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Time and frequency transfer lies at the heart of the field of metrology.Compared to current microwave dissemination such as GPS, optical domaindissemination can provide more than one order of magnitude in terms of higheraccuracy, which allows for many applications such as the redefinition of thesecond, tests of general relativity and fundamental quantum physics, precisionnavigation and quantum communication. Although optical frequency transfer hasbeen demonstrated over thousand kilometers fiber lines, intercontinental timecomparison and synchronization still requires satellite free space optical timeand frequency transfer. Quite a few pioneering free space optical time andfrequency experiments have been implemented at the distance of tens kilometersat ground level. However, there exists no detailed analysis or ground test toprove the feasibility of satellite-based optical time-frequency transfer. Here,we analyze the possibility of this system and then provide the first-stepground test with high channel loss. We demonstrate the optical frequencytransfer with an instability of $10^{-18}$ level in 8,000 seconds across a16-km free space channel with a loss of up to 70~dB, which is comparable withthe loss of a satellite-ground link at medium earth orbit (MEO) andgeostationary earth orbit (GEO).

Time-Frequency Transfer through a 70 dB Free Space Channel: Towards Satellite-Ground Time Dissemination