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Tracing climate change

When the microsatellite PICARD has reached its orbit in 2009, a series of filter radiometers will be on board that were calibrated by PTB. They will be part of the so-called PREMOS module that is to measure the total and the spectral solar irradiance as well as its variation with time. Six of these twelve filter radiometers will measure the solar radiation in the ultraviolet spectral range which, it is assumed, has a particularly large influence on the Earth‘s climate.

The microsatellite PICARD was built by the French Centre National d‘Etudes Spatiales. In it, besides the PREMOS module, there are other instruments for measuring solar activity (such as, for example, changes in the diameter of the Sun or in the surface structure).

The PREMOS module (Precision Monitoring of Solar Variability) was conceived in the Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center. In cooperation with the colleagues from Davos, PTB characterized the semi-conductor photodetectors necessary for the filter radiometers, in order to select those which showed a sufficiently homogeneous distribution of the spectral responsivity over the detector surface and are stable when being irradiated with hard UV radiation. Only in this way do they measure the solar irradiance exactly enough.

The filter radiometers were then assembled in Davos and subsequently calibrated at the UV and NIR cryogenic radiometer measuring facility of PTB. Since 2005, PTB has used this facility to realize the spectral responsivity of radiation detectors in the ultraviolet spectral range. Through the use of a unique high power plasma radiation source, a tunable radiation source with the highest spectral radiant power in the world is available in particular in the range between 200 nm and 300 nm. Thus in this spectral range during the calibration of the spectral responsivity of radiation detectors, extremely low measurement uncertainties (of about 0.1 %) are reached. This and the high spectral radiant power in the UV were crucial for the Physikalisch-Meteorologisches Observatorium in Davos in allowing PTB to carry out the extensive measurements. The solar irradiance in the ultraviolet spectral range measured in space by the filter radiometers with the low measurement uncertainty will offer a further metrological basis for the meteorological evaluation of climate change.