Ice radiance method for backscatter UV instrument monitoring

The radiance of large, ice covered land masses has been used to monitor TOMS instrument sensitivities. Greenland and Antarctica provide uniform and stable ice surfaces whose average albedo appears to be constant within the desired accuracy for instrument monitoring. Instrument radiance response will depend upon view and illumination angles, the sun-earth distance, and atmospheric conditions. Restriction to nadir views eliminates view angle dependence, and corrections are made for sun-earth distance. The effect of atmospheric conditions, such as ozone and clouds, is minimized by monitoring at wavelengths above 340 nm and by the high surface radiance. Relative instrument response is determined by the ratio of signals measured at different times using a binning technique to account for differences in solar illumination angles. The only remaining limit to long term monitoring accuracy is the albedo stability of the ice surface itself. Changes in the Nimbus-7/TOMS instrument response at long wavelengths are monitored within 1% accuracy over the lifetime of the instrument.