## 2 Scheme Framework  
A transponder subsystem connects a satellite's transmitting and receiving antennas. It is one of the most crucial subsystems in the space segment. There are two principal categories of transponders, i.e., single-conversion and regenerative transponders. For GEO communication satellites, the current remote time-frequency comparison uses a single conversion transponder, the principle of which is shown in Figure 1a. As Figure 1a shows, the onboard oscillator's frequency and phase are included in the signal returned to the ground. Consequently, the received signal always contains this unknown variable, i.e., conventional onboard single-conversion transponders introduce additional transmission errors caused by phase jitter and variations in the onboard local oscillator. As Figure 1b shows, this problem can be solved by a phase-locked frequency conversion method, by which the phases of the uplink and downlink signals are always locked. Once the signal traverses the bandpass filter (BPF), after undergoing a frequency division process, one of the signal paths is selected as the reference for the phase-lockeddielectric resonator oscillator (PDRO). The resulting PDRO output is then mixed with the other path of the original signal and filtered using a low-pass filter to generate the down-converted signal transmitted to the ground station. Concurrently, following the frequency division, the 10 MHz or 100 MHz signal generated by the PDRO output serves as the recovered frequency standard from the ground station to the satellite and from there is provided to other systems. Compared with the commonly used communication satellite transponders, the phase-locked transponder can eliminate the GEO satellite's onboard crystal oscillator's impact on the ground-satellite bidirectional link. This effectively solves the onboard crystal oscillator's phase drift problem and combines it with the proposed carrier-phase real-time compensation scheme to realize the precise onboard clock recovery (as discussed below). Figure 2 illustrates the architectural framework of the frequency transfer system based on the proposed GEO communication satellite's phase-locked transponder. The on-orbit payload is servo-controlled and tracked by the main channel over an extended period, with its corresponding ground station, labeled as Station A in the figure, equipped with an ultrahigh-precision atomic frequency standard. A two-way link is established between Station A and the satellite to measure the phase variations across the entire link, evaluated at the carrier signal's phase scale. Using real-time detection, the phase variations in the uplink and downlink are compensated separately to reproduce the terrestrial high-precision atomic frequency standard of Station A on the satellite. The down-converted PDRO of the phase-locked transponder is locked to and reproduces the frequency standard of Station A in real-time. Consequently, any other ground stations within the satellite's coverage area can utilize the proposed transponder as a single conversion transponder without requiring modifications to existing applications. Furthermore, this method supports using any other  
Figure 1: Schematic diagram of a geostationary Earth orbit (GEO) communication satellite transponder. The circle symbol represents a mixer. BPF: bandpass filter; LPF: lowpass filter; PDRO, phase-locked dielectric resonator oscillator. (**a**) Existing communication satellite transponder structure. (**b**) Novel GEO communication satellite phase-locked transponder structure. ground station within the GEO satellite's coverage area equipped with an atomic clock as the main channel, offering significant flexibility and scalability. Moreover, using a compensation approach similar to that of Station A, Stations B and C can each establish a two-way link with the satellite, enabling real-time atomic frequency standard comparisons between the two stations without introducing phase errors from the GEO satellite's oscillator.