## 6 Discussion, Conclusions and Future Work
### Conclusion and Future Work  
We studied the standard INS+LEO2 algorithm based on the LEO constellation, and the dual-satellite alternate navigation and positioning algorithm of the same orbit and adjacent orbits. Combining our simulation results and analysis, we can see that the standard INS+LEO2 algorithm has high navigation and positioning accuracy, the final convergence solution tends to 0, and the final trajectory coincides with the carrier's flight path, with very small errors. Scenarios with the same orbits and with adjacent orbits were compared, and it is concluded that the switching algorithm for adjacent orbital surfaces is better than the same orbital surface. However, they can effectively suppress the INS divergence problem, and at the same time, they are better than the corresponding MEO and IGSO constellation-based algorithms. Compared with traditional algorithms and some 3-satellite existing advanced algorithms, the performance is also greatly improved. In addition, our algorithm can show good robustness in different multipath or NSLO environments and noise environments, and can provide a reference solution for location services in harsh and challenging environments. Therefore, a dual-satellite alternate switching ranging/INS integrated navigation algorithm for the broadband LEO constellation has a low cost, high engineering ease of use, strong real-time and strong anti-interference ability. It can also complete the navigation and positioning service of a three-dimensional position with only two visible satellites without an altimeter and continuous observation.  
To improve our algorithm's adaptability, robustness and navigation and positioning accuracy, future research will aim to provide an algorithm based on business adaptive switching, anti-multipath or anti-NLOS, and carry out actual verification (if there is an open LEO constellation satellite experimental environment). It will also expand the application to extreme and challenging environments such as inside buildings, under metal roofs or underground. In addition, the research will also focus on addressing the problem of larger errors in the same orbit compared to adjacent orbits, especially the cumulative INS error under a longer switching time and the positioning instability caused by frequent switching. Specific we will consider combining LEO and MEO constellations to enhance the stability of the entire navigation system.  
[PERSON] and [PERSON] conceived the conceptualization and algorithm [PERSON] completed the implementation of the algorithm and the writing of the paper, and supported the writing--review and editing. [PERSON] completed some preliminary simulations, and did preliminary research and summary. [PERSON] provides theoretical guidance and suggestions for revision of the paper. [PERSON] and [PERSON] provide fund support and provide necessary assistance for thesis writing. All authors have read and agreed to the published version of the manuscript.  
FundingFundingFund was supported by National Key Research and Development Program of China (Grant Nos. 2017 YFC1500904, 2016 YFB0501301), National 973 Program of China (Grant Nos. 613237201506), Advance Research Project of Common Technology (No. 41418050201) and Open Research Fund of Southwest China Institute of Electronic Technology (No. H18019).  
Institutional Review Board StatementNot applicable.  
Informed Consent StatementNot applicable.  
Data Availability StatementNot applicable.  
Conflicts of InterestThe authors declare no conflict of interest.