Relativistic Time Transfer for Inter-satellite Links

<p>The Earth&#8217;s gravity potential (geopotential) field plays an important role in geodesy, for instance, it is the basis for defining the geoid and the International Height Reference System (IHRS). In chronometric geodesy, the main challenge for directly measuring geopotential differences between two stations lies in that a reliable link for time comparison is needed. Currently, most satellite links for time comparison are dealt with in the microwave domain, for which the ionospheric and tropospheric effects are major error sources that greatly influence the signal propagation compared to optical space links. Recently, accurate laser time transfer links between satellite and ground stations have already been planned and confirmed, such as Laser Time Transfer (LTT, China) on BeiDou satellites and Tiangong II / China's space station (CSS), Time Transfer by Laser Link (T2L2, French) on Jason-2 mission and European Laser Timing (ELT, Europe) on Atomic Clock Ensemble in Space (ACES). Therefore, in this study, we propose an approach for determining the geopotential difference between two ground atomic clocks based on the Two-way Laser Time Transfer (TWLTT) technique via a space station as a bridge, which could have potential applications in geoscience. This study is supported by the National Natural Science Foundations of China (NSFC) under Grants 42030105, 41721003, 41804012, 41631072, 41874023, Space Station Project (2020)228, and the Natural Science Foundation of Hubei Province of China under Grant 2019CFB611.</p>

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Relativistic Modelling for Accurate Time Transfer via Optical Inter-Satellite Links

Aerotecnica Missili & Spazio ◽

10.1007/s42496-021-00087-1 ◽

2021 ◽

Author(s):

Manuele Dassié ◽

Gabriele Giorgi

Keyword(s):

Orbit Determination ◽

Electromagnetic Waves ◽

Time Synchronization ◽

Relativistic Effects ◽

Satellite System ◽

Time Transfer ◽

Transfer Scheme ◽

Propagation Of Light ◽

Satellite Links ◽

The Impact

AbstractThe DLR Institute for Communication and Navigation is currently working on a new GNSS architecture that enables accurate autonomous inter-satellite synchronization at picosecond-level. Synchronization is achieved via time transfer techniques enabled by optical inter-satellite links (OISLs), paving the way for a system in which space (orbits) and time (synchronization) can be effectively separated, leading to a high level of synchronization throughout the constellation, which in turn greatly improves accurate orbit determination. This is possible provided that relativistic effects are adequately taken into account. This work focuses on a two-way time transfer scheme based on the exchange of time stamps via optical signals, which allows the synchronization of a GNSS satellite system with respect to a defined coordinate time with picosecond-level accuracy. We analyse the impact of relativistic effects in clock offset estimation between optically linked clocks: results show that to achieve synchronization at this level of accuracy it is necessary to account for terrestrial geopotential harmonics up to the third order while the gravitational influence of additional celestial bodies can be neglected. Relativistic delays in the propagation of electromagnetic waves through spacetime are also evaluated. It is shown that for a two-way synchronization method, the Euclidean expression for the propagation of light is sufficient to achieve picosecond synchronization, provided m-level orbit determination of both satellites is available, and the hardware delays are well calibrated to the targeted accuracy. Also, we show how to practically achieve autonomous synchronization via a sequence of pair-wise synchronizations across all satellites of the constellation.

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