Zhang Xiangbo, Guo Ji, Hu Yonghui, Zhao Dangli, He Zaimin
University of Chinese Academy of Sciences, Beijing 100039 China.
National Time Service Center, Chinese Academy of Sciences, Xi'an 710600, China.
Sensors (Basel). 2020 May 4;20(9):2622. doi: 10.3390/s20092622.
Time and frequency transfer through global navigation satellite system (GNSS) precise point positioning (PPP) based on carrier-phase measurements has been widely used for clock comparisons in national timing laboratories. However, the time jumps up to one nanosecond at the day boundary epochs of adjacent daily batches lead to discontinuities in the time transfer results. Therefore, it is a major obstacle to achieve continuous carrier phase time transfer. The day-boundary discontinuities have been studied for many years, and they are believed to be caused by the long-term pseudorange noise during estimation of the clock offset in the daily batches and are nearly in accordance with a Gaussian curve. Several methods of eliminating the day-boundary discontinuity were proposed during the past fifteen years, such as shift and overlapping, longer batch processing, clock handover, and ambiguity stacking. Some errors and new noise limit the use of such methods in the long-term clock stability comparison. One of the effective methods is phase ambiguity fixing resolution in zero-differenced PPP, which is based on the precise products of wide-lane satellite bias (WSB) provided by the new international GNSS Service (IGS) Analysis Center of Centre National d'Etudes Spatiales (CNES) and Collecte Localisation Satellites (CLS). However, it is not suitable for new GNSS, such as the Beidou Satellite System (BDS), GALILEO, and QZSS. For overcoming the drawbacks above, Multi-GNSS Experiment (MGEX) observation data of 10 whole days from MJD 58624 to 58633have been network processed by batch least square resolution. These observations come from several ground receivers located in different national timing laboratories. Code and carrier phase ionosphere-free measurements of GPS and BDS satellites are used, and the time transfer results from network processing are compared with PPP results provided by Bureau International des Poids et Mesures (BIPM) and used for international atomic time (TAI) computation (TAIPPP) and universal time coordination (UTC). It is shown that the time offsets of three different time links are almost continuous and the day-boundary discontinuities are sharply eliminated by network processing, although a little extent of day-boundary discontinuities still exist in the results of UTC(USNO)-UTC(PTB). The accuracy of time transfer has been significantly improved, and the frequency stability of UTC(NTSC)-UTC(PTB) can be up to 6.8 × 10 on average time of more than one day. Thus, it is suitable for continuous multi-GNSS time transfer, especially for long-term clock stability comparison.
基于载波相位测量的全球导航卫星系统（GNSS）精密单点定位（PPP）进行时间和频率传递，已在国家授时实验室的时钟比对中得到广泛应用。然而，相邻日时段在日边界历元处高达1纳秒的时间跳变会导致时间传递结果出现间断。因此，实现连续载波相位时间传递是一个主要障碍。日边界间断问题已研究多年，人们认为它是由日时段内时钟偏差估计过程中的长期伪距噪声引起的，且近似符合高斯曲线。在过去十五年中，提出了几种消除日边界间断的方法，如移位与重叠、更长时段处理、时钟交接和模糊度叠加。一些误差和新的噪声限制了这些方法在长期时钟稳定性比对中的应用。一种有效的方法是零差PPP中的相位模糊度固定解算，它基于法国国家空间研究中心（CNES）和法国卫星定位公司（CLS）的新国际GNSS服务（IGS）分析中心提供的宽巷卫星偏差（WSB）精确产品。然而，它不适用于新的GNSS，如北斗卫星系统（BDS）、伽利略系统和准天顶卫星系统（QZSS）。为克服上述缺点，对从MJD 58624到58633共10整天的多GNSS实验（MGEX）观测数据进行了批量最小二乘解算的网络处理。这些观测数据来自位于不同国家授时实验室的多个地面接收机。使用了GPS和BDS卫星的无电离层码和载波相位测量数据，并将网络处理得到的时间传递结果与国际计量局（BIPM）提供的PPP结果进行比较，用于国际原子时（TAI）计算（TAIPPP）和协调世界时（UTC）。结果表明，三种不同时间链路的时间偏移几乎是连续的，通过网络处理显著消除了日边界间断，尽管在协调世界时（美国海军天文台）-协调世界时（德国物理技术研究院）的结果中仍存在一定程度的日边界间断。时间传递的精度得到了显著提高，协调世界时（中国国家授时中心）-协调世界时（德国物理技术研究院）的频率稳定性在一天以上的平均时间内可达6.8×10。因此，它适用于连续的多GNSS时间传递，尤其适用于长期时钟稳定性比对。
