receiver would calculate that it was 186 miles from that satellite.) By checking its time against the time of three satellites whose positions are known, a receiver could pinpoint its longitude, latitude, and altitude.
The method just described would require that both the satellites and the receiver carry clocks of remarkable accuracy. However, having a receiver pick up a signal from a fourth satellite allows the receiver to get by with a relatively simple quartz clock—like that used in most watches. Once the receiver has made contact with four satellites, the system takes over and computes its position almost instantaneously.
For the system to work, the receiver has to know exactly where the satellites are and the satellites have to be able to keep reliable and extraordinarily accurate time. Accuracy is ensured by having each satellite carry four atomic clocks, the most accurate timing devices ever made. Reliability is ensured by the satellites' 11,000-mile-high orbits, which put them far above the atmosphere and keep them moving in very predictable trajectories. The Department of Defense monitors the satellites as they pass overhead twice a day and measures their speed, position, and altitude precisely. That information is sent back to the satellites, which broadcast it along with their timing signals.
A Tool to Study Nature
GPS itself was born as a military tool, but the atomic clocks that made GPS possible originated in basic research shortly before the Second World War. It was then that scientists found that high-precision techniques developed to study fundamental atomic structure could be used to make an atomic clock. Their inspiration had to do not with ultraprecise navigation, but rather with the dream of making a clock good enough to study the nature of time itself—in particular, the effect of gravity on time predicted by Einstein's theory of gravity and known as the gravitational red shift.
Until the late 1920s, the most accurate timepieces depended on the regular swing of a pendulum. They were superseded by more accurate clocks based on the regular vibrations of a quartz crystal, which could keep time to within less than one-thousandth of a second per day. Even that kind of precision, however, would not suffice for scientists who wanted to study Einstein's theory of gravity. According to Einstein, a gravitational field would distort both space and time. Thus, a clock on top of Mount Everest, for instance, was predicted to run 30 millionths of a second per day faster than an identical clock at sea level. The
1960
Ramsey and students Kleppner and Goldenberg operate hydrogen maser at Harvard University.
1960-1965
Rubidium optically pumped clock is introduced. Cesium frequency standards are installed in most international time-standard laboratories.
1964-1965
First position fix from a Transit satellite is computed aboard Polaris submarine.
1967
Transit system is made available to civilian community.
1968
Standards of a Defense Navigation Satellite System are defined.
1973
Development of Navstar GPS is approved by the Department of Defense.
1974
First GPS test satellite, from Timation program, is launched to test rubidium clocks and time-dissemination techniques.
1977
Test satellite incorporating principal features of later GPS satellites, including first cesium clocks in space, is launched.
1978-1985
Ten prototype GPS satellites are launched, built by Rockwell International.
1989-1993
Series of 24 satellites are launched at about 6 per year. Final satellite is launched on June 26, 1993.
1996
White House announces that a higher level GPS accuracy will be available to everyone.
