In February 2008 the space shutt le Atlantis transported the European space module Columbus to the International Space Station ISS where it was installed. The external platform SOLAR with different facilities for solar observation was also on board. Here, the solar-spectroradiometer SOLSPEC is the central measurement station, with which the spectral irradiance of the sun from the UV spectral range to the near infrared will be observed continuously over a long period of time. Before take-off , this instrument stopped by at PTB for its absolute calibration.

The international SOLSPEC team used the calculable spectral irradiance of a PTB high-temperature cavity radiator for its measurements. At radiator temperatures around 3100 K with high temperature stability (drift < 0.2 K/h) and an expanded temperature measurement uncertainty of only 1 K, the team could precisely determine the spectral irradiance and calibrate SOLSPEC with the highest accuracy.

The achieved uncertainties of this second version of the SOLSPEC experiment are therefore clearly lower than those of its predecessor, which provided reliable and precise measurement results during space missions in the 1990s. In the meantime SOLSPEC has started its work which is planned for 15 months and is observing the sun.