Establishing an ultra-high-precision link for time–frequency comparisons between satellites and ground stations is critically important. This endeavor is fundamental to the advancement of pioneering space science exploration and the development of a robust space-based time–frequency system featuring ultra-high-precision space atomic clocks. In response to the requirements for assessing the long-term stability of high-precision space atomic clocks, we have designed and implemented a satellite–ground microwave time–frequency comparison system and method based on a three-frequency mode. Ground-based experimental results demonstrate that the equipment layer can achieve a satellite–ground time comparison accuracy better than 0.4 ps (RMS), with the equipment delay stability (ADEV) for all three frequencies being better than 8 × 10⁻¹⁸ at 86,400. By leveraging the ground-based experimental results, we constructed a satellite–ground time–frequency comparison simulation and verification platform. This platform realizes ultra-high-precision satellite–ground time–frequency comparison based on the China Space Station (CSS). After correcting various transmission delay errors, the satellite–ground time comparison achieved an accuracy better than 0.8 ps and an ADEV better than 2 × 10⁻¹⁷ at 86,400. This validation of our novel satellite–ground time–frequency comparison system and method, capable of achieving an 10⁻¹⁷ magnitude stability, is not only a significant contribution to the field of space time–frequency systems but also paves the way for future advancements and applications in space science exploration.

High-precision time synchronization is a core technical challenge in modern space science, deep space exploration, and time-frequency metrology. Traditional microwave links struggle to achieve picosecond-level synchronization due to atmospheric delays (tropospheric and ionospheric effects). We propose a novel atmospheric delay correction technique using a triple-frequency microwave link that enables combined cancellation of atmospheric delays. This technique integrates a three-dimensional ray-tracing algorithm with microwave radiometer measurements to jointly correct non-dispersive and dispersive tropospheric delays. Simultaneously, first-and second-order ionospheric errors are precisely eliminated via dual-downlink inversion of the slant Total Electron Content (STEC). To validate this approach, we developed a theoretical analysis and simulation platform that systematically evaluates various factors -including different satellite types, transmission methods (carrier-phase versus pseudocode), and combinations of onboard and ground atomic clocks -to comprehensively verify the correction technique's effectiveness. Experimental results show that this method can achieve two-way satellite-ground time synchronization accuracy better than 0.3 picoseconds (RMS) when Low-Earth Orbit (LEO) satellites and ground stations are both equipped with high-stability optical clocks and carrier-phase transfer is used. The Allan deviation (ADEV) of the transfer link's clock difference is below 5.17 × 10 -15 at an averaging time of 100 s. These results highlight the significant potential of our approach, offering an innovative solution for an all-weather, high-reliability satellite-to-ground time and frequency transfer system.