*3.1. Comparison with Onshore Case*

Comparisons of the RCSs for floating-based monostatic and bistatic radars with those for onshore monostatic and bistatic cases are shown in Figures 4–6. It should be noted that only sway is considered in this part. From Figure 4a, the locations of the Bragg peaks of the onshore monostatic and bistatic HF radars are ω*mB* = ± 2*gk*<sup>0</sup> and ω*bB* = ± 2*gk*<sup>0</sup> cos φ0, respectively. It is apparent that the locations of the Bragg peaks of the bistatic HF radar are closer to zero frequency and the amplitudes are lower than those of the monostatic case. This is because a cos φ<sup>0</sup> term exists in Equation (47). From Figure 4b, it is seen that sway can result in some additional peaks symmetrically appearing in both monostatic and bistatic RCS curves. The amplitudes of these sway-induced peaks are generally lower than those of the Bragg peaks and the locations are respectively at ω*mB* + *n*1ω<sup>1</sup> and ω*bB* + *n*1ω<sup>1</sup> for the floating-based monostatic and bistatic cases. However, the locations of the Bragg peaks remain unchanged and the amplitudes are slightly lower than those of onshore cases.

**Figure 4.** Simulated first-order radar cross sections (RCSs), (**a**) for onshore monostatic and bistatic cases; (**b**) for floating-based monostatic and bistatic cases with sway.

**Figure 5.** Simulated second-order RCSs, (**a**) for onshore monostatic and bistatic cases; (**b**) for floating-based monostatic and bistatic cases with sway.

**Figure 6.** Simulated total RCSs containing first- and second-order RCSs, (**a**) for onshore and floating-based monostatic cases; (**b**) for onshore and floating-based bistatic cases. The black arrows represent additional sway-induced peaks.

As mentioned before, the second-order RCS mainly contains the hydrodynamic and electromagnetic contributions. For onshore bistatic radar, the locations of the hydrodynamic and electromagnetic peaks are respectively given as [15]

$$
\omega\_{hd} = \pm \sqrt{2} \omega\_{bB\prime} \tag{57}
$$

and

$$
\omega\_{\rm ad} = \pm 2^{3/4} \sqrt{\frac{\left(1 \pm \sin \phi\_0\right)^{1/2}}{\cos \phi\_0}} \omega\_{b\rm B}. \tag{58}
$$

For onshore monostatic case, the corresponding locations can be obtained by imposing φ<sup>0</sup> = 0 in Equations (57) and (58). The amplitudes of the second-order RCS for the onshore bistatic case are lower than those of the onshore monostatic case, as shown in Figure 5a. This is because there exists a cos φ<sup>0</sup> term in Equation (48) compared with the monostatic second-order RCS. Theoretically, additional sway-induced peaks will appear in the second-order RCS curves at frequencies ω*hd* + *n*1ω<sup>1</sup> and ω*ed* + *n*1ω<sup>1</sup> for the floating bistatic case. However, from Figure 5b, those additional sway-induced peaks are imperceptible. Although the effect of sway on the second-order RCS is not apparent, the additional peaks appearing in the first-order RCS curve may raise the second-order RCS, as shown in Figure 6.
