Fig. 1. (a) Satellite–ground comb-based optical TWTFT diagram. The satellite-to-ground communication is done via the downlink, while the ground-to-satellite communication is done through the uplink. z is the optical propagation distance. θ is the point-ahead angle. (b) Separate transmitting and receiving paths of the optical transceiver. MIR, a point-ahead mirror, is used to produce a point-ahead angle.

Fig. 2. Experimental setup. (a) Nashan and Gaoyazi located 113 km apart. Two independent free-space links of 1545 and 1563 signals and a fiber link are established between two transceivers. The core component of the transceiver is an optical bench. In the transceivers, the solid (dashed) green line is the path of the H (V)-polarized 1563 OFC signal. For the 1545 OFC signal, the solid red line is the H-polarized path with the left half blocked, and the dashed blue line is the V-polarized path with the right half blocked. SCA, semicircular aperture; FC, fiber collimator; PBS, polarizing beam splitter; DM, dichroic mirror; MIR, mirror; DMIR, deformable mirror; BE, beam expander; FSM, fast-steering mirror; USL, ultra-stable laser. x is the spatial displacement of the non-reciprocal 1545 link. (b) and (c) Figure of the optical bench in Transceiver A (B).

Fig. 3. (a) Comparing the links with and without T1NR,s by using PSD. (b) Comparing the links with and without T1NR,s by using TDEV. The error bar is 1σ. For PSD and TDEV, the measured T1NR,s combines the non-reciprocal 1545 link and the reciprocal 1563 link, and the contrast combines the reciprocal 1545 and the 1563 link.

Fig. 4. Fractional frequency instability of comparing two free-space links and comparing the fiber link with the free-space link. T1NR,s,t is constructed by adding a time delay to the non-reciprocal 1545 link. A 0.05 s delay (purple) corresponds to an MEO and a 0.1 s delay (green) corresponds to an GEO, while a 0.3 s delay (light blue) is two times that of a GEO.

Fig. 5. Systematic bias, or fractional offset, with 174 mm spatial displacement and 0.1 s delay of temporal displacement. Each label is led by date and duration. The uncertainty per point is the MDEV value at 4500 s. The red line is the weighted average.

Fig. 6. (a) The 1545 signal is transmitted from the optical transceiver at NS and received by the optical transceiver at GY. The telescope’s lower and upper apertures are alternately blocked. During 0–300 s and 720–960 s, the lower apertures of GY and NS are blocked. During 300–480 s and 960–1200 s, the upper apertures of GY and NS are blocked. No blocking occurs during 480–720 s and 1200–1440 s. (b) The 1545 signal is transmitted from the optical transceiver at GY and received by the the optical transceiver at NS. The lower and upper apertures of the telescope are blocked in the same way. During 0–240 s and 780–1020 s, the lower apertures of GY and NS are blocked. During 480–780 s and 1200–1440 s, the upper apertures of GY and NS are blocked. No blocking occurs during 240–480 s and 1020–1200 s.

Fig. 7. Thermal model of the optical bench at the GY.