[Writer Name]

[Institution name]

[Subject]

[Date]

Global Positioning System

Introduction

The use of radio signals for navigation began in the early twentieth century. Between 1912 and 1915, Reginald Fessenden of Boston, Massachusetts, transmitted radio waves from designated shore-based stations to correct ship chronometers and give mariners a sense of direction. The prospect of greater accuracy came in 1940 when Alfred Loomis of the National Defense Research Council suggested simultaneous transmission of pulsed signals from a pair of precisely surveyed ground stations to ship receivers and use of the difference in arrival times of the two signals to calculate a line of position. Developed during World War II by the Radiation Laboratory at the Massachusetts Institute of Technology, the system Loomis had proposed became known as Loran (LOng RAnge Navigation), and it supported convoys crossing the Atlantic. Subsequent improvements significantly increased Loran accuracy, but the technology was still limited to two dimensions—latitude and longitude. Not until 1960 did Ivan Getting of Raytheon Corporation in Lexington, Massachusetts, propose the first three-dimensional type of Loran system. Intended to solve navigational problems associated with rail-based, mobile intercontinental ballistic missiles, Raytheon's system was called Mosaic (MObile System for Accurate ICBM Control). That system was never developed, however, because a new technology—artificial satellites—provided the basis for more precise, line-of-sight radio navigation.

NAVSTAR

The NAVSTAR (NAVigation System Timing And Ranging) Global Positioning System (GPS) provides an unlimited number of military and civilian users worldwide with continuous, highly accurate data on their position in four dimensions—latitude, longitude, altitude, and time—through all weather conditions. It includes space, control, and user segments (Figure 6). A constellation of 24 satellites in 10,900 nautical miles, nearly circular orbits—six orbital planes, equally spaced 60 degrees apart, inclined approximately 55 degrees relative to the equator, and each with four equidistant satellites—transmits microwave signals in two different L-band frequencies. From any point on earth, between five and eight satellites are “visible” to the user. Synchronized, extremely precise atomic clocks—rubidium and cesium—aboard the satellites render the constellation semiautonomous by alleviating the need to continuously control the satellites from the ground. The control segment consists of a master facility at Schriever Air Force Base, Colorado, and a global network of automated stations. It passively tracks the entire constellation and, via an S-band uplink, periodically sends updated orbital and clock data to each satellite to ensure that navigation signals received by users remain accurate. Finally, GPS users—on land, at sea, in the air or space—rely on commercially produced receivers to convert satellite signals into position, time, and velocity estimates.

Drawing the best concepts and technology from several predecessor navigation systems, engineers synthesized one that became known in the early 1970s as GPS. Those previous efforts began with Transit or the Naval Navigation Satellite System (NNSS), developed in the late 1950s by the Applied Physics Laboratory at Johns Hopkins University in Baltimore, Maryland, and used operationally by both the U.S. Navy and commercial mariners from 1962 to 1996. Also contributing to the mix was the U.S. Naval Research Laboratory's Timation satellite project that began during 1964 to provide very ...