# _remote sensing_
## 2 Scientific Background  
The main scientific purpose of the HORUS mission is to scan the Earth surface in the optical band to acquire data for ocean chlorophyll concentration monitoring, detection of surface characteristics, reflectance, land vegetation indices and the study of albedo, aerosols and clouds' features. Chlorophyll-a (Chl-a) concentration in ocean waters can be measured by analyzing imagery acquired in the visible spectrum. The most used wavelengths linked to this application belong to blue and green bands. Results can be improved by observations in the near-infrared (NIR) band [32]. Consequently, acquisitions in blue and green bands permit to quantify ocean chlorophyll concentration, important for estimating the phytoplankton abundance in the oceans and used as a health indicator of marine ecosystems. The concentration of this species affects the water color and increases the light backscattering. Vegetation and land cover classification can be performed by analyzing data acquired in red and NIR bands. Generally, the normalized difference vegetation index (NDVI) is calculated as a ratio difference between measured canopy reflectance in the red and near infrared bands, respectively, to be used as a simple numerical indicator to allow the delineation of the distribution of vegetation, agricultural area, thick forest, etc. [33]. Indeed, the NDVI of a densely vegetated area will tend toward positive values, whereas water and built-up areas will be represented by near zero or negative values. Data acquired in the blue and green bands can be used to improve the obtained results. The retrieval of heterogeneous lands can be attempted if there are features with different brightness in the acquired scene by assuming that although the surface brightness changes, the surface bidirectional reflectance distribution function (BRDF) does not significantly change. Routine and global observations permit to measure the aerosols' climate. Moreover, atmospheric aerosols from space should be ideally observed by imagining them over a surface that is completely dark. Indeed, the amount of light reflected in all directions from the ocean surface can be calculated in the red and NIR bands if the features (wind speed, direction, etc.) of the wind blowing at the ocean surface is known because the ocean surface is dark. For what concerns the surface reflection, this is estimated from the fraction of incoming light seen in different directions--by means of the BRDF of surface--especially over thick forests that tend to be dark in the blue and red bands (not in green). Consequently, the characterization of different atmosphere properties and both ocean and land aerosol measurements, such as the estimate of the related optical depth, etc., can be obtained by combining the data acquired in all the bands (red, green, blue, and NIR) [34; 35; 36]. Red and NIR acquisitions allow observing the effects of clouds on infrared radiation on cloud-free nights (radiative effects), useful to assess how the global climate is affected by the long-term average of all the effects linked to this phenomenon. Additionally, different types of cloud forms and their physical structure (shape, heights, thickness, and roughness of cloud tops) can be obtained by studying the change in reflection at different view angles combined with stereoscopic techniques. Finally, the total amount of light reflected by the cloud (\"albedo\") can be obtained by collecting reflectivity measurements from more than one view angle. Indeed, clouds do not reflect solar radiation equally in all directions and albedo retrieval is expected to be 10 times more accurate than those obtained from similar measurements with only a single camera looking at nadir [37; 38]. The combination of multiangle and multispectral data allows improving the type and level of information details to be obtained from each image or from the combination of more than one, acquired in multiple bands or view-angles.