; their modern counterparts have access to receivers that calculate their positions using signals broadcast from satellites.
Early navigation aids
Ancient travelers used the position of prominent stars and constellations, such as the North Star and the Southern Cross, to gauge their approximate direction of travel. In the mid-18th century, this method was refined by the use of sextants—optical devices that help measure the precise angle of stars above the horizon. An observer could measure latitude or position north or south of the Equator by comparing the data from a sextant with charts of the stars’ positions. A means of measuring longitude remained elusive for a while.
In 1761, the problem of measuring longitude was solved by the invention of the chronometer—an accurate timepiece that could be used aboard ships. By taking a chronometer with them, travelers had a way of keeping track of the time at home or in some place of reference. By noting the times of sunrise and sunset and comparing them with records of the times of the same events at the place of reference, travelers had a means of calculating their positions east or west of that place.
Since 1884, the principal time standard for travelers has been Greenwich Mean Time—the time kept by high-accuracy chronometers at the Royal Greenwich Observatory, England. The observatory is situated on the prime meridian, an imaginary north–south line of zero longitude. In the early 20th century, time signals broadcast by radio from Greenwich were the traveler’s principal reference for calculating longitude.
Radio navigation
The main drawback in navigation based on star positions is its dependence on a clear night sky. Close-range navigation using sightings of beacons and other reference points suffers from a similar dependence on good visibility. Navigation using radio beacons was the first attempt to overcome such problems by using transmissions of low-frequency radiation that is not obscured by atmospheric conditions in the way that light is. Radio navigation has the weakness that it is difficult to determine the position of radio beacons with great precision over long distances.
Satellite GPS
In 1973, the U.S. government resolved to establish a network of satellite-mounted beacons that would beam high-frequency signals down to Earth as part of a high-technology replacement for navigation by stars. In fact, the first of these satellites were called Navstars, but the system later became known as GPS (Global Positioning System). The U.S. GPS is owned and operated by the Department of Defense, but most of its features are available for civilian use.
The first of the 10 development satellites that formed the so-called Block-I GPS was launched in 1978. Building on experience gleaned from the Block I project, a total of 24 Block-II satellites were put into orbit between 1989 and 1993. The satellites orbit 10,900 miles (17,500 km) above Earth’s surface, giving them an orbital period of 12 hours. The orbits are coordinated so that most points on Earth’s surface are in direct line of sight of six satellites at any given time. In fact, only three or four contacts are necessary to establish a position, but the extra satellites reduce the probability of the system being rendered useless by "outages" (failures) of several satellites all occurring at the same time.
