GNSS多站组网原子钟实时比对算法研究

The performance of GNSS is greatly influenced by the accuracy and stability of atomic clocks.The remote realtime monitoring of atomic clocks in the monitoring stations plays an important role in GNSS. The frequency accuracy and frequency stability of atomic clocks have reached to the order of E-13 or even better, the precision of realtime monitoring should be better than nanosecond correspondingly. Two way satellite time and frequency transfer(TWSTFT) is expensive and interrupted in time because it needs to rent satellite channel. Precision of satellite comnnon view time transfer is five nanoseconds approximately. Precise point positioning(PPP) relies on precise orbit and clock products, which lead to seven hours’ delay. This reaserch promotes an algorithm of GNSS multi-stations networking for atomic clocks realtime comparison ，whose precision can reach sub-nanosecond order.The researching contents includes: the model and algorithm design of GNSS networking solution based on ultra rapid orbit prediction,anomaly detections of clock difference using robust estimation theory,the construction of equivalent weight functions basing the assumption of normal distribution to eliminate the influence of anomaly and promote monitoring accyracy,and the model of smoothing window and smoothing factors to denoise clock difference and optimize results of medium and short term frequency stability of atomic clock.The key methods and models have great reference importance in time difference monitoring and precise timing service of the BeiDou system.

原子钟是导航系统的核心，其准确度和稳定度直接影响系统性能，监测站原子钟远程实时监测对导航系统具有重要意义。目前原子钟的频率准确度和日频率稳定度已经达到了E-13和E-15量级，要求原子钟实时监测精度达到亚纳秒量级。卫星双向比对需租用通信卫星信道，成本高且难以连续实施；卫星共视比对只能实现5纳秒左右的精度；PPP比对依赖于精密轨道和钟差产品，存在约17个小时延迟。为此，本项目拟提出基于GNSS的原子钟实时比对多站组网算法，实现亚纳秒量级的远程实时时间比对能力。研究内容包括：基于超快速预报轨道的GNSS网解模型构建与算法设计；引入抗差估计理论进行钟差异常探测处理，采用正态分布统计量构造等价权函数抑制异常观测，提高监测准确性；构建钟差数据加窗平滑计算模型并引入平滑因子进行钟差降噪，以期改进原子钟中短期频率稳定度评估水平。本项目的研究成果对北斗系统监测站钟差监测以及精密授时具有重要参考价值。

针对传统GPS PPP时间比对算法依赖精密星历产品、时间延迟较长和实时性差等问题，提出了一种多站组网实时时间比对算法。采用IGU超快速星历产品作为解算输入条件，利用多站联测增加多余观测，将测站钟差和卫星钟差作为未知数统一解算。实验结果表明，比对结果与IGS最终钟差的一致性达到了0.3ns以内，比对结果的天频率稳定度优于2.5×10-15。在钟差参数求解中引入了抗差估计方法，给出了详细计算方法，并通过钟差计算验证了抗差方法的有效性。同时考虑到原子钟的测量噪声会被引入到频率稳定度评估过程，尤其是对中短期频率稳定度的影响，通过对原子钟的数学模型和噪声特性进行分析，提出了原子钟频率稳定度评估的加窗平滑噪声处理方法，通过对时差观测序列合理的加窗设计和平滑处理，抑制测量噪声对评估结果的影响。实验结果表明，利用铷钟观测序列的100点平滑可以有效地提高短期频率稳定度结果（100s~1000s）的可信度。本项目的研究成果对北斗系统监测站钟差监测以及精密授时具有重要参考价值。