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Objectives: With the ultra-high precision time and frequency signals provided by space high precision time and frequency system, series of space-to-ground time and frequency transfer and fundamental physics experiments can be carried out to support high precision validation of relativity and related theories. Methods: First, theoretical derivation and simulation analysis are conducted on the time and frequency transfer algorithm of space laser links. Theoretical formula of space-to-ground laser two way time transfer are reviewed, and the advantages and disadvantages of X-type and Lambda-type approaches are compared and analyzed. Then, the impact of relativistic effects on the conversion between proper time and coordinate time is discussed for the specific orbital height of China space station. Finally, the impact of space-to ground time and frequency transfer discontinuity on the stability evaluation of high-precision atomic clocks onboard space stations is discussed. Results: The results show that: (1) Lambda-type two-way time transfer based on satellite laser ranging can make up the distance difference of first-order Doppler effect on the uplink and downlink, and increase of Sagnac effect can be ignored. (2) The relativistic effect of the conversion between proper time and coordinate time for satellite ground time transfer makes high demands on the velocity accuracy of space station positions, in order to achieve 1×10-18 frequency transfer accuracy, geocentric distance accuracy of 1 dm and speed accuracy of 0.1 mm/s of space station is required; (3) Influenced by the visibility of space stations to the ground, it is not possible to evaluate the medium term stability of atomic clocks with only the observation data of domestic stations satellite for space-to-ground links. Conclusions: High precision navigation solution is important to ensure high-precision time and frequency transfer between space and ground. How to optimize navigation algorithms and use various payloads onboard to achieve high-precision position and velocity solution is a key issue that needs to be studied. [ABSTRACT FROM AUTHOR]

Einstein's general relativity theory provides a successful understanding of the flow of time in the gravitational field. From Einstein's equivalence principle, the influence of the Sun and Moon masses on clocks is given in the form of tidal potentials. Two clocks fixed on the surface of the Earth, compared to each other, can measure the tidal effects of the Sun and Moon. The measurement of tidal effects can provide a test for general relativity. Based on the standard general relativity method, we rigorously derive the formulas for clock comparison in the Barycentric Celestial Reference System and Geocentric Celestial Reference System, and demonstrate the tidal effects on clock comparison experiments. The unprecedented performance of atomic clocks makes it possible to measure the tidal effects on clock comparisons. We propose to test tidal effects with the laboratory clock comparisons and some international missions, and give the corresponding estimations. By comparing the state-of-the-art clocks over distances of 1000 km, the laboratory may test tidal effects with a level of 1%. Future space missions, such as the China space station and FOCOS mission, can also be used to test tidal effects, and the best accuracy may reach 0.3%. [ABSTRACT FROM AUTHOR]

Objectives: Gravitational redshift test using high precision space-borne/sattelite-payload atomic clocks is always a hot topic in physics. With the improvement of the accuracy of atomic clocks, it is possible to use high-precision time-frequency signals to test the gravitational redshift and determine the gravitational potential. Methods: The experiment module of China space station (CSS) is expected in orbit in October 2022, which lays a foundation for high-precision gravitational redshift experiment. The dual-frequency combination method is applied to eliminate the propagation frequency shifts by using an uplink and a downlink with the same frequency and different polarization direction. The gravitational redshift simulation experiment is carried out by using the microwave links of CSS time-frequency comparison system. Results: The results show that the stability of atomic clock achieves 2 × 10−15 / τ, the long term stability reaches 10−18, and the accuracy of gravitational redshift reaches 10−7 .Conclusions: It is 1− 2 orders of magnitude higher than the highest accuracy of gravitational redshift in the world. [ABSTRACT FROM AUTHOR]