The need is identified for a unified approach to the preflight and in-flight absolute radiometric calibration of satellite sensors, which does not depend on the accurate transfer of lamp and detector calibrations from the laboratory to orbit. Such an approach is described that uses the sun to provide the link between preflight solar-radiation-based calibration, in-flight solar- diffuser-based calibration, and vicarious calibration. An example is given of each of these methods and uncertainty budgets are provided. It is shown that an uncertainty, with respect to solar exo-atmospheric irradiance, of less than 3%, one sigma, can be attained for each method and that each can, if needed, be related to national laboratory standards. In addition to providing the link between preflight and in-flight on-board calibrations, this unified approach also fills the critical need to relate these calibrations to those of the radiometers used for vicarious calibration and data-product, field-validation measurements, by also referencing them to the same solar-irradiance scale.