Characterization and evaluation of GPS PPP techniques for optical clock comparisons

The suitability of an improved GPS time and frequency transfer technique, Integer Precise Point Positioning (IPPP), for the frequency comparison of optical clocks on the 1e−16 level was investigated. An optical fiber link established between Physikalisch-Technische Bundesanstalt (PTB) and Laboratoire national de métrologie et d’essais - Système de Références Temps-Espace (LNE-SYRTE), which due to its superior stability and accuracy serves as the ground truth, was used to characterize the PPP and IPPP links between both institutes. For two different GPS stations at LNE-SYRTE, the agreement between the IPPP link and the optical fiber link was (−0.34 ± 2.66)e-16 and (−0.23 ± 0.57)e−16 for an averaging time of 1.2e6 s and 0.9e6 s respectively. Direct remote frequency comparisons of 171-Yb ion clocks developed at PTB and National Physical Laboraty (NPL) via IPPP links were performed. The impact of frequent dead time due to the limited duty cycle of the clocks on the statistical uncertainty of the clock comparison via GPS link was studied. A method to optimize the statistical uncertainty of the clock comparison was applied that uses flywheel oscillators to bridge the frequent dead time. The average fractional frequency deviation of the clocks based on the electric quadrupole transition in the 171-Yb ion was y(PTB)−y(NPL)=(−1.46 ± 0.55)e−15 including the systematic uncertainties of the optical clocks. A systematic frequency offset, not accounted for in the systematic uncertainty budgets was observed. For the clocks based on the electric octupole transition, the average fractional frequency deviation was y(PTB)−y(NPL)=(−0.1 ± 3.1)e−16, no systematic frequency offset was observed