Number 165 (Story #4), February 17, 1994 by Phillip F. Schewe and Ben Stein THE GLOBAL POSITIONING SYSTEM (GPS) can determine latitude and longitude for any spot on Earth with an uncertainty of only 10 meters; the distance between two points hundreds of km apart can be determined to within 1 cm. GPS does this by relaying timing signals from a network of satellites, each carrying an atomic clock, to a receiver (sometimes a hand-held device) which calculates the position from the relative time delay of the signals. In an essay in the January 1994 Physics Today, MIT physicist Daniel Kleppner uses GPS as a case study for demonstrating why science is a good investment. He recounts the slow, painstaking march of scientific and technological advances---e.g., hydrogen-maser clocks, microelectronics, high- speed data processing---that culminated in GPS.