ION GNSS 2012
Session E5: Precise Point Positioning 1

Title: Real-time Clock Jump Detection and Repair for Precise Point Positioning
Author(s): F. Guo, X. Zhang, Wuhan University, China
Date/Time: Friday, September 21, 2012, 11:26 a.m.
Room: 209/210 (NCC)

Most GPS receivers use inexpensive quartz clocks. The frequency stability of the quartz clocks is on average two orders worse than that of atomic clock in 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 by periodically 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 dangerous to neglect the impact of clock jumps. Failures in properly detecting and accounting for receiver clock reset could sometimes cause unexpected behavior of the GPS software and gross errors in PPP solution. Particularly troublesome would be there that irregular millisecond or microsecond clock jumps exist in GPS observations in different ways. In this study, we first provide a comprehensive classification of clock jumps based on GPS raw data analysis. We summarize and define them three categories, i.e. type I (clock jump generates code discontinuity), type II (clock jump generates phase discontinuity), and type III (clock jump generate code and phase discontinuity simultaneously). Influence of different clock jump on GNSS data preprocessing and PPP solution is investigated in detail. It is concluded that clock jump of type I and type II are prone to cause the failure of gross error and cycle slip detection algorithms, result in repeated re-initialization or even nonconvergent solution, which leads to gross errors in PPP solution. According to the nature of clock jump, a simple but robust method is proposed for real-time clock jump detection and repair. Numerous validation tests with different practical GNSS data show that the method is applicable to all types of clock jumps. By applying the propose method, all clock jumps presented in GPS data can be identified and repaired accurately, and re-initialization in PPP will not triggered by receiver clock jumps any more, which results in significant improvement of PPP accuracy and reliability.

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