Fast cycle slip determination for high-rate multi-GNSS RTK using modified geometry-free phase combination

Abstract

Cycle slip determination plays an important role in continuous real-time kinematic (RTK) positioning. Since cycle slips should be processed at each epoch, a fast determination method is needed for high-speed RTK applications, such as autopilot and airborne positioning, in which case high-sampling-rate GNSS data (such as 50 HZ) are required to be processed in real time. The geometry-free (GF) phase combination is one of the most commonly used combinations to deal with the cycle slips. However, the GF phase combination cannot distinguish the frequency on which cycle slips occur. Therefore, other methods, such as the Hatch–Melbourne–Wubbena (HMW) combination, are usually adopted to determine the frequency and the size of the cycle slips. Since noisy code pseudorange measurements are introduced, it is difficult to identify the small cycle slips, especially when cycle slips occur in multiple satellites and multiple frequencies simultaneously. In this contribution, a modified geometry-free (MGF) phase-only linear combination is proposed to quickly determine cycle slips for high-sampling-rate multi-GNSS RTK. The MGF algorithm utilizes the decimal part of the time difference GF phase combination to determine cycle slips satellite by satellite. It can directly estimate the cycle slips on a specific frequency, rather than a combined cycle slip as in the GF combination. The MGF method is tested against airborne and other kinematic situations. The results show that the MGF method can quickly determine the small cycle slips on each frequency even when all carrier phases suffer from cycle slips simultaneously. Compared with the combined method of GF and HMW and using a 24-h 1-Hz GPS/BDS dataset, the MGF method is 144 times faster and reduces the number of incorrectly repaired cycle slips from 4005 to 141.