Objectives  To integrate processing of BDS-2 and BDS-3 satellites observations and to conduct the deeply merged strategy for enhancing the parameter solutions, the differences between BDS-2 and BDS-3 should be considered and analyzed.

  Methods  Given the differences of BDS-2 and BDS-3 satellites, to fully utilize and fuse the observations, the inter-system bias (ISB) model of BDS-2/BDS-3 combined precise orbit determination is analyzed and improved. Firstly, to improve the accuracy and reliability of ISB, the singular value decomposition is introduced to extract the observation information of adjacent epoch. Secondly, the characteristics of ISB are analyzed, which indicts that the ISBs are significantly presented and related with the types of receivers based on the new signals of BDS-3 and BDS-2. Then, the correlations between ISB and orbit parameters are analyzed; the results suggest that the BeiDou orbit accuracy is affected more significantly than that of GPS. Finally, based on the estimated ISB series, the short-term prediction of ISB model is established by spectral analysis, which is inserted into the ultra-rapid precise orbit determination.

  Results  With ultra-rapid orbit determination experiments, the proposed ISB prediction and estimation strategies can improve -0.4–1.0 cm and 0.8–4.1 cm for BDS-2 and BDS-3 orbit one-day overlapping, respectively.

  Conclusions  Therefore, it is meaningful for improving the BDS-2/BDS-3 ultra-rapid combined precise orbit determination with the improved ISB estimation and prediction model.