Global Navigation Satellite System (GNSS) timing is a main service function. Each GNSS has its own time performance specification. However, a uniform timing performance assessment methodology and its outcomes do not exist. Firstly, the timing performance specifications of each GNSS are analyzed. Then, time transfer accuracy is considered as the key GNSS timing performance indicator. Secondly, an assessment method for the Coordinated Universal Time (UTC) published by the Bureau International des Poids et Mesures (BIPM) and UTC kept by the National Time Service Center of China (UTC(NTSC)) is proposed. Thirdly, the uncertainty budget of the assessment method is given. The timing performances of BDS, GPS, GLONASS, and Galileo are assessed and compared. The results show that the time transfer accuracy of BDS, GPS, GLONASS, and Galileo was 13.8 ns, 4.5 ns, 16.8 ns, and 4.2 ns, respectively, in 2021, meeting their performance requirements specified by GNSS (30 ns or 40 ns). Meanwhile, the assessment results of GPS and Galileo are much better than requirements, while the assessment results of BDS and GLONASS show fixed time offset and can still be improved further. If the local reference time of GNSS users can be connected with UTC, this assessment method can be used.

In response to the need for on-orbit performance evaluations of atomic clocks in low-orbit spacecraft, we integrated the operational conditions of the atomic clock ensemble onboard the China Space Station with the dual-mixing time difference measurement payload. In addition, we proposed a dual-verification framework that consists of on-orbit and ground-based simulations. Furthermore, this study elucidates the coupling mechanism between the thermal environment encountered during the orbit of the space station and the performance fluctuations of atomic clocks and their corresponding on-orbit measurement systems. The experimental results revealed that the periodicity of the on-orbit comparison data was correlated with periodic variations in the thermal environment of the space station orbit. Specifically, temperature change resulted in a phase-difference fluctuation of 1.51E–11 °C, indicating that temperature control plays a pivotal role in improving the on-orbit stability of atomic-clock measurement systems. This study provides a reference for the temperature control of on-orbit measurement systems and the on-orbit assessment of atomic clocks in low-orbit spacecraft.

随着科学技术的进步和发展，许多基础前沿领域要求时间比对的精度为几十皮秒甚至更高.空间站上的原子钟系统比地面钟性能更优，但传统的共视时间比对方法应用于空间站共视存在一定的局限性.本文基于广义相对论分析了精度为几十皮秒的空间站共视时间比对原理，考虑了所有的皮秒级以上的时延项；结合空间站共视时间比对原理，仿真分析了空间站对于中国几大主要地理城市的可见性，分析结果表明部分地区存在共视时间比对的工作盲区.结合理论和仿真研究了空间站轨道误差对传统共视时间比对方法的影响，研究结果表明传统共视时间比对方法不能有效地抵消轨道误差，其对共视时间比对的影响在几百皮秒量级.提出了空间站分时共视时间比对新方法，介绍了该方法的主要原理和优势.通过仿真实验验证了新方法的有效性，能够实现几十皮秒精度的两地面站远距离共视时间比对，同时解决了传统共视方法的工作盲区问题.

There will be better atomic clock system and micro-wave time comparison link in the near earth space station, like Chinese Space Station and European ACES(Atomic Clock Ensemble in Space) system, than those in the GNSS(Global Navigation satellite System) satellites. Therefore, the space station common-view (CV) will realize more accurate time comparison than GNSS CV in theory. But due to the orbit characteristic of the space station, there are some limitations if traditional GNSS CV time comparison method is applied to the space station. In order to solve these problems, the GNSS CV method is optimized and the method that is appropriate for the space station is proposed. First, the basic CV principle is analyzed, and the delay items which are needed to be considered for GNSS and space station CV are compared and analyzed. Then, the differences between GNSS and space station CV are studied, and the influences of orbit error on these two CV methods are analyzed in detail. The GNSS CV method is optimized to be fit for the space station next. Finally, the performance of the optimized method is validated by simulated experiments. The simulation results show that the space station time comparison accuracy of several tens of picoseconds can be obtained by the optimized method. Furthermore, the problem of CV blind area is solved by the optimized method effectively.