Lunar Orbit VLBI Experiment Ground Validation System.

China is planning to launch a relay satellite in the coming years with a payload for the Earth-Moon very-long-baseline interferometer observation research and scientific applications, known as the lunar orbit very-long-baseline interferometry (VLBI) experiment (LOVEX) system. This system will perform the LOVEXs for in the world, and will improve the accuracy of planetary spacecraft orbit determination and the scientific observation capabilities of astrophysics and astrometry. Prior to launch, key technologies of the system must be verified, so we have established a LOVEX ground validation system with similar performance to the LOVEX system. The LOVEX ground validation system consists of a 4.5-m ring-focus antenna, X -band receiver, frequency converter and digital backend, hydrogen maser, and monitoring and control software. Using this system, we have successfully conducted VLBI differential observation experiments with the Shanghai 65 m radio telescope and Beijing 50 m radio telescope. The experimental results show that, after calibration with NRAO150, the standard deviations (STDs) of delay error of Tianwen-1 are within 0.5 ns, consistent with the theoretical timing accuracy, verifying the feasibility of the LOVEX time-delay calibration.