## 3 Results  
### Laboratory-Based BRDF Measurements of Tarps  
The BRDF measurement is used to infer reflectance at a given viewing geometry and to compute a hemispheric albedo from a reflectance measurement made from a specific direction [37]. The accuracy of such tarp-based field calibrations depends on accurate knowledge of the tarps' laboratory-measured BRDF at a variety of source illuminations and detector scatter angles [38], since the BRDF effect is considered an unknown error quantity in the reflectance values [39]. In the case of KOMPSAT-3 and -3A, the observed surface reflectance from optical sensors are affected by the surface BRDF effect on deployed tarps even though they were carefully constructed to be flat and chemically treated to have a constant reflectance [11]. When measuring laboratory BRDF for the four field campaign cases using a hyperspectral gonioreflectometer, we only use the first case field campaign of KOMPSAT-3 as an illustration in Table 2. This is because the last two KOMPSAT-3 and -3A cases, performed at Zuunmod, Mongolia, are close to the nadir direction viewing zenith angle, which results in negligible BRDF effects of surface radiometric tarps due to an observation geometry similar to that of the ASD Fieldspec(r) 3. Figure 2: Flowchart showing the vicarious radiometric calibration of KOMPSAT-3 and -3A using the 6S radiative transfer model and related input parameters. Moreover, in the second case using KOMPSAT-3, acquired on 27 July 2017, at Naju, a non-negligible BRDF effect is caused by the reflectance of radiometric tarps because KOMPSAT-3 had the largest the viewing zenith angle (21.833\({}^{\circ}\)). However, the two-dimensional hyperspectral gonioreflectometer could not measure the BRDFs of sample targets when the illumination and reflection angle was within 2\({}^{\circ}\) (under backward scattering geometry conditions) because the illuminator and detector sensors can become overlapped and thus impossible to measure (Figure 3). Therefore, we consider only the BRDF effect of the first field campaign on 14 May 2017 by reflecting the angle of incidence and reflection using a hyperspectral gonioreflectometer designed to measure two-dimensional BRDFs for the principal plane with a 3-nm radiometric resolution in the 380-1000 nm wavelength range [40]. In this study, anisotropy factors (ANIFs) of each radiometric tarp were measured using the following equation [22] to normalize sensor viewing geometry to ASD Fieldspec\({}^{\circledR}\) 3 geometry:  
\[\text{ANIF}(\theta_{i},\theta_{r},\lambda)=\frac{BRDF(\theta_{i},\theta_{r}, \lambda)}{BRDF_{\theta_{N}=0}(\theta_{i},\theta_{N},\lambda)}, \tag{1}\]  
In the equation, \(\theta_{i}\), and \(\theta_{r}\) are the illumination and reflection angle, respectively, of the corresponding wavelength \(\lambda\). \(BRDF(\theta_{i},\theta_{r},\lambda)\) is the BRDF value according to the incident illumination and reflected elevation directions in the principal plane. It reflects the observation geometry of KOMPSAT satellites according to solar and viewing zenith angles. \(BRDF_{\theta_{N}=0}(\theta_{i},\theta_{N},\lambda)\) is designed for the nadir direction of ASD FieldSpec\({}^{\circledR}\) 3; therefore, the reflection angle was fixed as the nadir direction (\(\theta_{N}\) = 0\({}^{\circ}\)). ANIF is an intuitive reflectance anisotropy measure that varies in theory between 0\({}^{\circ}\) and \(\infty\)[24]. Figure 4 presents the measured ANIF values of the four tarps according to KOMPSAT-3 geometry, which is the first case in Table 2. The geometry conditions of the laboratory BRDF measurement include an incident angle of 28.500\({}^{\circ}\) and a detection angle of 17.202\({}^{\circ}\). According to Table 2, forward not backward scattering of the reflectance radiance is expected, because the relative azimuth angle between the sun and the sensor is 102.4\({}^{\circ}\). Figure 4a,c,e,g shows backward scattering and Figure 4b,d,f,h shows forward scattering geometry.