# Estimating BDS-3 Satellite Differential Code Biases with the Single-Frequency Uncombined PPP Model
## 5 Conclusions  
This study endeavors to estimate BDS-3 satellite DCBs utilizing the SF uncombined PPP model. The experimental setup involved the utilization of BDS-3 B1 observations from a network of 25 globally distributed stations during the month of March 2023. Independently estimated satellite DCBs at each station unveiled latitude-dependent variations in accuracy. The RMS of the absolute offsets in satellite DCB estimation spanned from 1.31 ns to 6.81 ns at stations situated in low-latitude regions, suggesting relatively lower stability. In contrast, stations located in mid- and high-latitude regions predominantly exhibited RMS values below 2 ns, indicating improved stability and accuracy compared to their low-latitude counterparts. Moreover, the stations located in the Northern Hemisphere exhibit a relatively higher level of consistency in the RMS values compared to those in the Southern Hemisphere. To mitigate the influence of regional characteristics on ionospheric modeling and address the disparities in modeling accuracy across different regions, a strategy is employed where ionospheric observables acquired from a number of selected stations are used to obtain a final satellite DCB value for each satellite. When utilizing estimates from all 25 stations, the RMS of the absolute offsets in satellite DCBs estimation remained consistently below 0.8 ns throughout March 2023, with a monthly average of 0.68 ns, exhibiting better accuracy compared to the satellite DCBs estimation by single receiver. By excluding 8 low-latitude stations and utilizing the data from the remaining 17 stations to estimate the final satellite DCBs, a notable improvement in accuracy was observed. The RMS of the absolute offsets in satellite DCBs estimation was further reduced to below 0.63 ns, with a monthly average of 0.49 ns. This process helps to minimize the biases and uncertainties introduced by localized ionospheric effects, ensuring a more balanced and reliable estimation of the satellite DCBs. This improvement in accuracy highlights the effectiveness and necessity of excluding low-latitude stations in enhancing the accuracy of satellite DCB estimates, especially when employing SF receivers for this purpose. The SF receiver offers a cost-effective solution for estimating satellite DCBs; however, the SF uncombined observations is susceptible to the influence of ionospheric variability, which substantially impacts the accuracy of satellite DCB estimates. To address this issue, it is strongly advised to employ observations acquired from an extensive network of uniformly distributed monitoring stations situated in mid- and high-latitude regions, particularly when aiming to accurately estimate satellite DCBs. Given the cost-effectiveness advantage of SF receivers, it indeed becomes feasible to augment the density of the ground network by deploying affordable SF receivers. This strategic approach not only fulfills the demands of space-atmosphere studies but also serves as a substantial enhancement to observational capabilities for various other GNSS applications. Conceptualization, [PERSON] and S.G.; methodology, [PERSON]; software, S.G.; validation, S.G. and D.L. ; formal analysis, [PERSON]; investigation, S.G.; resources, D.L. ; data curation, S.G.; writing--original draft preparation, S.G.; writing--review and editing, [PERSON]; visualization, J.W. ; supervision, J.W. ; project administration, J.W. ; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 41504024. Not applicable. The ISG stations observation files, precise orbits and clocks products can be downloaded from the IGS Data Center of Wuhan University ([[ftp://igs.gnsswhu.cn/pub/](ftp://igs.gnsswhu.cn/pub/)]([ftp://igs.gnsswhu.cn/pub/](ftp://igs.gnsswhu.cn/pub/)) (accessed on 1 May 2023)).