Over the last three decades, the demand for Earth-observation products has increased tremendously, particularly for the management of natural resources and the environment in general. Moreover, surveillance of the Earth’s environment at the local, regional, continental, or global scales, using various satellite sensors, requires adequate radiometric calibration in order to produce accurate and reproducible geophysical and biophysical surveys over time. Radiometric calibration of sensors is a critical prerequisite for creating high-quality science data and, consequently, higher-level downstream products. Without accurate calibration, significant errors can spread through all subsequent image processing operations, including spatial and multitemporal analyses, thematic classifications, and the generation of vegetation indices. To get the maximum information from satellite-data-derived products, sensors must be constantly calibrated, data must be validated, and the stability and quality of data must be ensured.