A method with differential inter-system bias calibration and wide-lane linear combinations for multi-system GNSS RTK positioning under severely constrained observational conditions

Calibrating differential inter-system bias (DISB) in global navigation satellite system (GNSS) real-time kinematic positioning can effectively reduce the problem of insolvability in a severely constrained observational (SCO) condition. However, even though the problem can be overcome, poor satellite geometry is often inevitable. Meanwhile, the impact of significant multipath effects on ambiguity resolution (AR) is hardly reduced by the degraded satellite geometry. Past studies indicate that employing GNSS measurements and wide-lane (WL) combinations in a cascading AR (CAR) can be effective in reducing the multipath effect on AR. Therefore, this study aims to incorporate a unified DISB-calibrated method into a CAR strategy and proposes a WL-assisted DISBfixed (WADF) method. To maintain the same level of solvability as the unified DISB-calibrated method, the WADF method defines a selection of WL combinations to ensure that the number of overlapping frequencies does not decrease. In the analyses, static and kinematic data are given with high cutoff angles (>= 50 degrees) to simulate SCO conditions. Experimental results indicate that, under SCO conditions, apart from the importance of combining multiple systems and implementing DISB calibration, utilizing multiple frequencies and WL combinations is crucial to increase AR performance. Moreover, the AR performance result using kinematic data shows that the WADF method outperforms the unified DISB-calibrated method by 9 % in the percentage of reliable epochs. Meanwhile, the result indicates that the WADF method is effective in achieving AR even in the presence of large measurement noise, including random noise and significant multipath effects.