基于GNSS的通信星座系统的时间同步研究

为近地通信卫星星座系统提出一个将星座各星上时钟同步到地面系统时钟的新技术方案,以GNSS导航系统时钟作为高精度、高稳定的时间基准,利用地面站和星座星上的GNSS接收机基于定位原理测得各自时钟相对于GNSS系统时钟的时差量（秒脉冲相位差）,进一步得出各星时钟相对于地面系统时钟的时差观测量。每个星上时间管理单元利用一个Kalman滤波器估计星地时差模型参数,周期性更新星上时差预报,并据此估计秒脉冲调相时机,以及按照最小调相残差原则估算的调相幅值,并令PPS发生器完成相关调相操作。还设计了上述时统方案在一个典型Walker（24/3/1）星座中的应用研究,并开展了数学仿真和半物理仿真。文章还设计了一个半实物仿真方案,通过数学仿真和半物理仿真,表明该时统方法可将星地秒脉冲相位差自动控制在指定误差范围内,证实其在近地通信卫星星座系统全网时间同步任务中应用的可能性和有效性。

Study on GNSS-based Time Synchronization for a Communications Satellite Constellation System

The paper proposes a GNSS-based time synchronization method and a hardware-in-the-loop simulation scheme for a low-earth communications satellite constellation system. The me-thod takes the GNSS baseline clock as a quite precise and stable baseline clock and uses the GNSS receivers mounted on the ground station and each communication satellite to measure the phase error of the pulse per second （PE of PPS） of the ground/onboard clock with respect to the GNSS baseline clock. The PE of PPS of the ground clock relative to the GNSS clock is sent to each onboard time manager unit （OTMU） and used with the onboard PE of PPS measurements to compute the coarse PE of PPSs of the onboard clock relative to the ground clock. A Kalman filter is designed to estimate the variables of the phase error prediction model which is used to predict the PE of PPS of each satellite’s clock with respect to the ground clock. The predicted value is used by the OTMU to estimate the right time to demand its PPS generator to adjust the phase error and compute the demanded adjusting size according to the principle of the least residual phase error.In the paper the time synchronization method is applied to a classical Walker system （24/3/1） and researched by using a computer-based simulation and a hardware-in-the-loop simulation. The simulated results show that the PE of PPS of the satellite’s clock relative to that of the ground clock is successfully controlled into a desired error range. This indicates the proposed method is a promising time synchronization way for the low-earth communications satellite constellation systems.