Checking the Radiometric Stability of Infrared Imagers with Venus

Observations of the Moon with imagers in the thermal infrared are often affected by saturation, and the large uncertainties of hitherto published observations of Mercury with such instruments rendered them useless for calibration. We suggest Venus close to inferior conjunction as an alternative flux reference for inter-calibration and checks of radiometric stability. Observations of this planet with the Advanced Himawari Imager (AHI) and the Spinning Enhanced Visible Infra-Red Imager (SEVIRI) are used to demonstrate the concept. By averaging the measured radiances at similar phase angles of Venus we achieved a precision better than 2% at wavelengths larger than 7μm. Brightness temperatures measured six years apart varied by less than 0.6K with both AHI and SEVIRI in most of these channels.