Abstract: A Space-based Calibration Transfer Spectroradiometer (SCATS) instrument that establishes a maintainable SI traceable radiometric calibration on-orbit with long-term stability accuracy, such as is required for long term climate monitoring missions, is provided. The SCATS instrument includes sensors that directly measure the highly stable solar irradiance and/or the lunar irradiance, and the Earth radiance through an invariant instrumental optical path. This common path measurement concept eliminates uncertainties associated with the calibration and degradation of intervening solar attenuation elements that represent the current state of the art. The SCATS instrument can be used to calibration of hyperspectral and multi-spectral sensors operating from the ultra-violet to long wave infrared. A suitable selection of detectors and filters enables this wide wavelength application.