Abstract
An inter-system differencing model between two Global Navigation Satellite Systems (GNSS) enables only one reference satellite for all observations. If the associated differential inter-system biases (DISBs) are priori known, double-differenced (DD) ambiguities between overlapping frequencies from different GNSS constellations can also be fixed to integers. This can provide more redundancies for the observation model, and thus will be beneficial to ambiguity resolution (AR) and real-time kinematic (RTK) positioning. However, for Global Positioning System (GPS) and the regional BeiDou Navigation Satellite System (BDS-2), there are no overlapping frequencies. Tight combination of GPS and BDS needs to process not only the DISBs but also the single-difference ambiguity of the reference satellite, which is caused by the influence of different frequencies. In this paper, we propose a tightly combined dual-frequency GPS and BDS RTK positioning model for medium baselines with real-time estimation of DISBs. The stability of the pseudorange and phase DISBs is analyzed firstly using several baselines with the same or different receiver types. The dual-frequency ionosphere-free model with parameterization of GPS-BDS DISBs is proposed, where the single-difference ambiguity is estimated jointly with the phase DISB parameter from epoch to epoch. The performance of combined GPS and BDS RTK positioning for medium baselines is evaluated with simulated obstructed environments. Experimental results show that with the inter-system differencing model, the accuracy and reliability of RTK positioning can be effectively improved, especially for the obstructed environments with a small number of satellites available.
Highlights
With the existing Global Navigation Satellite Systems (GNSS) being modernized or new being ones developed (i.e., Global Positioning System (GPS), GLONASS, BDS, Galileo), many more satellites and frequencies are available for precise positioning
We find that the improvement for the ambiguity resolution (AR) in the real-time kinematic (RTK) initialization period is very small
We can see that the time to first fix (TTFF) for the two models are very close, and just have small difference when a small number of satellites are visible
Summary
With the existing Global Navigation Satellite Systems (GNSS) being modernized or new being ones developed (i.e., GPS, GLONASS, BDS, Galileo), many more satellites and frequencies are available for precise positioning. One is the classical loose combination in which each system uses its own reference satellite with no double differences being formed across systems This model will be called classical differencing in this paper. The other one is the tight combination in which two systems use the common reference satellite and utilizes double differencing across different systems [10] This model will be called inter-system differencing in this paper. Using the inter-system differencing model can help to maximize the redundancy if the inter-system biases (ISBs) in range and phase observations can be handled properly [11] This is essential for positioning in severe observation environments, such as urban areas where signals are blocked by high buildings or trees [8,12]. In the inter-system differencing model, it can still be used when a priori ISB information is available, the positioning model can be strengthened
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