The multi-GNSS fusion makes positioning more reliable and accurate. Considering the signal difference of different systems, GPS + Galileo + QZSS + BDS tightly combined double-difference model (TCDDM), including function and stochastic model, is proposed. The proposed model fully utilizes the overlapping frequency signals of various systems, and thus to enhance positioning model when DISBs are known beforehand. The observations of 3 ultra-short (1~10 m) and 3 short (4~10 km) baselines were processed by self-programming software, and the single-epoch single-frequency RTK performance using different system-combined models was evaluated by ambiguity-fixed correctness rate (ACR) and positioning accuracy. It demonstrated that three- and four-system TCDDM were superior to their corresponding loosely combined double-difference model (LCDDM) for ACR and positioning accuracy especially at high cut-off elevation. Moreover, four-system TCDDM had the best RTK performance obtaining average ACRs of 100% and 97.6% even at 25° cut-off elevation for ultra-short and short baseline, respectively.

多重GNSS融合使定位更加可靠和准确。针对不同系统的信号差异，提出了GPS + Galileo + QZSS + BDS紧密结合的双差分模型（TCDDM），包括功能模型和随机模型。所提出的模型充分利用了各种系统的重叠频率信号，从而在事先知道DISB时增强了定位模型。通过自编程软件处理了3个超短（1〜10 m）和3个短（4〜10 km）基线的观测值，并通过不同的系统组合模型评估了单周期单频RTK性能歧义固定正确率（ACR）和定位精度。结果表明，三系统和四系统TCDDM在ACR和定位精度方面优于其相应的松散组合双差模型（LCDDM），尤其是在高截止高度时。此外，即使在超短基线和短基线的25°截止高度处，四系统TCDDM的RTK性能也最佳，获得的平均ACR分别为100％和97.6％。