Time and frequency transfer lies at the heart of the field of metrology. Compared to current microwave dissemination such as GPS [1], optical domain dissemination can provide more than one order of magnitude in terms of higher accuracy [2, 3], which allows for many applications such as the redefinition of the second, tests of general relativity and fundamental quantum physics, precision navigation and quantum communication [4][5][6][7][8]. Although optical frequency transfer has been demonstrated over thousand kilometers fiber lines [2], intercontinental time comparison and synchronization still requires satellite free space optical time and frequency transfer. Quite a few pioneering free space optical time and frequency experiments have been implemented at the distance of tens kilometers [9, 10] at ground level. However, there exists no detailed analysis or ground test to prove the feasibility of satellite-based optical time-frequency transfer. Here, we analyze the possibility of this system and then provide the first-step ground test with high channel loss. We demonstrate the optical frequency transfer with an instability of 10 −18 level in 8,000 seconds across a 16-km free space channel with a loss of up to 70 dB, which is comparable with the loss of a satellite-ground link at medium earth orbit (MEO) and geostationary earth orbit (GEO).

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