Time Dissemination

Various timed systems are being kept within narrow tolerances of the USNO Master Clock. The LORAN chains covering North America had been kept within about 100 nanoseconds (ns) rms, whereas the overseas chains had larger tolerances, the largest in the case of the Mediterranean chain (1 microsecond).

The Navy Navigation Satellite System (NNSS, also known as TRANSIT ) ceased broadcasting valid navigation messages and timing on December 31, 1996 after 32 years of service.

The Global Positioning System (GPS) , with the correction given in the navigation message (A0 and A1), is typically within 15 ns rms with selective availability removed. Including selective availability, observed with a single frequency receiver, the rms error has been about 70 ns, with a maximum error of 291 ns. These measurements include all available satellites with a 13-minute observation per pass. By obtaining the small residual difference between UTC(USNO,MC) and UTC from the Automated Data Service (ADS) of the USNO, a near real time access to UTC is, therefore, possible via the GPS at the level of accuracy given above. By averaging over all available satellite passes per day, a fixed station with a cesium frequency standard can increase this precision to below 10 ns with appropriate filtering. The obtainable accuracy will usually be limited by the stability and calibration of the local antenna-receiver delays.

For highest accuracy, the USNO has extended the use of its two-way satellite time transfer instrumentation. Regular time transfers have been continued with the National Institute of Standards and Technology (NIST) in Boulder, Colorado, with the NRC in Ottawa, Canada, and with the AMC , the USNO Alternate Master Clock in Colorado Springs, Colorado. Since 1992, experiments have also been conducted with the Technical University in Graz, Austria, and with OCA in Grasse, France. An additional high precision time reference station has been established on the island of Oahu, Hawaii, and initial two-way time transfers have been started with that station. Some problems with the spread-spectrum modems have limited the obtained precision of these measurements to about 3 ns. The mobile Earth station has been used to make relative delay calibration between USNO and several other sites.

The instrumentation at the USNO consists currently of two 4.5-meter VERTEX antennas, the mobile Earth station, one VSAT, and a new "Fly-Away" small terminal that is used for the quick calibration of remote stations because this terminal can be easily transported by air and assembled by one person in a few hours.

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Dr. Demetrios Matsakis, Director, Time Service Dept. (Matsakis.Demetrios@usno.navy.mil)