## 3 Materials and Methods
### Experimental Setup  
To verify the feasibility of the proposed method, we designed an indoor ground experiment to evaluate its performance. Because the tropospheric and ionospheric path characteristics in the free-space link were lacking, this experiment aimed to evaluate whether the system had demonstrated sufficient dynamic performance for installation on geostationary orbit satellites affected by orbital perturbations, and what levels of accuracy and stability could be obtained. We required a clock with high stability to maintain the frequency standard and ensure that all RF devices and field-programmable gate arrays (FPGAs) could be referenced to it. Therefore, this study used an active hydrogen maser, whose stability is shown in Figure 7.  
In contrast to the 1.8 m parabolic antenna typically used at earth stations, this experiment used two horn antennas, owing to indoor space limitations. In the laboratory, the antennas were placed 20 m apart. As shown in Figure 8, the specific hardware implementation included a moving platform, a C-band transceiver, and baseband processing devices. The C-band was selected to avoid crosstalk with other satellite communication equipment operating in the laboratory during ground experiments, ensuring minimal interference from other signals. The signal was generated by the PDRO, conveyed through a high-precision phase shifter, and subsequently through a bandpass filter and power amplifier before being transmitted through free space to the transponder. The PA used in the experiment had a gain of 35-40 dB, with a 1 dB compression point output power of \(\geq\)10 dBm. The signal was then converted at the transponder using a phase-locked mechanism, and the downlink frequency was set to 4.5 GHz. At the experimental ground station, the received signal was amplified by LNA and converted from the downlink frequency to an intermediate frequency (IF) using a customized downconverter (D/C), which featured a 4.5 GHz to 1 MHz conversion range. Two identically structured, hardware phase-locked transceivers were placed at the remote and ground ends for the common-mode elimination of additional noise introduced by the devices. Two 1 MHz signals were input into the  
Figure 7: Stability of active hydrogen maser. baseband processing system realized by the FPGA hardware from National Instruments (NI) Corp. The phase difference extracted from the IQ complex signal in the FPGA was sent to the NI PC controller as an error input. Given that the carrier phase is used as the frequency transfer medium, the portion of the uplink and downlink that are proportionally compensated can be effectively managed through a phase delay, i.e., a phase shifter. Consequently, as shown in Figure 9, the generated phase control quantity was proportionally distributed and directly applied to the 5G and 4.5G signals through high-precision analog phase shifters. The phase shift rate was 1 kHz with an accuracy of 0.1\({}^{\circ}\). The clock reference for all the RF devices was an active hydrogen maser in a laboratory setting. The FPGA's global clock network also used this hydrogen maser as the input clock source. Data sampling, including A/D conversion, was precisely synchronized with the reference signal through the design of the phase shifter control process, thereby circumventing the host computer's potential clock interference introduced during signal processing. These approaches ensured that all links within the system were referenced to the same active hydrogen maser. We also designed a dynamic terrestrial experiment to verify whether the system had sufficient dynamic performance to track satellite maneuvers and whether the phase change introduced by the satellite Doppler effect was still proportionally distributed for the uplink and downlink. The synchronous satellite was only theoretically stationary relative to the ground.