**5. Conclusions**

This paper presented an efficient solution to the problem of tracking an unknown and time-varying number of marine ships from multiple sensors with unknown clutter rate and probability of detection. Particularly, these two unknown parameters are parallel estimated based on the *λ*−CPHD and the *pD*−CPHD filters, then bootstrapped into the cutting-edge GLMB filter. By using the bootstrapping method, the proposed filter utilizes the advantages of the two former estimators in accommodating the unknown backgrounds and reduces the computational cost from tracking algorithm of the latter filter. The effectiveness and correctness of the proposed method are demonstrated in Section 4. From our best knowledge, this is the first principled online algorithm for tracking marine ships via multiple sensors with unknown backgrounds in Doppler measurements. For future work, one of the forcuses would be investigating the combination of multi-scan GLMB [28] and multisensor GLMB [9] filters for multisensor multitarget tracking.

**Author Contributions:** In this work, T.T.D.N. and W.L. contributed in software and discussion. C.-T.D. conceived and planned the conceptualization, data curation, formal analysis, investigation, and methodology. C.-T.D. also designed and directed the project, resources, supervision, validition and visualization. C.-T.D. wrote, revised and edited the original draft as well as the manuscript. All authors discussed the comments from reviewers before sending their responses.

**Funding:** This research was sponsored by the Joint CIPRS-MOET Scholarship.

**Acknowledgments:** The Authors acknowledge the administrative and technical support from their Supervisors and Universities.

**Conflicts of Interest:** The authors declare no conflict of interest in this study.
