Measuring and Modeling TIR Directional Effects based on the Thermal Infrared Anisotropy Measurements in India and Southern Europe (Tiramisu) Project: Preparation to the Trishna Mission

TRISHNA is a thermal mission led in collaboration by CNES (Centre National d'Etudes Spatiales) and ISRO (Indian Space Research Organisation). It will collect optical and thermal datasets globally at a spatial resolution of 57m. The revisit period of 3 times per 8 days at low latitudes, with additional acquisitions towards the poles, and, suggests a high field of view (HFOV) around 35° to fill mission objectives. This will infer angular effects on the time series of measurements.TRISHNA overpass is 12:30 local time, which is highly prone to capture the hot spot phenomena corresponding to a radiometric peak when the view and sun geometries coincide. Previous studies (Lacaze et al.,2000 [1]; Roujean, 2000 [2]; Duffour et al., 2016 [3]) showed that the maximum directional effect arises in the hot spot geometry, thus adding significance to anisotropy effects in LST (Land Surface Temperature) measurements beside HFOV. Hot spot features can lead to LST measurement errors up to 10K. This uncertainty propagates in the estimation of evapotranspiration, which is a significant deliverable of the TRISHNA mission. Therefore, paying attention to the observations that are impacted by the hot spot is mandatory, either for discarding them or to perform an angular correction. Such a decision depends on the width of the hot spot peak and its intensity. On the one hand, data located very near the hot spot geometry may not be processed as hotspot characteristics are susceptible to the architecture of the canopy, and such information will not be available globally.On the other hand, regarding the cloudiness in certain regions, clear data, even in the hot spot, should be harnessed. Therefore, acquiring knowledge through measurements and simulations is mandatory for preparing the TRISHNA mission to be launched in 2026 in the best conditions.Hitherto, several studies about the impact of the hot spot phenomenon on radiometry were broadly concentrated in the optical domain and inadequately in the thermal infrared (TIR) range. The reason for that is the difficulty to accumulate information for TIR over time as the TIR signal is ephemeral and depends on environmental factors (wind, soil wetness, atmospheric humidity) in addition to the structure of the medium, which is the only driver for optical.Directional effects, either thermal or optical, can be resumed by the BRDF (Bidirectional Reflectance Distribution Function) (Julien et al., 2023 [4]). The starting point is collecting directional measurements for various environmental conditions and canopy types. This sustained the implementation of the TIRAMISU (Thermal InfraRed Anisotropy Measurements in India and Southern eUrope) project.