Visible infrared spin-scan radiometer atmospheric sounder radiometric calibration: an inflight evaluation from intercomparisons with HIRS and radiosonde measurements.

The Visible IR spin-scan radiometer Atmospheric Sounder (VAS) is the first sounding instrument placed in geostationary orbit. The calibration procedure for determining VAS radiances requires modeling the contributions of the telescope foreoptics components to the background radiation. A simple relation between target radiance and detector output voltage is derived. The calibration procedure is successfully tested by comparisons with HIRS and radiosonde data. After removing a negative offset of up to 3 degrees C in three of the upper atmospheric CO(2) bands of VAS, the capability to produce accurate soundings (within 2 degrees C of radiosonde) was demonstrated.