Winkler, R; (2013) Cryogenic Solar Absolute Radiometer - a potential SI Standard for Solar Irradiance. Doctoral thesis , UCL (University College London).

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Abstract

This thesis reports the development of an instrument which could act as a future standard for Solar Irradiance. The instrument is called Cryogenic Solar Absolute Radiometer (CSAR), and it exploits the advances made in the field of cryogenic radiometry in the last few decades. The aim is to significantly reduce the measurement uncertainty as compared to the current standard (the World Radiometric Reference) and to guarantee the long-term stability of the measurement record. Several tests were carried out in order to verify the performance of CSAR. In a first test, CSAR was found to agree within 0.01% with the National Physical Laboratory’s SI standard for radiant power. In a second test, CSAR and the World Radiometric Reference were compared on the World Radiation Center’s solar tracker in Davos/Switzerland. In this comparison, the World Radiometric Reference measured 0.309% higher than CSAR; the relative standard uncertainty of the comparison was 0.028%. This difference between the current Solar Irradiance standard and CSAR is able to explain the offset between the two space experiments VIRGO/SOHO and TIM/SORCE. The CSAR result is further confirmed by the fact that a similar offset between the World Radiometric Reference and the SI-scale has been determined through experiments independently performed at the Laboratory for Atmospheric and Space Physics (University of Colorado Boulder). CSAR has also been designed with space flight in mind. Although no full evaluation of the space-worthiness has been carried out, thermal tests indicate that CSAR could cope with the limited cooling power provided by readily available space coolers. The relative standard uncertainty of space-based Total Solar Irradiance measurements by CSAR is estimated to be 0.011%.

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