The Antarctic and Greenland snow surfaces as calibration targets for the visible channel of the Advanced Very High Resolution Radiometer

The snow surfaces of the high plateaus of the East Antarctic and Greenland ice sheets are used to determine multi-year drift in the sensitivity of the visible channel of the Advanced Very High Resolution Radiometer (AVHRR) instruments on the polar-orbiting satellites NOAA-9, 10, and 11. Bidirectional reflectance distribution functions (BRDFs) are empirically derived for the months of October-February (Antarctica) and April-August (Greenland) using a simplified atmospheric model. The BRDF of the snow surface should not change from year to year for near-nadir satellite views. Therefore, drift in normalized monthly averages of the derived BRDFs is interpreted as showing a change in channel sensitivity. Using this method, the authors show that the visible channel on NOAA-9 degraded linearly over the lifetime of the instrument (February 1985-0ctober 1988) by 5.2%/year. The detrended monthly average BRDFs have a standard deviation of 1%, indicating that the sensitivity changed at a constant rate. The drift of channel 1 on NOAA-10 (December 1986 August 1991) was nonlinear, but could be fitted with a fourth order polynomial. The visible channel on NOAA-11 was much noisier, with a detrended standard deviation of ~3%, indicating that this satellite experienced month-to-month sensitivity changes. Data processed for November 1988-February 1991 showed a linear increase in sensitivity of 0.5%/year