A Very-Short-Baseline Time Transfer Experiment Using Two Geodetic-Quality GPS Receivers and Carrier Phase Techniques
NATIONAL PHYSICAL LAB TEDDINGTON (UNITED KINGDOM)
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For over 10 years primary timing laboratories have used signals from the Global Positioning System GPS to compare atomic timescales. The method used, the common-view of GPS satellites, does not use the GPS system for its full potential. Recently, there has been much interest in using geodetic-quality GPS receivers for time transfer. The result has been a substantial improvement in the precision of the resulting time and frequency transfers. In this paper, a detailed account is presented of common-clock comparisons made at NPL between a recently purchased Ashtech Z12-T receiver and an older Allen Osborne Associates AOA TTR-4P receiver. Data collected from these geodetic-quality GPS receivers were processed using analytical software developed at NPL. Direct comparisons are made between the two receivers using both PI and P2 coded signals, and Ll and L2 phase measurements. Measured ionospheric delays were obtained from both receivers and compared. Multipath is one of the major causes of errors in the surveying applications of geodetic GPS receivers. Code-phase differences were used to estimate the magnitude of code multipath present in both receivers. The principal sources of error present in the common clock measurements are discussed and possible improvements are considered. The future direction of geodetic time transfer work at NPL is also outlined.
- Test Facilities, Equipment and Methods