**4. Conclusions**

In this study, a PC-based optical and SAR image registration algorithm ROS-PC is proposed to address the matching difficulties caused by complex nonlinear radiation differences and speckle noise.

We designed a novel feature detector named UMPC-Harris, comprising an overlapping block strategy, Harris on multi-moment of the PCM, and the vote strategy to obtain uniformly distributed keypoints and increase the repeatability rate of the keypoints. The experimental results on simulated images demonstrated that the proposed UMPC-Harris method achieved a good performance in keypoints detection. The proposed HOSMI descriptor is constructed using the histograms of the PC orientation on the multi-scale MIM. The image registration experiments prove that the ROS-PC is robust to nonlinear radiation variations of optical and SAR images and it can tolerate a small amount of rotation and scale changes.

**Author Contributions:** Conceptualization, L.W. and M.S.; methodology, L.W.; software, L.W.; validation, L.W., M.S., and J.L.; formal analysis, L.W. and M.S.; writing—original draft preparation, L.W.; writing—review and editing, L.C. and G.M.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported in part by the National Key R&D Program of China under Grant NO. 2017YFC0822402.

**Acknowledgments:** The authors would like to thank Dr. Xiang for providing the OS-SIFT and Pair A images and the Beijing Institute of Radio Measurement for providing the SAR images in Pair B-H.

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