**5. Conclusions**

A new model we have named "shore-to-air bistatic HF radar" has been proposed for ocean observation. The first-order and second-order scattering coefficients for the shore-to-air bistatic radar are derived using the perturbation method. The scattering coefficient of shore-to-air bistatic HF radar can be reduced to the case of monostatic radar when the scattering angle and the azimuth angle are 90◦ and 180◦ respectively, which is identical to the result in [29].

The scattering mechanism presented herein between the radar electromagnetic waves and the sea surface may be the foundation for shore-to-air bistatic HF radar development and validation. The Doppler spectra are simulated for various operating conditions and sea states based on the proposed scattering coefficients, which may provide a guideline on the extraction of sea state information using shore-to-air bistatic HF radar. The characteristic of the simulated echo spectrum is similar to the characteristic of the Doppler spectrum for the monostatic and land-based bistatic radar, which verifies the correctness of the developed scattering coefficients. In addition, the singularities in the Doppler spectrum for shore-to-air bistatic radar are analyzed using the normalized constant Doppler frequency contours. Further research will be conducted to apply the proposed scattering coefficients to practical situations.

**Author Contributions:** Conceptualization, Z.C. and C.Z.; funding acquisition, Z.C. and C.Z.; investigation, J.L., F.D.; Methodology, C.Z., J.L. and F.D.; project daministration, Z.C. and C.Z.; software, J.L.; supervision, Z.C. and C.Z.; Validation, J.L., F.D. and X.C.; visualization, J.L.; writing-Original draft, J.L.; writing-review&editing C.Z.

**Funding:** This research was supported in part by the National Natural Science Foundation of China under Grant 61871296, Grant 41506201, and Grant 41376182, in part by the National Key Research and Development Program of China under Grant 2017YFF0206404 and Grant 2016YFC1400504, in part by the Public Science and Technology Research Funds Projects of Ocean under Grant No. 201205032 in part by Fundamental Research Funds for the Central Universities under Grant 2042018kf1009.

**Acknowledgments:** The authors would like to thank anonymous reviewers for their valuable comments in improving the quality of this paper.

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