Using the Navstar Global Positioning System as a Global Timing System

The Navstar Global Positioning System GPS, although primarily designed for three dimensional position and velocity determination, is uniquely capable of providing highly accurate and stable timing information to users. This capability is derived from the time dejay pseudo ranging concept which farms the basis for GPS navigation. The combination of navigation and timing together in one system will make GPS the Precise Time and Time Interval PTTI source of choice for the forseeable future. Only GPS can supply continuous timing data world wide, in any weather, to a dynamically moving user at a previously unsurveyed site, with submicrosecond accuracy traceable to an established reference. This paper discusses the application of Navstar GPS to the problems of PTTI dissemination. A short review of the GPS concept leads to a detailed description of the implementation of time transfer through Navstar GPS. Time is followed from the U.S. Naval Observatory USNO through the ground control, satellite, and receiving segments of GPS to the users clock system. The three options by which a users system can receive from the GPS receivers, currently under development by the DOD, are defined in detail. The electricaldigitalmechanical interface parameters along with suggested methods for their use are outlined for each option. A detailed error model is also presented for the traceability of UTG GPS to UTC USNO. Although absolute accuracy of UTC GPS provided to a user is specified to be slightly over 100 nanoseconds rms, substantially better accuracies can be easily achieved. By understanding and working around some of the GPS error uncertainties, real time synchronization between stationary users on the same continent can be controlled to within a few nanoseconds, and absolute post processed time offset with UTC USNO measured within 25 nanoseconds or better.