## 4 Conclusions  
A hybrid DGTDPO method is proposed for accelerating the calculation of broadband scattering multi-scale and multi-object composite scatters. The hybrid method divides the composite target into a DGTD region and an asymptotic region. The DGTD method is used for the multi-scale and multi-object region, which needs to be solved precisely, and the TDPO algorithm is utilized to calculate the electrically large rough sea surface quickly. As a result of the TDPO method, the shadow region and free space do not need to discretize in a large space; it greatly reduces the computational memory requirement and improves the computational speed. The numerical results show that the NRMSD of the DGTD method in the time domain, frequency domain and spatial domain can reach below 0.0685--that is, the accuracy of DGTDPO for multi-scale and multi-target regions is good enough. Compared with the DGTD method, the CPU time and memory requirement of DGTDPO can be reduced by 99.46% and 70.27% respectively, or even more. Meanwhile, the NRMSD of the time-domain, high-frequency approximation methods is over 0.2, and DGTDPO's NRMSD is only 0.0971. That is, the accuracy of the hybrid method is more than 64% higher than that of the approximate methods. Numerical results demonstrate that, compared with the DGTD method, the hybrid DGTDPO algorithm is more efficient in various composite scattering scenarios, and has better accuracy than time-domain, high-frequency approximation methods, including TDSBR and TDPO methods. Therefore, the hybrid method is of great significance for the scattering calculation of multi-scale and multi-target composite targets. Additionally, the DGTDPO method is also vital for the simulation of electromagnetic environments in engineering applications. Moreover, further work will also be devoted to more complex background environments, such as the attenuation of electromagnetic wave in various meteorological environments, and electromagnetic scattering in complex geographical environments, such as canyons, forests and so on.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **DGTD** & **DGTDPO** & **Decreasing Rate** \\ \hline CPU Time & 1142.85 h & 4 h & 99.65\% \\ Memory & 16.71 G & 4.12 G & 75.34\% \\ \hline \hline \end{tabular}
\end{table}
Table 7: The CPU time and memory of DGTD and DGTDPO for multi-object-rough sea surface composite objects.  
Figure 15: The RCS of the multi-object and sea surface composite computed at (a) 1 GHz and (b) 1.5 GHz from the solutions of the DGTDPO method and MoM.  
**Author Contributions:** Conceptualization, J.Z., Y.W. and C.M.; methodology, J.Z. and C.M.; validation, J.Z. and C.M.; investigation, J.Z. and Y.W.; writing--original draft preparation, C.M.; writing--review and editing, J.Z. and Y.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China--the Joint Fund for Basic Research of High Speed Railway--Research on the Electromagnetic Environment and System Level EMC Theory and Applications to High Speed Railway (number U1734203); Basic Research Business Expenses of Beijing Jiaotong University--Special Project of Frontier Science Center of Smart High Speed Railway System (number 2020 JBZD010); and Enterprise project (number W20L00370).  
**Acknowledgments:** The authors wish to thank the reviewers for their valuable comments and suggestions concerning this manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.