Accurate TWSTFT Time Transfer with Indirect Links

The conventional wisdom suggests that a direct Two-Way Satellite Time and Frequency Transfer (TWSTFT or TW) time link should result in a smaller uncertainty than that of an indirect TW link over the same baseline [12]. This is why all Coordinated Universal Time (UTC) TW links are direct and therefore assumed to be the most precise. This paper shows an exception to this logic. Following the use of the Global Positioning System (GPS) all-in-view [1], the UTC links, including Global Navigation Satellite Systems (GNSS) and TWSTFT, became the direct links between Lab(k) and the PTB (Physikalisch-Technische Bundesanstalt, Germany), which is the single pivot laboratory of the international UTC network. Currently, the daily variation (diurnal) in TW differences is the dominant source of instability in TWSTFT. This, at least in the case of the inner-European network, may be significantly reduced by an indirect link through a third intermediate laboratory in the USA, acting as a relaying laboratory [2, 3]. In this study, we examined the direct and indirect Europe-Europe TW links with data collected by SATRE modems and using the NIST (National Institute of Standards and Technology) or the USNO (US Naval Observatory) laboratory as an intermediate laboratory. We applied the Time deviation (Tdev), triangle closures, and the gains obtained by comparing with the results of the GPS Precise Point Positioning (GPSPPP) [4] and the TW software-defined radio (SDR) receiver [5, 6, 7] as indicators to verify the improvements in the indirect links versus the direct links. Considerable improvements were obtained by a factor of two or three. On the same TW baselines, we further investigated the TW SDR indirect links and gains continued to be observed. In addition, the rapidly developing new time and frequency transfer techniques, such as optical fibres, are often not linked directly with the present pivot laboratory, the PTB. Therefore, indirect links or the multi-pivot UTC time transfer network configuration for future UTC generation is meaningful and has to be reconsidered. Finally, one of the most important characteristics of the indirect link is that it does not require new equipment or additional measurements. Only minor modifications were required to the software and this has already been carried out in the BIPM UTC/TAI computation software package, Tsoft.