  Time and frequency transfer lies at the heart of the field of metrology.
Compared to current microwave dissemination such as GPS, optical domain
dissemination can provide more than one order of magnitude in terms of higher
accuracy, 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. Although optical frequency transfer has
been demonstrated over thousand kilometers fiber lines, 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
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).
