## 9 Conclusions  
Aiming at the urgent requirement for high-precision tracking and reliable catalog of non-cooperative targets in cislunar space, in this paper, a BeiDou-3 Inter-Satellite Link and Connected Element Interferometry(CEI)-based method is proposed for effective monitoring and measurement of high-value cislunar space targets. The main work of this paper is as follows:  
(i) A non-cooperative cislunar space targets measurement method based on CEI and BDS-3 inter-satellite link signals is proposed, with its general scenario and specific flow.  
(ii) The mathematical models of the proposed method are put forward, which mainly includes four parts: dynamical constraint equations for non-cooperative targets in cislunar space; BDS-based interplanetary link for irradiation of non-cooperative targets; transmission loss equation of BDS-3 inter-satellite link signal in cislunar space; CEI-based precision measurements of targets in cislunar space.  
(iii) Based on the previous analysis, the measurement errors of the entire process has been analyzed, followed by the error correction equations.  
(iv) To give the feasibility demonstration of the proposed method, the link budget analysis and performance evaluation are presented.  
(v) Simulation results demonstrate that the proposed method could have potential in achieving long-term cataloging, effective tracking, and high-precision measurement of high-value unidentified targets in cislunar space, indicating a promising potential in the increasingly critical cislunar space situational awareness missions in the future.  
Further works could be focused on the application of the proposed method to actual unidentified Earth-Moon space target tracking missions.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/rs15153744/s1](https://www.mdpi.com/article/10.3390/rs15153744/s1)]([https://www.mdpi.com/article/10.3390/rs15153744/s1](https://www.mdpi.com/article/10.3390/rs15153744/s1)), Text S1-S13: \"Chain-BD3-lunarprobe-CEI Access AER\" to \"Satellite-lunarprobe-1-To-Satellite-GEO1 AER\"; Excel X1 to X4:\"SN0-link budget-1\" to \"SN0-link budget-4\".
* [PERSON] and [PERSON] conceived the idea; [PERSON], [PERSON] and [PERSON] conducted the experiments; [PERSON] and [PERSON] completed the original manuscript; [PERSON] and [PERSON] analyzed the results; [PERSON], [PERSON], [PERSON] and [PERSON] checked the original manuscript. All authors reviewed the submitted manuscript. All authors have read and agreed to the published version of the manuscript.
* This work was supported in part by the Major Science and Technology Projects of Beijing under Grant: Z181100002918004, and in part by the National Key Laboratory of Science and Technology on Space Microwave under Grant: HTKJ2021 KL504012.
* **Data Availability Statement:** Some of the data are publicly available at [[http://www.beidou.gov.cn/xt/gfx2/](http://www.beidou.gov.cn/xt/gfx2/)]([http://www.beidou.gov.cn/xt/gfx2/](http://www.beidou.gov.cn/xt/gfx2/)) (accessed on 1 May 2023) and [[http://www.csno-tarc.cn/system/constellation](http://www.csno-tarc.cn/system/constellation)]([http://www.csno-tarc.cn/system/constellation](http://www.csno-tarc.cn/system/constellation)) (accessed on 4 May 2023). The data presented in this study are available on request from the corresponding author valuable suggestions to improve the quality of this work.  
**Acknowledgments:** The authors would like to thank the anonymous reviewers and editors for their valuable suggestions to improve the quality of this work.  
**Conflicts of Interest:** The authors declare no competing interests.