Sky-scanning radiometer for absolute measurements of atmospheric long-wave radiation.

Atmospheric long-wave radiation is one of the most promising parameters for observations of climate change that are greenhouse effect related. Long-wave irradiance is usually measured with pyrgeometers that consist of a flat thermopile and a hemispherical dome acting as a filter and protecting the absorbing receiver surface. Pyrgeometers are blackbody calibrated, but uncertainties, mainly related to the silicon hemisphere, arise from thermal effects, inadequate spectral transmission, and cosine-response errors. The new absolute sky-scanning radiometer (ASR) allows absolute measurements of atmospheric long-wave radiation and is suggested as a future reference standard for pyrgeometer field calibration. The calibration of the ASR is based on a reference blackbody source traced to absolute temperature standards. The pyroelectric detector has no window to prevent thermal and spectral transmission effects. Scanning the sky with a narrow viewing angle and integrating with the Gaussian quadrature, rather than taking hemispherical measurements, prevent errors related to the cosine effect.