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GPS for Precise Time and Time Interval Measurement


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 (1ms) precision and/or accuracy and time interval (frequency) to better than one part in ten to the ninth (1X10-9). 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 distance.

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 the each satellite and is an integral part of the ground monitor stations. This relationships between clocks and navigation is not unique. It goes back to the eighteenth century when John Harrison (1693-1776) developed this famous clock. Harrison’s clock solved the longitude problem for the Royal Navy by allowing a ship to carry Greenwich time with it to sea. The navigator then determined his own local time. The difference between the navigator’s local time and the Greenwich time, which he was carrying with him, was his longitude difference from Greenwich. The GPS NAVSTAR satellites are similar to the Royal Navy H.M.S. Deptford. They carry a standard reference time onboard. The navigator then uses the difference between his local time and the reference time onboard the satellite to help him determine his position.

The importance of the GPS to the PTTI community can be neither understated nor underestimated. The GPS is and will be the primary means by which time, that is Universal Coordinated Time, U.S. Naval Observatory [UTC(UNSO)], the time scale maintained at the U.S. Naval Observatory and the reference for all timed DOD systems, will be distributed within the DOD. The GPS provides time it the one-way mode (OWM), easily to a precision and accuracy of 100 ns in real-time. With a modest amount of care, it is possible to reach 25 ns. In the OWM, the GPS is considered to be akin to a clock on the wall. The output from the receiver provides time as if looking at a clock on the wall. In addition, the OWM also allows the user to determine the difference between a local clock and UTC(USNO) or GPS time. Correction can be applied to the local clock in real time or after the fact, so that can be set on time to UTC(USNO) within the specifications of the system.

Through the GPS, PTTI users can also compare clock in the common-view mode (CVM) over large distances to a precision and accuracy better than 10 nanoseconds. In the CVM, two users make measurements of their local clock with respect to the same GPS satellite at the same instant of time. If a user differences the values obtained at each site, he or she can determine the offset between the clocks at each site. However, this method requires the exchange of data by at least one of the participants.

The melting-pot method (MPM), which is similar to the OWM and requires an exchange of data with the CVM, also allows clocks at remote sites to be synchronized and, more importantly, to be steered. In the MPM, a control station determines both the remote clock offset and rate from GPS time or UTC(USNO) and its own clock offset and rate from GPS time or UTC(USNO) by some from of regression to the observations of as many satellites as possible during the day. by comparing the two clock offsets and rates with respect to GPS time or UTC(USNO), corrections to the remote clock can be estimated. Then, corrections to the remotely located clock can be sent via a dial-up modem at any desired time. This last mode has the advantage of allowing automatic operation, and it is not dependent upon any one satellite.

The ability to use the GPS in different modes to derive timing information ensures its prominence as a critical contributor to all timed system. However, a word of caution is necessary. Prudent systems engineering requires that adequate and alternate back-up systems for PTTI be factored into the overall design of the system. This point must be emphasized.