## 3 Results
### Hyperspectral Reflectances of Tarps Using Dual ASD FieldSpec(r) System  
Reflectance values of four reference tarps, including wavelength spectral ranges (350-950 nm) of KOMPSAT-3 and -3A, were obtained using the ASD FieldSpec(r) 3 hyperspectral radiometer (Figure 5). This radiometer has been widely used for vicarious calibration of satellite imaging sensors because it is reliable and can measure a broad spectral range between 350 nm and 2500 nm [41; 42; 43]. The full width at half maximum (FWHM) of the radiometer is 3 nm in the range 350-700 nm and 10 nm from 700-2500 nm. Sampling spectral intervals are 1.4 nm (350-1000 nm) and 2 nm (100-2500 nm), respectively. The primary field of view for the optical fiber to measure the reflected radiance is 25\({}^{\circ}\) at a height of 1.5 m.  
Before measuring the tarp reflectance, the radiometer sensor should be calibrated under the field conditions at the time of measurement to prevent saturation from downwelling irradiance, including optimization and dark current correction processes. This procedure is also the process to convert a digital number of the radiometer into the reflected radiance (W cm\({}^{-2}\) nm\({}^{-1}\) sr\({}^{-1}\)). To obtain  
Figure 4: Measured ANIF values (red points) of KOMPSAT-3 for the Goheung field campaign sites. The uncertainty of BRDF measurement is shown by the vertical error bar and the blue dashed line means that no BRDF effect is expected due to an ANIF value of 1. (**a**,**c**,**e**,**g**) show ANIF values of 3%, 24%, 36%, and 57% tarp under backward scattering conditions, and (**b**,**d**,**f**,**h**) show the same values under forward scattering conditions. the downwelling irradiance information for the calibration, a spectralon white reference panel was used (30.5 \(\times\) 30.5 cm), which has an almost Lambertian reflectance of close to approximately 99%. The calibration should be regularly repeated whenever lighting conditions change, even by a small amount. Under non-clear sky conditions, this requires substantial time and effort because the user has to wait for clouds to pass and frequent calibrations are required to maintain constant lighting conditions at the time of calibration and measurement. Therefore, it is preferable to perform the field campaign in clear sky conditions with no clouds. However, it is rare that ideal sky conditions and the satellite image acquisition time coincide in practice. Hence, the error caused by the difference between the calibration and measurement times is the most problematic issue with the ASD FieldSpec(r) 3 radiometer. In this study, a dual radiometer system was constructed to reduce these inherent errors and to obtain stable and reliable reflectance values for the tarps. The main aim is to minimize the error due to the variation in lighting conditions by simultaneously measuring the white reference and the target using two radiometers. The system consists of three procedures. First, we measured the white reference using two ASD FieldSpec(r) 3 radiometers simultaneously for inter-calibration, which calibrates the difference between the sensor responses of both radiometers based on the white reference panel. Second, one radiometer measured the white reference (fixed FieldSpec(r) 3, FFS) whilst the other measured the tarps (mobile Fieldspec(r) 3, MFS) at the same time (Figure 5). Through this process, simultaneous downwelling and reflecting radiance values were obtained. Third, the inter-calibrated reflectances were calculated during post-processing using the following equation:  
\[\text{reflectance}=\left(MFS\_Rad_{wr,t=0}/FFS\_Rad_{wr,t=0}\right)\cdot\left( MFS\_Rad_{target,t=i}/FFS\_Rad_{wr,t=i}\right), \tag{2}\]  
where \(FFS\_Rad_{wr,t=0}\) and \(MFS\_Rad_{wr,t=0}\) are the radiances of the white reference measured by FFS and MFS, respectively, and \(FFS\_Rad_{wr,t=i}\) and \(MFS\_Rad_{wr,t=i}\) are the radiances of the white reference and the tarps measured by FFS and MFS, respectively.