Performance evaluation of real-time carrier-phase GPS time transfer

Abstract

A real-time GPS time transfer system was developed to measure the phase difference between clocks at the sub-nanosecond level. The relative performance of the system was assessed with other contemporary time transfer methods and several issues regarding real-time GPS time transfer were addressed, including the use of a sequential data processor and establishment of a real-time data link. By use of the Kalman estimator, a two-state clock model is incorporated into system dynamics and the classical client–server model is utilized to establish GPS data links via TCP/IP communication. To reduce part of the time transfer errors related to satellite position and clock biases, IGS ultra-rapid products are used. Validation of the real-time GPS time transfer system was performed in two ways, through the use of a common-clock signal and through direct measurement of the phase difference with a time interval counter. Finally, a 5 month long time transfer session was conducted among four national timing laboratories and the results were compared against those obtained with TWSTFT and Circular T issued by the Bureau International des Poids et Mesures.