Retooling the Global Positioning System.

Abstract

The article focuses on the capabilities of the Global Positioning System (GPS). Underlying improved capabilities is a series of system upgrades that include additional satellite signals, increased broadcast power, performance monitoring, guaranteed error bounds, smart antennas that can selectively direct and receive signals, and integration with television and cellular-phone networks. GPS got its start when the U.S. Department of Defense launched the first Navstar satellite in 1978. Today some 30 million people regularly track their whereabouts using GPS. The receiving units assist in guiding road vehicles, ships and boats, as well as in fleet management for rental cars and buses, and recreational uses. The American GPS Navstar satellites are not alone in orbit, however. Russian GLONASS navigation satellites share that physical and functional space, and in a few years so will the European Galileo constellation. Each time a GPS receiver locates itself on the planet's surface, it trilaterates (a cousin of triangulation) its precise distance from at least four GPS satellites using ranging signals broadcast from overhead. In essence, the specially coded radio signals serve as invisible rulers that measure the path from the satellites to the receiver. Because mobile users change position rapidly, current GPS receivers also monitor the Doppler shift of the incoming signals--that is, motion-caused shifts of the signal's wavelengths. Transmitters onboard GPS satellites broadcast their information through standard radio-frequency (RF) waves. Starting in 2005, GPS satellites will begin to broadcast new signals that will boost the robustness of services and help fine-tune their positioning accuracy by eliminating the ionospheric errors. INSET: Overcoming GPS Signal Interference.