## 2 Materials and Methods
### Sentinel-2
Planet CubeSat satellites are notably smaller and lighter (0.10 m \(\times\) 0.10 m \(\times\) 0.30 m and 4 kg, respectively) compared to their counterparts, like Landsat-8 and Sentinel-2. Operating in a sun-synchronous orbit with an altitude ranging from 450 km to 580 km, these satellites pass over the equator between 9:30 and 11:30 a.m. local time. We used imagery files from 11:30 a.m. since it was the closest to the other time of RS data acquisition regarding the other RS sensors in this study. Planet CubeSat's capabilities are exclusively limited to multispectral imagery, as detailed in Table 5. Unfortunately, thermal imagery is still not provided by these constellations of microsatellites. The surface reflectance images obtained by Planet CubeSat undergo pre-processing and calibration, also incorporating a scaling factor of 10,000. The imagery pre-processing and calibration process encompass adjustments for radiation scattering due to atmospheric gases, aerosol concentration, and their altitude-dependent variations between Earth's surface and the satellite's at-sensor camera in space. To enhance their calibration, data from MODIS (Moderate Resolution Imaging Spectroradiometer), such as water vapor, ozone, and aerosol quality control products, are utilized, along with the 6 SV2.1 radiative transfer model. However, the atmospheric correction process is still evolving, primarily because Planet's imagery calibration does not currently address effects like stray light, haze, and the influence of thin cirrus clouds. The approach by Planet Labs assumes that Earth's landscapes behave as Lambertian surfaces, scattering light uniformly in all directions, and that all scenes are effectively at sea level. Following image acquisition, geometric corrections are meticulously executed using sensor telemetry, ground control points (GCP), and finely detailed digital elevation models (DEM). Furthermore, the Planet Team released a harmonized version of Planet imagery, including CubeSat data, aligning the quality of multispectral data with (calibration to) Sentinel-2 standards. For further details on the image harmonization processes, refer to the works of [44; 45]. In this study, the primary data source used was the CubeSat harmonized images. In our study using the data from the LIRF and IIC using all listed spaceborne RS sensors, we specifically focused on clear-sky images. This deliberate choice was made to ensure that cloud cover conditions did not interfere with the accuracy of ground-based surface reflectance and temperature values in our research fields. For Landsat-8, the evaluation of evapotranspiration (ET\({}_{\text{a}}\)) included two different datasets. The first dataset involved the use of the original Landsat-8 platform, encompassing imagery data from OLIS \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Bands**} & **Central Wavelength** & \multirow{2}{*}{**Bandwidth (nm)**} & **Spatial Resolution** \\  & **(nm)** & & **(m)** \\ \hline BLUE & 491 & 60 & 3 \\ \hline GREEN & 566 & 90 & 3 \\ \hline RED & 666 & 80 & 3 \\ \hline NIR & 867 & 80 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Planet CubeSat multispectral bands used in this study at LIRF and IIC research sites. (multispectral) and TIRS (thermal). The second Landsat-8 dataset combined OLIS data with ground-based infrared temperature (IRT) data. The utilization of TIRS and ground-based T\({}_{\rm s}\) data was specific to the Landsat-8 spaceborne platform since Planet CubeSat and Sentinel-2 relied on ground-based nadir-looking IRT T\({}_{\rm s}\) data for input in the TSEB RS of the ET\({}_{\rm a}\) algorithms. This choice, in the case of the Landsat-8 imagery, was made due to the pivotal role of its TIRS data, albeit its original pixel size, in providing original spaceborne T\({}_{\rm s}\) information for maize ET\({}_{\rm a}\) estimation. The Planet Cubesat pixel data that had the measurement stations at LIRF and IIC were used for the estimation of maize ET\({}_{\rm a}\) using the TSEB algorithms.