## 5 Conclusions and Discussion  
This paper introduces a method of using the B\({}_{1}\) single-frequency carrier phase and pseudorange observations to calculate the relative ionospheric TEC, then reconstructs the algorithm of carrier-phase observation B\({}_{3}\). The quasi-carrier-phase observation B\({}_{3}^{*}\) is reconstructed from the pseudorange and carrier-phase observation of the frequency B\({}_{1}\). The refractive index product is inverted, and the accuracy of the relTEC, the reconstructed excess phase B\({}_{3}^{*}\), and the refractive index product in the single-frequency inversion algorithm is evaluated. On the whole, the reconstructed excess phase Doppler and dual-frequency excess phase Doppler not only have good frequency consistency but also have a strong monotonic correlation. The relative average deviation of the reconstructed excess phase Doppler is less than 1%, and most deviations are less than 0.2%. The relative standard deviation is less than 1%, and most standard deviations are less than 0.4%, indicating that the reconstructed excess phase Doppler quality is good. The refractive index of single-frequency and dual-frequency inversion shows good consistency, and the deviation from  
Figure 8: Deviation degree between single-frequency reconstruction and dual-frequency refractive index products and ERA5 products, (**A**) is occultation event 1 and (**B**) is occultation event 2. ERA5 results is less than 1%. The single- and dual-frequency inversion products have comparable accuracies, and the inversion results are satisfactory. The accuracies of dual- and single-frequency inversion products are on the same order of magnitude, with overall comparable accuracy of products, except for the slight decrease in the standard deviation accuracy of single-frequency inversion products over 25 km. The use of dual-frequency receivers for single-difference or non-difference technology for occultation data processing is maturing. The BDS B1I signal is a high-precision and robust signal that covers the whole world. This paper studies the correctness of the single-frequency occultation inversion algorithm and product availability by taking the occultation observation of the BDS B1I signal by the FY3E satellite as an example. On the one hand, single-frequency occultation processing has the advantage of simple instruments. On the other hand, in practical applications, due to cost considerations or other accidents, the occultation receiver can only rely on single-frequency occultation measurement for GNSS atmospheric occultation detection. At this time, the ionospheric error cannot be eliminated by dual-frequency combination, which requires algorithm verification and accuracy analysis of the single-frequency processing method under current differential technology based on BDS occultation observations. Overall, the development of a single-frequency receiver for occultation data processing using reconstruction methods can reduce satellite-related costs. With the further advancement of global navigation satellite signals, low-cost and fast atmospheric occultation detection data processing will provide more support for meteorological business applications. Conceptualization, Q.D. and X.M. ; methodology, Q.D., X.M. and W.B. ; formal analysis, R.L., M.Y., H.T., X.W., G.T. and P.H. ; resources, Q.D., X.M. and Y.S. ; writing--original draft preparation, R.L., M.Y. and H.T. ; writing--review and editing, Q.D. and X.M. ; supervision, Y.S. All authors have read and agreed to the published version of the manuscript. This work is supported by the Youth Cross Team Scientific Research Project of the Chinese Academy of Sciences (JCTD-2021-10) and in part by National Natural Science Foundation of China 42074042 and 42104032. Not applicable. The datasets analyzed and generated during the current study are available from the corresponding author on reasonable request.