## 8 Conclusions  
To address the requirement of high-precision ranging/orbit determination and high-capacity data transmission in BDS-3 ISLs, this study innovatively proposes a novel two-way measurement method based on a CCFD system. This method achieves significant ranging accuracy improvement through simultaneous same-frequency transceiving. The main work of this paper is as follows:  
1. Analyzing the self-interference performance of the CCFD system from three aspects: spatial, RF, and digital domains, and evaluating the approximate total self-interference suppression in ISL scenarios;
2. Based on the BDS-3 navigation constellation configuration, constructing a dynamic link constraint model from two aspects of satellite visibility and antenna directivity;
3. Analyzing the link budget of BDS-3 ISL in the whole process, and giving the calculation methods of signal quality and link margin;
4. Providing the fundamental principle, mathematical model, and key error analysis of the inter-satellite two-way measurement method in CCFD, with particular emphasis on the theoretical derivation and analysis of inter-satellite timestamp reckoning accuracy;
5. Through experimental validation and analysis, establishing the BDS-3 ISL scenario using STK software to analyze inter-satellite topology characteristics. Based on the link budget results, verifying the feasibility of the CCFD system and demonstrating that its pseudo-code ranging accuracy achieves over 50% improvement compared with traditional systems, indicating promising application prospects.  
This study systematically demonstrates the application value of the CCFD system in BDS-3 ISLs through theoretical analysis, algorithm design, and experimental verification. The continuous innovation of CCFD technology is expected to advance satellite navigation systems toward higher precision and greater autonomy.  
**Author Contributions:** Conceptualization, H.F. and H.M ([PERSON]).; methodology, Z.Y.; software, H.F. and Y.J.; validation, Q.C. and H.F.; formal analysis, H.F. and Z.Y.; resources, H.F. and H.M. ([PERSON]); data curation, H.F. and H.M. ([PERSON]); writing--original draft preparation, H.F.; writing--review and editing, Z.Y. and T.W.; funding acquisition, H.M ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Data Availability Statement:** Some of the data are publicly available at [[https://celestrak.org/NORAD/elements/](https://celestrak.org/NORAD/elements/)]([https://celestrak.org/NORAD/elements/](https://celestrak.org/NORAD/elements/)) (accessed on 1 November 2024) and [[http://www.beidou.gov.cn/](http://www.beidou.gov.cn/)]([http://www.beidou.gov.cn/](http://www.beidou.gov.cn/)) (accessed on 14 March 2025). The data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** The authors gratefully acknowledge the constructive comments and insightful suggestions provided by the anonymous reviewers and editorial team, which have substantially enhanced the scholarly rigor and presentation quality of this research.  
**Conflicts of Interest:** The authors declare no competing interests.