Review of GPS Carrier-Phase and Two-Way Satellite Time Transfer Measurement Results between Schriever Air Force Base and the United States Naval Observatory

We are investigating the use of GPS carrier-phase for high accuracy time transfer measurements. We have conducted a long-term time transfer experiment between the United States Naval Observatory (USNO) Master Clock and Alternate Master Clock (AMC). These clocks are located at the USNO in Washington D.C. and Schriever Air Force Base near Colorado Springs, Colorado, respectively. At both locations two-way satellite time transfer (TWSTT) measurements are made nearly every hour and geodetic dual-frequency GPS carrier-phase data are collected every 30 seconds. The data are analyzed using a geodetic software package developed by the Jet Propulsion Laboratory combined with precise satellite orbits determined by the International GPS Service (IGS) global tracking network [1,2]. The radial accuracy of these orbits is approximately 10 cm. Instead of computing the difference between the GPS observables, as is common in highaccuracy geodetic GPS analyses, the software explicitly estimates the receiver and satellite clocks at each data epoch. The technical GPS issues investigated include the importance of carrier-phase ambiguity resolution, troposphere modeling, thermal effects, and multipath noise. Given that the error spectra for the GPS and TWSTT systems are quite distinct, we hope to learn more about each system's accuracy potential.