Global Positioning System
As time measurement technology improves, so does our ability to use that technology. Not since the 14th century, when the pendulum replaced the sundial, have we looked to the skies for state-of-the-art time measurement. Since then, the spring has replaced the sundial and the atomic clock has yet to meet its match. But with the availability of the GPS and its traceability to atomic clocks, we are again looking skyward for the ultimate in precision time measurement.
The Navigation Satellite Timing and Ranging (NAVSTAR) GPS is an all-weather, radio-based satellite navigation system that enables users to accurately determine 3-dimensional position, velocity, and time. It is composed of a constellation of 21 satellites (plus three spares) operating in 12-hour orbits at an altitude of 20,183 km (10,898 nautical miles). The satellites are arranged in six orbits, each orbital plane equally spaced about the equator and inclined at 55 degrees. The system is managed by a master control center and a number of widely spaced monitoring stations. The ground control network tracks the satellites, precisely determines their orbits, and periodicaly uploads data to all satellites for transmission to the user.
Only recently has the U.S. Military allowed civilian use of the GPS. Although mainly intended as a navigation/positioning system, the GPS offers time information traceable to the USNO's battery of cesium atomic clocks. ESE acknowledges the GPS for allowing us to further our commitment to provide accurate, reliable and cost-effective methods for worldwide timekeeping.