Optical clock technologies for global navigation satellite systems

Thilo Schuldt,Evgeny Kovalchuk,Sven Herrmann,Ahmad Bawamia,Matthias Lezius,Andreas Wicht,Kai Voss,Markus Oswald,Tim Blomberg,Klaus Döringshoff,Martin Gohlke,Claus Braxmaier,Markus Krutzik,Jan Wüst,Achim Peters

doi:10.1007/s10291-021-01113-2

Abstract

Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies. Especially optical clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies—in combination with optical inter-satellite links—enable new GNSS architectures, e.g., by synchronization of distant optical frequency references within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized setups have been developed, showing frequency instabilities at the 10–15 level for averaging times between 1 s and 10,000 s. We introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references.

Highlights

Over the last decades, optical clock technologies evolved, recently demonstrating frequency instabilities at the 10–18 level for integration times of a few thousand seconds (Ushijima et al 2015; McGrew et al 2018)
The satellites are equipped with frequency references based on optical resonators, providing the required high short-term stability where the round-trip time of the light within one orbital plane is relevant
Technology development with respect to transportable setups has been initiated (Koller et al 2017; Cao et al 2017; Brewer et al 2019; Hannig et al 2019), and a compact setup of a 88Sr lattice clock has been realized, where space-related design criteria have been considered (Bongs et al 2015; Origlia et al 2018). Such optical clocks require several lasers, a vacuum chamber and a cavity pre-stabilization of the clock laser to achieve their outstanding frequency instability enabled by millihertz linewidth transitions

Summary

Introduction

Optical clock technologies evolved, recently demonstrating frequency instabilities at the 10–18 level for integration times of a few thousand seconds (Ushijima et al 2015; McGrew et al 2018). While becoming more and more widespread technology in and outside laboratories on Earth, space applications—including GNSS—can benefit from the recent advancement of optical technologies. The satellites are equipped with frequency references based on optical resonators, providing the required high short-term stability where the round-trip time of the light within one orbital plane (of about 0.1 s) is relevant. The LEO satellites carry mid- to long-term stable absolute optical clocks based on Doppler-free spectroscopy of molecular iodine for the definition of the system time. We first give a short overview of optical clock technologies for space, together with their current technology development status. We detail our developments with respect to iodine-based optical frequency references for applications in space and present the mission

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