## 4 Conclusions  
In this paper, we investigated a wide range of two-dimensional radio refractivity using an IDW interpolation and high-altitude radio refractivity data from multiple meteorological observatories. The two-dimensional regional refractivities of the entire Korean Peninsula were obtained using the IDW method in consideration of the Earth's curvature. The refractivities for the four seasons in 2019 were estimated at the locations of seven meteorological observatories in the Korean Peninsula with the IDW and conventional interpolation methods: linear, cubic, and nearest-neighbor. The summer season had the highest average RMSEs for the IDW, linear, cubic, and nearest-neighbor methods at the four inland area sites, which were 29.1, 33.0, 35.0, and 41.0, respectively. On the other hand, the winter season had the lowest average RMSEs for IDW, linear, cubic, and nearest-neighbor at the four inland area sites, which were 13.3, 16.1, 17.2, and 18.6, respectively. Then, the atmospheric refractivities on 15 February 2019 were observed across the entire Korean Peninsula using the atmospheric data collected from 20 meteorological observatories so that the proposed IDW interpolation had smoother and more continuous results than other methods. To confirm the resulting two-dimensional refractivity interpolation for  
Figure 7: Illuminated electromagnetic field intensities on the terrain in relation to the atmospheric conditions: (**a**) the standard atmospheric conditions and (**b**) the observation-based atmospheric conditions on 15 February 2019.  
airborne SAR applications, all the propagation path losses across Pohang and Heuksando were obtained using the standard atmospheric conditions of \(\
abla M=118\) and the actual atmospheric conditions on 15 February 2019. On the terrain surface ranging from 90 km to 190 km, the average path losses in the standard and the observation-based conditions were 179.7 dB and 182.1 dB, respectively. Finally, based on the air-to-ground scenario in the SAR application, two-dimensional illuminated field intensities on the terrain surface were estimated. These results demonstrated that the SAR illumination could be degraded by atmospheric conditions, especially in long-range applications. In the near future, we expect these results to be more useful when more meteorological observatories for the high-altitude atmospheric data are built near the Korean Peninsula. For future work, we aim to investigate the propagation characteristics of SAR images or SAR signals while considering the amplitude and phase structures.  
Conceptualization, H.C. and S.W.; methodology, T.H.L.; software, S.W. and T.H.L.; validation, H.C. and S.W.; formal analysis, S.W.; investigation, S.W.; data curation, S.W.; writing--original draft preparation, S.W. and T.H.L.; writing--review and editing, H.C.; visualization, S.W.; supervision, H.C.; project administration, C.S.; funding acquisition, C.S. and K.O. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a grant-in-aid of HANWHA SYSTEMS, the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF-2015R1A6A1A03031833), and the NRF grant funded by the Korean government (No. NRF-2017R1A5A1015 596).  
Not applicable.  
The authors declare no conflict of interest.