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The Global Positioning System (GPS) has quickly evolved into the primary system for the distribution of Precise Time and Time Interval (PTTI). This is true not only within the Department of Defense (DOD) but also within the civilian community, both national and international. The users of PTTI are those who maintain and distribute time (epoch) to better than one-millisecond (1 ms) precision and/or accuracy and time interval (frequency) to better than one part in ten to the ninth (1 10 ?9). The GPS is very effective not only in meeting these modest requirements of the PTTI community but also meeting more stringent ones, such as synchronizing clocks to tens of nanoseconds over large distances.
It is not surprising that this is the case. As with all navigation systems, the heart of the GPS is a clock. In the GPS, it controls the transmission of the navigation signals from each satellite and is an integral part of the ground monitor stations. This relationship between clocks and navigation is not unique. It goes back to the eighteenth century when John Harrison (1693 1776) developed his famous clock. [1] Harrison's clock solved the longitude problem for the Royal Navy by allowing a...
