Remote atomic clock synchronization via satellites and optical fibers

Abstract

In the global network of institutions engaged with the realization of International Atomic Time (TAI), atomic clocks and time scales are compared by means of the Global Positioning System (GPS) and by employing telecommunication satellites for two-way satellite time and frequency transfer (TWSTFT). The frequencies of the state-of-the-art primary caesium fountain clocks can be compared at the level of 10 -15 (relative, 1 day averaging) and time scales can be synchronized with an uncertainty of one nanosecond. Future improvements of worldwide clock comparisons will require also an improvement of the local signal distribution systems. For example, the future ACES (atomic clock ensemble in space) mission shall demonstrate remote time scale comparisons at the uncertainty level of 100 ps.To ensure that the ACES ground instrument will be synchronized to the local time scale at PTB without a significant uncertainty contribution, we have developed a means for calibrated clock comparisons through optical fibers. An uncertainty below 50 ps over a distance of 2 km has been demonstrated on the campus of PTB. This technology is thus in general a promising candidate for synchronization of enhanced time transfer equipment with the local realizations of UTC .Based on these experiments we estimate the uncertainty level for calibrated time transfer through optical fibers over longer distances. These findings are compared with the current status and developments of satellite based time transfer systems, with a focus on the calibration techniques for operational systems.

Optical atomic clocks

“…It is based on a frequency comparison between two remote optical clocks via optical fibers (see Sec. III.B) or free-space microwave (Salomon et al, 2001Levine, 2008Piester et al, 2011;Delva et al, 2012;Fujieda et al, 2014) or optical (Fujiwara et al, 2007;Djerroud et al, 2010;Exertier et al, 2013;Giorgetta et al, 2013) satellite links to provide a direct height difference measurement between two remote locations. Alternatively, a mobile clock (operating during transport) together with careful modeling of its speed and geopotential trajectory can be used.…”

Synchronization of Distant Optical Clocks at the Femtosecond Level

“…In these applications, the local optical time scale would be either compared against or synchronized to a distant time scale via terrestrial or satellite free-space links. Existing rf-based techniques can support time transfer over free-space links but are limited to 10-to 100-ps accuracy and ∼1-ps stability, 100 to 1000 times worse than optical oscillators [21][22][23]. The highest performance rf system to date is planned for the ACES mission and will support 300-fs timing stability at 300-s integration and <6-ps timing stability over days from ground to space [24,25].…”

Precise UTC dissemination through future telecom synchronization networks

“…Frequency synchronization is also required by mobile system to facilitate handover between base stations and meet the regulatory requirement [12]. Currently, the base stations are usually equipped with the global navigation satellite system (GNSS) equipment, which can support sub-microsecond accuracy for general receivers [13], and reach a accuracy of less than 5 ns with respect to UTC with dual-frequency receivers for time-keeping purposes [14]. Although most synchronization requirements can be easily satisfied by using GNSS signals, the reception of GNSS antenna needs a good view of sky.…”