Since 2015, the new generation global BDS system, i.e., BDS-3, has started its development with five experimental satellites demonstration system and has announced its initial global service officially on December 27, 2018. Among the various characteristics to be analyzed for the new generation BDS satellites, the differential code bias (DCB) is of special attention since that it has a direct dependence on the new signals, i.e., B1C and B2a, and it is one of the most intricacy problems in the ionosphere sensing and positioning with multi-GNSS and multi-frequency observations. To take the full capability of the triple-frequency BDS signals, this paper proposed a new method for the DCB estimation in which the undifferenced uncombined observations are processed in PPP mode. In addition, with the intention to estimate all the unknowns, including the DCB, in a single filter, the DESIGN (deterministic plus stochastic ionosphere model for GNSS) method is applied for the ionospheric delay constrains in this method. In the formula derivation, special attention is given to the DCB and clock parameters due to different frequencies for B1I/B1C, etc. Then, the efficiency of the new method is assessed with observations of 23 iGMAS stations capable for BDS triple-frequency tracking and 21 IGS stations capable for GPS triple-frequency tracking during DOY 001 to 090, 2019. Moreover, the traditional DCB estimation method by employing the geometry-free (GF) combination with the ionospheric delay removed by global ionosphere map product is also performed for comparison purpose. The experimental results suggest that by using the undifferenced uncombined solution, rather than the GF combination, the BDS-2 DCB on B1IB2I and B1CB3I can be improved, especially for the MEO satellites. Regarding to the DLR products, the undifferenced uncombined DCB solution presents a RMS of 0.32 ns and 0.27 ns for B1IB2I and B1CB3I, respectively. Concerning the BDS-3 satellites DCB, it is GF combination that performs better by a factor of $$12.7\%$$ 12.7 % and $$15.2\%$$ 15.2 % for B1CB2a and B1CB3I, respectively. This is mainly due to the fact that the undifferenced uncombined DCB solution is sensitive to the limited precision of the BDS-3 orbit and clock. This conclusion is further confirmed by the improvement in the GPS DCB solution with the new method. Compared with the GF combination solution, the STD for daily repeatability improves from 0.088 to 0.061 ns and 0.119 to 0.090 ns for satellite on C1WC2W and C1WC5X, respectively, by using the undifferenced uncombined model.

中文翻译：

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基于三频观测的无差分非组合模型BDS-3差分码偏估计

2015年以来，新一代全球北斗系统北斗三号系统开始研制五颗实验卫星示范系统，并于2018年12月27日正式宣布全球首发。在一代 BDS 卫星中，差分码偏差 (DCB) 受到特别关注，因为它直接依赖于新信号，即 B1C 和 B2a，它是多信号电离层传感和定位中最复杂的问题之一。 -GNSS 和多频观测。为了充分利用三频BDS信号，本文提出了一种新的DCB估计方法，其中以PPP模式处理无差异的未合并观测值。此外，为了估计所有未知数，包括 DCB，在单个滤波器中，DESIGN（用于 GNSS 的确定性加随机电离层模型）方法应用于该方法中的电离层延迟约束。在公式推导中，由于B1I/B1C等频率不同，对DCB和时钟参数给予了特别关注。 然后，通过对23个具有北斗三频跟踪能力的iGMAS台站的观测来评估新方法的效率。 2019年DOY 001-090期间21个能够进行GPS三频跟踪的IGS站。此外，还进行了传统的DCB估计方法，即采用几何自由（GF）结合全球电离层地图产品去除电离层延迟的方法进行比较目的。实验结果表明，通过使用无差未组合解，而不是 GF 组合，B1IB2I 和 B1CB3I 上的 BDS-2 DCB 可以改进，特别是对于 MEO 卫星。对于 DLR 产品，无差未组合 DCB 解决方案的 RMS 分别为 B1IB2I 和 B1CB3I 的 0.32 ns 和 0.27 ns。对于 BDS-3 卫星 DCB，GF 组合的性能更好，B1CB2a 和 B1CB3I 的性能分别提高了 12.7 % 和 15.2 % 12.7 % 和 15.2 % 15.2 %。这主要是由于无差分未组合DCB解对BDS-3轨道和时钟的有限精度很敏感。新方法对 GPS DCB 解决方案的改进进一步证实了这一结论。与 GF 组合解决方案相比，C1WC2W 和 C1WC5X 上卫星的每日重复性 STD 分别从 0.088 提高到 0.061 ns，从 0.119 提高到 0.090 ns，