Optical clock technologies include a variety of different implementations, all having their own assets and drawbacks. When being based on an absolute and known frequency, such a system is often also referred to as a “frequency standard”. Please note that we refer to a “clock” for a system which delivers a countable frequency signal in the radio frequency range, and where a time signal can be derived from, as it is e.g., the case in GNSS applications. We then detail our developments with respect to iodine-based optical frequency references for applications in space and present the mission concept COMPASSO, a proposed DLR in-orbit verification mission on the International Space Station (ISS). In the following, we first give a short overview of optical clock technologies for space, together with their current technology development status. The LEO satellites additionally carry mid- to long-term stable absolute optical clocks based on Doppler-free spectroscopy of molecular iodine for the definition of the system time. In the current baseline, 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. All Kepler satellites carry optical frequency references which are intra-system synchronized using bi-directional optical inter-satellite links together with time and frequency transfer techniques. 2019) which foresees 24 satellites in medium-earth orbits (MEO, in three orbital planes, similar to the current Galileo system), together with 6 satellites in low-Earth orbit (LEO). One example is the proposed Kepler architecture (Giorgi et al. 2006), on the other hand, optical clock technologies-in combination with optical inter-satellite links-enable new GNSS architectures. On the one hand, optical clocks could back-up or replace the currently used microwave clocks (Droz et al. Optical clocks surpass the performance of the currently used GNSS microwave clocks by several orders of magnitude. While becoming more and more widespread technology in and outside laboratories on Earth, also space applications-including GNSS-can benefit from the recent advancement of optical technologies. 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. We introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references. 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. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. 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. 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. Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies.
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