On the accuracy of two-way satellite time and frequency transfer: A study of triplet closures

Two-way satellite time and frequency transfer has become an important technical component in the process of the realization of International Atomic Time and also for comparisons among the best primary frequency standards. This paper contains results of studies aimed at improving the knowledge of the uncertainty for true time transfer between remote sites. Our study addresses mostly the collection of so-called time transfer triplets between sites 1, 2, and 3. Systematic deviations from zero in the combination (1-2) + (2-3) + (3-1) would point to incomplete understanding and handling of systematic offsets in the individual time transfers. We show that such offsets exist, but that they are only in specific cases significant (up to about 2 ns), and we discuss possible causes.