## 2 Materials and Methods
Table 2 shows the selected geometric conditions of KOMPSAT-3 and -3A images used for radiometric calibration. We obtained only four valid field campaign measurements, despite attempting at least ten. Additionally, for the KOMPSAT-3 images observed on 14 May 2017 and 27 July 2017, the viewing zenith angles of the satellite sensor were greater than 10\({}^{\circ}\); this means that bidirectional reflectance distribution (BRD) effects of the surface target are expected. Thus, the BRD function (BRDF) of the reference radiometric tarps should be measured, thereby reducing the uncertainty of the BRDF effect, in order to use them as radiometric calibration samples. Primary processing of the vicarious calibration using a radiative transfer model depends on the simulation quality of the theoretical top of atmosphere (TOA) radiance received by spectral sensors of KOMPSAT-3 and -3A. In this study, the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) radiative transfer model (RTM) was mainly used to simulate TOA radiance according to the SRF of satellite sensors [33] by considering various advantageous input parameters, predominantly related to surface reflectance and atmospheric effects. The 6S model was validated using other RTMs and was found to be consistent with other RTMs to within 1%, according to previous reports [34; 35], meaning that it is useful for calibrating satellites to the requested accuracy. Figure 2 presents a flowchart of the vicarious radiometric calibration of KOMPSAT-3 and -3A using the 6S model. For the optical sensors, surface reflectance is the largest term contributing to the determination of the TOA radiance observed [36]. When acquiring the reference reflectance, an ASD Fieldspec(r) 3 hyperspectral radiometer (Malvern Panalytical Ltd., Malvern, UK) was used to measure each radiometric tarp under nadir direction geometry. We also attempted to measure tarp reflectance \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Date** & **Satellite** & **Site** & \begin{tabular}{c} **Observed** \\ **Time (UTC)** \\ \end{tabular} & \begin{tabular}{c} **Solar** \\ **Zenith** \\ \end{tabular} & \begin{tabular}{c} **Solar** \\ **Azimuth** \\ \end{tabular} & \
\begin{tabular}{c} **Viewing** \\ **Zenith** \\ \end{tabular} &
\begin{tabular}{c} **Viewing** \\ **Azimuth** \\ \end{tabular} \\ \hline
14 May 2017 & KOMPSAT-3 & Goheung & 04:27:16 & 28.500 & 242.883 & 17.202 & 140.507 \\
27 July 2017 & KOMPSAT-3 & Naju & 04:34:59 & 20.091 & 221.489 & 21.833 & 184.074 \\
19 September 2017 & KOMPSAT-3 & Zunmmod & 05:38:57 & 48.209 & 200.921 & 3.112 & 259.113 \\
20 September 2017 & KOMPSAT-3 & Zunmmod & 05:47:39 & 48.040 & 197.588 & 0.762 & 257.866 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Selected geometric conditions of KOMPSAT-3 and -3A imagery for radiometric calibration. Figure 1: (**a,c,e**) KOMPSAT-3 and -3A RGB images of Goheung County, South Korea acquired on 14 May 2017, Naju, South Korea acquired on 27 July 2017, and Zunmmod, Mongolia, acquired on 19 September 2017, respectively. (**b,d,f**) cropped satellite RGB images showing the locations of the deployed four radiometric tarps corresponding to red squares in (**a,c,e**) respectively. using this radiometer within 30 min of the satellite passing time. Both of these ground measurement conditions were intended to reduce the effects of the bidirectional reflectance distribution function (BRDF) using relative sun-target-sensor geometry. Nevertheless, the BRDF effects caused by changing the satellite position remained, especially in the tilted viewing zenith angle (Table 2). Therefore, we calculated the laboratory-based BRDF using a gonioradiometer to compensate for the geometric discrepancies between the ASD Fieldspec(r) 3 and satellite-only cases of sizeable tilted sensor geometry.