The lack of accurate calibration sources is preventing current sensor technology from achieving the minimum possible uncertainty levels in remote-sensing radiometric measurements. This has been established in a recent study undertaken at the National Institute of Standards and Technology (NIST) following the identification of the problem at the Calibration Source Requirements Workshop at the NIST in 1997. This paper presents the concept of establishing high-accuracy calibration sources in space, utilizing the International Space Station (ISS) as a platform. For example, a retrievable total solar irradiance radiometer standard could be deployed on the ISS in order to establish the Sun as a calibration source of the required accuracy. Periodic measurements of solar irradiance with this radiometer standard would help to establish the Sun as a radiometric standard with the accuracy needed for various radiometric sensors on other platforms in space. As the radiometer on the ISS would be retrievable, its traceability to the International System of Units (SI) could be maintained by periodically bringing it down for calibration with SI-traceable standards maintained in ground laboratories. Other suitable radiometric standards on the ISS could also establish the Moon, the stars and various ground sites as calibration sources.