**5. Conclusions**

By combining the advantages of coast-based bistatic HFSWR and shipborne monostatic HFSWR, a coast–ship bistatic HFSWR system has potential in anti-electronic interference and expanded detection range by fully exploiting the advantage of the flexibility of a shipborne platform. To investigate the characteristics of the first-order sea clutter spectrum, and the related blind area and its influence on target detection, the theoretical formulas for a coast–ship bistatic HFSWR system were derived. Moreover, simulations of the first-order sea clutter spectrum under different radar parameters and different shipborne platform velocities were analyzed. From the simulation results and its influence on target detection, it can be concluded that the Doppler shift of both the first-order sea clutter and the moving target were affected mainly by the velocity of the moving platform and the bistatic angle in a coast–ship bistatic HFSWR. Broadening of the first-order sea clutter spectrum will cause a blind area for target detection. When the shipborne platform was anchored, the broadening ranges of the right sea clutter spectrum and left sea clutter spectrum were symmetrical, and the widths of the right sea clutter and left sea clutter were equal. When the shipborne platform was navigating, the broadening ranges of the first-order sea clutter spectrum on the left and right were asymmetrical, and their widths were not equal, owing to the combined influence of the bistatic angle and the platform motion. In addition, the broadening range of the first-order sea clutter spectrum of a coast–ship bistatic HFSWR increased with the increase of platform velocity in most cases. At the same platform velocity, both the

broadening range of the first-order sea clutter spectrum and the width of the sea clutter blind area of a coast–ship bistatic HFSWR changed with radar frequency, and the width of the first-order sea clutter of a coast–ship bistatic HFSWR was less than that of a shipborne monostatic HFSWR. Under the condition of higher platform velocity, although the detection blind area caused by sea clutter was wider, the echo amplitude of sea clutter was lower, and the lower SNR was beneficial for target detection. Although the comparative relationship between the amplitude of the left first-order spectrum and the right first-order spectrum changed under different wind conditions, the range of broadening of the first-order sea clutter spectrum remained unchanged. The analysis of measured experimental data revealed that the broadening range of the first-order sea clutter and of the frequency shift of a moving target are consistent with theoretical and simulation results. The findings of this study further the understanding of the theoretical formulas and measured spectrum data of the two types of coast-ship bistatic HFSWR.

It should be noted that under the condition of a moving platform, irrespective of whether a monostatic shipborne HFSWR or a coast–ship bistatic HFSWR is used, the first-order sea clutter will be broadened markedly in the channel spectrum. In general, beam data rather than channel data are used in target detection to obtain a better SNR. Therefore, the influence of sea clutter broadening within the beam data of coast–ship bistatic HFSWR should be considered in the process of motion compensation or sea clutter suppression for target detection, and related research will be undertaken in the future.

**Author Contributions:** Conceptualization, Y.J.; methodology, Y.J. and C.Y. (Chao Yue); validation, W.G., and H.S.; formal analysis, J.L.; investigation, Y.W., C.Y. (Chao Yue) and W.G.; resources, Y.J. and C.Y. (Changjun Yu); Data Curation: W.G. and J.L.; writing—original draft preparation, Y.J.; writing—review and editing, Y.W. and M.L.; visualization, H.S.; supervision, J.Z. and M.L; project administration, Y.J.; funding acquisition, Y.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R & D Program of China (grant number 2017YFC1405202); and the National Natural Science Foundation of China (grant number 61671166).

**Acknowledgments:** The authors thank the anonymous reviewers for their comments and suggestions that have helped to improve the quality and the readability of this article. We thank James Buxton MSc from Liwen Bianji, Edanz Group China (www.liwenbianji.cn./ac), for editing the English text of this manuscript.

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