Time Transfer Project

Jim R. Ray
United States Naval Observatory,
USA

Using GPS Measurements for Time and Frequency

The IGS/BIPM Pilot Project to Study Accurate Time and Frequency Comparisons using GPS Phase and Code Measurements was authorized in December 1997 jointly by the International GPS Service (IGS) and the Bureau International des Poids et Mesures (BIPM). A Call for Participation was issued shortly afterwards and a working group formed from the respondents began in Spring 1998.

A number of groups have been working for several years to develop the capability of using geodetic GPS techniques for accurate time transfer. A variety of convincing demonstrations have already been performed showing the potential for determining clock differences at the level of a few hundred picoseconds. The current state of maturity of both the global tracking network and data analysis techniques now allows practical applications to be considered. The central goal of this pilot project is to investigate and develop operational strategies to exploit GPS measurements for improved availability of accurate time and frequency comparisons worldwide. This will become especially significant for maintaining the international Coordinated Universal Time (UTC) timescale as a new generation of frequency standards emerges.

Pilot Project Participation and Objectives

Investigators have been invited to participate in one or more of the following areas or to indicate others:

• Deployment of GPS receivers, including new receivers at timing laboratories and upgrading of existing tracking stations for better timing performance.
• GPS data analysis, including novel strategies for analyzing GPS phase and pseudorange observations from a large number of stations, consistent with other IGS products.
• Analysis of instrumental delays to relate GPS-derived clock estimates to external timing standards.
• Time transfer comparisons with simultaneous, independent techniques.

• Accurate satellite clock estimates fully consistent with other IGS products, with sampling at least every 5 minutes and possibly every 30 seconds.
• Accurate station clock estimates for as many IGS sites as possible, fully consistent with other IGS products, together with accurate monitor data to relate some of them to external timing standards.
• An accurate and stable reference ensemble timescale for use in IGS products to improve upon GPS time.