Although most GPS receivers use the time codes that are transmitted by the satellites for both position determination and for time transfer, it is also possible to use the carrier itself for similar purposes. Since the frequency of the carrier is about 1000 times higher than the frequency of the C/A code, carrier-phase methods have much greater resolution in principle. In order to realize this potential advantage, it is usually necessary to analyze the received data after-the-fact using post-processed ephemeredes of the satellites and detailed models of the ionosphere and troposphere.
The job of post-processing has been greatly simplified by the availability of precise orbits and other parameters calculated by the International GNSS Service, and by the development of analysis software by a number of groups. Most of our work has been done using the GIPSY-OASIS software developed at JPL (e-mail: hurst [at] cobra.jpl.nasa.gov).
We have conducted a number of tests of this technique, including short-baseline experiments at NIST, longer-baseline tests between NIST in Colorado and the US Naval Observatory in Washington, DC, and a very-long baseline test between NIST and the Physikalisch Technische Bundesanstalt (PTB) in Braunschweig, Germany. The latter experiment is particularly significant, since it can provide a direct comparison between the primary frequency standards located at NIST and PTB.
Our results to date show that the method is capable of providing frequency comparisons with a fractional uncertainty of about 2 x 10-15 using one day of averaging. This uncertainty is smaller than the combined uncertainties of the primary frequency standards in NIST and PTB, and it will therefore support a comparison of these primary frequency standards without degrading their capabilities with the noise of the transfer system.
Our current work is focused on reducing the noise in the measurement process. This noise arises from a number of sources, including the noise in the clocks of the reference stations, problems with cycle slips (in which the phase-measurement in the receiver responds to a noise pulse by jumping an integral number of cycles of the carrier), and inadequacies in the models of the ionosphere and the troposphere.
Kristine Larson and Judah Levine, Time transfer using the phase of the GPS Carrier, IEEE Trans. On Ultrasonics, Ferroelectrics and Frequency Control, vol. 45, pp. 539-540, 1998.
Kristine Larson and Judah Levine, Carrier-Phase Time Transfer, IEEE Trans. On Ultrasonics, Ferroelectrics and Frequency Control, vol. 46, pp. 1001-1012, 1999.
See the Time and Frequency publication database for a complete list of Judah Levine's publications.
For questions or more information contact Judah Levine at jlevine [at] boulder.nist.gov.