The attempt to trace radiometric units and calibrations back to a common radiance Source at the United States National Institute of Standards and Technology (NIST) in order to compare measurements taken by different methodologies in absolute terms. The stability and precision requirements for remote sensing from space are often higher than that of (1) the transfer Calibration from NIST to transfer lamps, (2) the calibrated radiometers for vicarious measurements and (3) the calibrated on-board sources. For this reason, some sensors reference their calibration to presumably more quantifiable and more stable solar irradiance rather than to NIST-traceable pre-launch calibration of on-board calibrators. However, in the absence of any absolute national radiance standard other than at NIST, it is still necessary to reference radiometric calibrations to NIST standards. Also see radiometric reflective band calibration. (Source: Dr. John Barker)