Fig. 1. Free space frequency reference transfer device of Observatoire de Paris^[15]

Fig. 2. Free space phase-stabilized frequency transfer system^[16]

Fig. 3. Schematic of frequency reference transfer of UWA^[18]

Fig. 4. Schematic of frequency reference transfer of KAIST. (a) Schematic of extracting patterns of light^[19]; (b) schematic of optical frequency reference transfer in 18 km free-space link^[20]

Fig. 5. Schematic of time reference transfer of NIST. (a) Schematic of clock synchronization on 12 km link^[23]; (b) schematic of clock synchronization on mobile platform^[26]

Fig. 6. Schematic of optical two-way time-frequency reference transfer of NIST^[29]

Fig. 7. Schematic of optical time-frequency reference transfer in three-node network^[31]

Fig. 8. Schematic of time-frequency reference transfer of USTC. (a) Schematic of simulated ground to satellite link^[32]; (b) schematic of 113 km link time-frequency reference transfer^[33]

Fig. 9. Schematic of time-frequency reference transfer of UESTC^[36]

Fig. 10. Comparison chart of transfer distance and accuracy in each research unit