Real-time Clock Jump Detection and Repair for Precise Point Positioning

Most GPS receivers use inexpensive quartz clocks. The frequency stability of the quartz clocks is on average two orders of magnitude worse than that of an atomic clock in a GPS satellite. The receivers attempt to keep their internal clocks synchronized to GPS time to prevent the clock bias becoming too large. This is done periodically by clock resets (clock jumps). The actual mechanism of how the clock in a particular manufacturer's receiver is adjusted varies from one manufacturer to another. Such events are generally ignored in double-differenced positioning. For Precise Point Positioning (PPP) techniques, it is unwise to neglect the impact of clock jumps. Failure to properly detect and account for receiver clock reset may sometimes cause unexpected behavior of the GPS software and large errors in PPP solution. Particularly troublesome would be when irregular millisecond or microsecond clock jumps exist in GPS observations from different sources. In this study, we first provide a comprehensive classification of clock jumps based on raw GPS data analysis. We summarize and define them in three categories, i.e. type I (clock jump that generates a code discontinuity), type II (clock jump that generates a phase discontinuity), and type III (clock jump that generates both code and phase discontinuities simultaneously). Influence of different clock jumps on GNSS data preprocessing and PPP solution is investigated in detail. It is concluded that clock jumps of type I and type II are prone to cause the failure of gross error and cycle slip detection algorithms, and so result in repeated re-initialization or even nonconvergent solutions, which lead to gross errors in the PPP solution. According to the nature of clock jump, a simple but robust method is proposed for real-time clock jump compensation. Validation tests with different practical GNSS data show that the method is applicable to all types of clock jumps. By applying the proposed method, all clock jumps presented in the GPS data can be identified and repaired accurately, and re-initialization of the PPP will no longer be triggered by receiver clock jumps, which results in significant improvement of PPP accuracy and reliability.