Precise estimation of satellite differential code biases (DCBs) plays a crucial role in precise ionospheric modeling, positioning, and timing. Due to the rank deficiency, a constraint or a datum is required in order to separate the satellite DCBs from the receiver DCBs. A common practice is to impose a zero-mean constraint on all the visible satellites. However, datum selection is affected by satellite replacement and variation of the DCBs. As a result, the long-term variations of current DCB products vary significantly. Taking the DCBs of SVN 44 (PRN 28) as a reference, we analyzed the long-term variations of DCBs over a period of 20 years, between 2000 and 2019. Based on this reference, the results indicate that the change of the zero-mean datum is responsible for the variation of current DCB products. The datum change is attributed to the satellite replacement as well as the discontinuities and their variations. We found that discontinuities for the same satellite vehicle reach 1.8 ns, which is related to satellite changes announced in the Notice Advisory to Navstar Users message and to flex power. The magnitude of the DCBs depends on the satellite type. DCBs for Block IIR-A and IIR-M satellites are close to each other, while DCBs of Block IIR-B satellites are approximately 5 ns larger and DCBs for the Block IIF are 8 ns smaller. In addition, the satellite biases between GPS P1 and C1 are also briefly examined, and the results show that they are also affected by the satellite replacements and discontinuities. However, the satellite bias differences between P1 and C1 for different satellite types are minor.

卫星差分代码偏差（DCB）的精确估算在精确的电离层建模，定位和定时中起着至关重要的作用。由于等级不足，为了将卫星DCB与接收器DCB分开需要约束或数据。通常的做法是在所有可见卫星上施加零均值约束。但是，基准的选择受卫星替换和DCB变化的影响。结果，当前DCB产品的长期变化显着。以SVN 44（PRN 28）的DCB为参考，我们分析了2000年至2019年20年间DCB的长期变化。基于此参考，结果表明零值的变化平均数据负责当前DCB产品的变化。基准变化归因于卫星替换以及不连续性及其变化。我们发现同一颗卫星飞行器的不连续性达到1.8 ns，这与在Navstar用户通告公告中宣布的卫星变化以及弹性功率有关。DCB的大小取决于卫星类型。IIR-A块和IIR-M块的DCB彼此接近，而IIR-B块的DCB大约大5 ns，IIF块的DCB减小8 ns。此外，GPS之间的卫星偏差 DCB的大小取决于卫星类型。IIR-A块和IIR-M块的DCB彼此接近，而IIR-B块的DCB大约大5 ns，IIF块的DCB减小8 ns。此外，GPS之间的卫星偏差 DCB的大小取决于卫星类型。IIR-A块和IIR-M块的DCB彼此接近，而IIR-B块的DCB大约大5 ns，IIF块的DCB减小8 ns。此外，GPS之间的卫星偏差还对P 1和C 1进行了简要检查，结果表明它们也受到卫星替换和不连续性的影响。但是，对于不同类型的卫星，P 1和C 1之间的卫星偏差差异很小。