**6. Conclusions**

This paper explores the use of calibration targets with known positions to refine the inaccurate transmitter/receiver positions and thus enhance target localization accuracy in MPR systems. We start our research by evaluating target localization CRLB in the presence of calibration targets, which justifies the potential of calibration targets in enhancing localization accuracy. Then, in order to fulfill this potential, a novel closed-form solution was designed for target localization using BR measurements from the unknown target as well as the calibration targets. The proposed solution was shown both analytically and numerically to attain the CRLB under some mild conditions, and verified to outperform existing methods in terms of localization accuracy. Furthermore, from the view of engineering practice, if the employed calibration targets are off-the-shelf, such as the commercial aircrafts broadcasting an ADS-B signal, the use of calibration targets would bring little added cost or complexity to the MPR system, but could bring a significant enhancement to target localization accuracy.

**Author Contributions:** Conceptualization, Y.Z. (Yongsheng Zhao); methodology, Y.Z. (Yongsheng Zhao) and D.H.; software, Z.L.; validation, Y.Z. (Yongjun Zhao); formal analysis, Y.Z. (Yongsheng Zhao); writing—original draft preparation, Y.Z. (Yongsheng Zhao); writing—review and editing, Y.Z. (Yongsheng Zhao); supervision, D.H.; project administration, C.Z.; funding acquisition, D.H. and Y.Z. (Yongjun Zhao).

**Funding:** This research was supported by National Natural Science Foundation of China, grant number 61703433. **Conflicts of Interest:** The authors declare that they have no conflict of interest to this work.
