A study on the effects of viewing angle and solar geometry variation in crop EO observation

This work aims to assess the effects of various acquisition geometries devoted to the crop’s studies using the PRISMA ("PRecursore IperSpettrale della Missione Applicativa") hyperspectral satellite data. PRISMA is a mission of the Italian Space Agency Agenzia Spaziale Italiana (ASI) aiming at qualifying space-based hyperspectral technology and providing imaging spectroscopy data to promote a variety of resource management and environmental monitoring applications. The satellite's payload instruments include a VNIR-SWIR imaging spectrometer and a high-resolution panchromatic camera (PAN). The satellite was launched on 22 March 2019, with an expected operational mission lifetime is 5 years. PRISMA is in a Sun-Synchronous Low Earth Orbit flying at an altitude of 615 km with an inclination of 97.85°and local time of equator crossing on Descending Node (LTDN) of 10:30, with a re-look capacity of 7 days and off-nadir observation, the nominal orbit revisit time is 29 days (from nadir). Off-nadir observations (±21°) are performed through platform roll manoeuvres (across-track or along track). Typical image size is of 30 x 30 km with a Ground Sampling Distance (GSD) of 30 m for (VNIR-SWIR) and 5 m for (PAN). Variations in the geometry of the sun and the view can lead to unwelcome brightness gradients throughout an image. Image brightness gradients can seriously impact on the analysis in research where reflectances from many images will be compared. These effects related to the bidirectional reflectance distribution function (BRDF) are also impacting on imaging spectroscopy data (Gu et al. 2021; Moriya, Imai, and Tommaselli 2018; Zhang et al. 2021). The BRDF describes the reflectance of a surface by considering the incoming and outgoing light direction. The function is parameterized by the zenith and azimuth angles of the incoming (solar) and outgoing (sensor) directions, in total 4 parameters. The BRDF effects in imagery result from different sunlit/shaded portions of the same surface target seen by the sensor under different solar and view geometries (Roujean, Leroy, and Deschanps 1992; Queally et al. 2022). BRDF effects are most evident when using a wide field of view sensors such as MODIS (Roujean, Leroy, and Deschanps 1992; Queally et al. 2022). Time series of sensor data characterized by a large range of view or sun angles show the same effect. BRDF correction aims to minimize such effect by normalizing the reflectance to the same solar and view geometry, this same solar and view geometry are defined by a constant view zenith angle (θv) and solar zenith angle (θs) This study was motivated by the observation that, even though the brightness gradients for PRISMA hyperspectral imaging inside an image don't change greatly due to its small FOV (2.77°), the various acquisition geometry across images may produce unfavorable artifacts. This study intends to investigate the impact of the BRDF effect on PRISMA images when utilized to retrieve biophysical parameters of different crops such as cereals and sugarcane. This study aims to assess the effect on the retrieval of crops biophysical variables like Leaf Area Index (LAI) and Chlorophyll retrieved by hybrid procedures utilizing the PROSAIL radiative transfer model. Two BRDF models are considered in this work: i) a simple kernel multiplicative correction, in which surface reflectance is viewed as a combination of two different components, diffuse reflection and volume scattering and ii) the Flexible BRDF correction (FlexBRDF) (Queally et al. 2022), in which the image pixel is pre-classified using the Normalized Difference Vegetation Index (NDVI). The current study area location is the Maccarese farm (Rome, Italy), while other sites will be selected during the coming days on the base of the results of the ongoing PRISMA and the contemporary field collection in China.