This induces additional limitations on the spatial resolution of accurate ground-based measurements of all solar radiation components.Ĭonversely, geostationary satellites have been used to overcome these limitations because they provide global coverage with enough spatial resolution (~2–5 km) and moderately high temporal resolution (~10–30 min) that allows the determining of some cloud properties (cloud cover, cloud type and top height), its daily evolution and, consequently, the solar radiation throughout the day. Unfortunately, in most ground-based stations, only the global horizontal irradiance (GHI) component is measured and the other components may be only estimated considering other atmospheric factors (e.g., clearness index) and using different parametric, physical or statistical methods. In addition, several methods to estimate the cloud cover, optical depth and aerosol optical depth (AOD) have been developed from the combination of different components of broadband solar radiation. This instrumental setup is widespread, especially in the most populated areas around the world. These radiometers are commonly installed on solar trackers and the different components of the solar radiation are measured with moderate–high temporal resolution (~1–15 min). The measurements of solar radiation on the Earth’s surface usually rely on broadband radiometers, covering the entire solar spectrum (280–3000 nm). Consequently, different instruments and measurement techniques have been developed. For short time scales, the solar energy forecast is strongly related to the atmospheric conditions and especially with the number of clouds and their temporal evolution. The success of these projections highly depends on improving solar radiation measurements, both in accuracy and worldwide availability. ![]() It is especially important for photovoltaic and thermosolar plants, due to their increasing development and perspectives over the coming years. Therefore, there is now a need to improve the solar energy projections of these plants for different time scales. It was mainly triggered by the large increase in solar energy plants that have been deployed all over the world as part of the most ambitious strategies defined by different international administrations to mitigate climate change. In recent years, interest in the accurate measurements of solar radiation on the Earth’s surface has resumed. Thus, the remote sensing techniques described here will undoubtedly be of great help for solar and atmospheric research. These results support the developed methodology and allow us to glimpse the great potential of sky-cameras to carry out accurate measurements of sky radiance and solar radiation components. The SONA sky radiance shows a difference of an RMBD < 10% while the broadband diffuse radiation shows differences of 2% and 5% over a horizontal plane and arbitrarily oriented surfaces, respectively. Our results show a very good agreement with the independent measurements of the AERONET almucantar for sky radiance and pyranometers for broadband retrievals. An extensive validation of our radiometric retrievals has been performed in all sky conditions. As a result, a 1 min time resolution database of geometrically and radiometrically calibrated HDR images has been created and has been available since February 2020, with daily updates. This approach is based on a detailed instrumental characterization of a SONA sky-camera in terms of image acquisition and processing, as well as geometric and radiometric calibrations. ![]() ![]() We propose a methodological approach to provide the accurate and calibrated measurements of sky radiance and broadband solar irradiance using the High Dynamic Range (HDR) images of a sky-camera.
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