# Evaluating Optical Clock Performance for GNSS Positioning
## 7 Conclusions  
This paper overviews the current development of optical clocks in the PNT. Due to the emerging optical clock technology and the ongoing necessity for maintaining or improving GNSS capability, the potential contribution of optical clocks to GNSS positioning is discussed based on the literature review and the stability analysis conducted. Throughout the development of timing devices, optical clocks have been the most stable to date, achieving the ultimate uncertainty of \(10^{-18}\) and beyond [4; 28], which exceeds the preceding atomic clocks by two orders of magnitude in performance. The critical improvement of optical clocks was the source of the oscillator--that is, an ultra-stable laser emitting optical transitions. In contrast, a typical atomic clock configuration has a quartz crystal oscillator generating microwaves. In terms of clock characterisation, we briefly described clock stability, noise components, ADEV measures, and SWAP parameters. It was noted that these characteristics are crucial in evaluating any atomic or optical clocks. Furthermore, in some cases, like satellite missions, both ADEV and SWAP requirements are essential because spacecraft payload criteria are thoroughly considered, whereas for clock-based applications on the Earth, especially in metrology, the SWAP requirement is less essential. With the clock stability analysis, three successfully developed optical clocks, Iodine MTS, CROC, and Sr lattice, were compared to the current GNSS satellite atomic clocks, 439 atomic clocks, onboard 122 satellites of the four main constellations to discuss the optical clock potentiality. Amongst the satellite clocks, the ensemble of two PHM and two  
Figure 5: The standard deviation of GNSS range errors resulting from the clock ADEV values presented with different colours of lines. The upload intervals of navigation data for each constellation are highlighted additionally. Rb atomic clocks onboard Galileo FOC satellites were the most stable atomic clocks with ADEV of 1.2 \(\times\) 10\({}^{-13}\) at 30 s and 1.7 \(\times\) 10\({}^{-14}\) at 900 s. Whereas for the optical clocks, the Sr Lattice optical clock was the most stable having 7.5 \(\times\) 10\({}^{-17}\) and 1.4 \(\times\) 10\({}^{-17}\) stability over 30 s and 900 s, respectively--which showed a few orders of magnitude better stability than the Galileo FOC satellite clocks. Lastly, the optical clock potentiality in GNSS positioning is discussed by evaluating the clock stability contributions to range errors for GNSS PPP real-time and SPP applications. With the PPP application, optical clocks could decrease the clock contribution to the range errors to 1 mm over 30 s, assuming the clock drift and drift rates are flawlessly determined. Regarding the SPP application, which relies on broadcast navigation messages, users can expect 10 times better errors than the Galileo FOC satellite clocks. But considering the clock TRL and SWAP parameters, the optical clocks have not matured yet. However, in the authors' view, the current development of optical clocks is auspicious in GNSS positioning given that its primary purpose of timekeeping is superb; nevertheless, the technology needs to mature further before being deployed on GNSS satellites. Conceptualization, E.B., D.G., S.C., S.Z. and L.H. ; methodology, E.B. ; software, E.B. ; validation, E.B. ; formal analysis, E.B. ; investigation, E.B. ; data curation, E.B. ; writing--original draft preparation, E.B. ; writing--review and editing, E.B., D.G., S.C., S.Z. and L.H. ; visualization, E.B. ; supervision, S.C., S.Z., L.H. and D.G. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The IGS clock products were provided by GeoForschungsZentrum Postdam (GFZ) at [[https://cddis.nasa.gov/archive/gnss/products/mgex/](https://cddis.nasa.gov/archive/gnss/products/mgex/)]([https://cddis.nasa.gov/archive/gnss/products/mgex/](https://cddis.nasa.gov/archive/gnss/products/mgex/)) (accessed on 3 March 2023).