## 3 Study Area and Satellite Data
### Overview of Sensors and Satellite Data  
The NAOMI-1 sensor provides multispectral and panchromatic images with a periodicity of 37 days at-nadir conditions. Considering the satellite's roll capability of \(\pm 30^{\circ}\), the revisit period is reduced to 3-5 days. The technical specifications of NAOMI-1 are reviewed in Table 1, and more details can be found in [PERSON] et al. [9] and Aerial Photogrammetric Service (SAF) [10]. RapidEye is a five-satellite constellation launched on 29 August 2008, able to provide data on a daily basis at off-nadir conditions and every five days at-nadir [73]. Each satellite of the mission, named from RE1-RE5, carries identical MSI instruments. The requirements of the mission are radiometric stability over time and between spacecraft. The response between the same bands is within 5% over the whole mission, providing directly comparable images [74; 75]. Since 2009, annual field campaigns have been conducted at Ivanpah Playa, Railroad Valley Playa (RVP) and Brookings calibration sites [76]. The Cal/Val activities have been performed by Blackbridge, in conjunction with the Universities of \begin{table}
\begin{tabular}{c c c} \hline \hline
**Specifications** & **RapidEye-MSI (RE4)** & **FAST-C NAOMI-1** \\ \hline Orbital height (km) & 630 & 620 \\ Nominal GSD at-nadir (m) & 6.5 & 5.8 \\ Dynamic range (bits) & 12 & 10 \\ Swath (km) & 77 & 10 \\ Descending node & 11:00 a.m. & 10:30 a.m. \\ Spectral Bands (nm) & & \\ Blue & 440–510 & 455–520 \\ Green & 520–590 & 528–588 \\ Red & 630–685 & 625–695 \\ NIR & 760–850 & 758–881 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical specification of the MSI RapidEye and NAOMI-1 FASat-C sensors. Figure 1: Location of the Frenchman Flat calibration site, Nevada, United States (36\({}^{\circ}\)48\({}^{\prime}\)24\({}^{\prime\prime}\)N; 115\({}^{\circ}\)56\({}^{\prime}\)27\({}^{\prime\prime}\)W), where the study area is highlighted in red (**a**,**b**); (**c**) The specific areas for the cross-calibration of FASat-C against RapidEye were determined based on the criteria of high reflectivity, relative homogeneity and degree of spatial association (please refer to Section 4.1). Preliminary evaluation samples were distributed across the shown area. Arizona and South Dakota State. Since 2011, simultaneous collections for the whole constellation have been considered. The results of a validation performed over RVP indicated that, after the calibration updates of early 2014, the percentage differences between field measurements and MSI imagery were below 3% for the whole constellation (personal communication) [77]. In the case of RapidEye-4 (RE4), the sensor that acquired the data involved in the present work, the maximum differences were in the order of 1.25% [78]. More details about MSI are presented in Table 1, and a comparison of the relative spectral response (RSR) of both sensors is presented in Figure 2.  
Data acquired by the EO-1 Hyperion Hyperspectral Imager (HSI), a pushbroom sensor of 198 calibrated bands of \(\sim\)10 nm Full Width at Half Maximum (FWHM) in the range of 400-2500 nm [79], were used to obtain SBAF. The Hyperion spatial resolution is 30 m, and the dimensions of the scans are 7.5 km \(\times\) 100 km. On 25 July 2014, simultaneous acquisitions over Frenchman Flat were obtained by FASat-C and RapidEye (Table 2). According to [PERSON] et al. [15] and [PERSON] et al. [19; 20], this approach helps minimize the uncertainties associated with differences in the atmospheric conditions and illumination geometry. The FASat-C image was acquired by the Space Operations Group (GOE), and the respective Level 1A product was provided to the authors by SAF. More details about the data request can be found in SAF [10]. The RapidEye L1B imagery was provided as demo data by Blackbridge, to support this cross-calibration study. EO-1 Hyperion data of the study area in Hierarchical Data Format (HDF)