Absolute calibration of AVHRR visible and near-infrared channels using ocean and cloud views

Abstract

Methods for absolute calibration of visible and near-infrared sensors using ocean and cloud views have been developed and applied to channels 1 (red) and 2 (near-infrared) of the Advanced Very High Resolution Radiometer (AVHRR) for the NOAA-7, -9 and -11 satellites. The approach includes two steps. First step is intercalibration between channels 1 and 2 using high altitude (12 km and above) bright clouds as ‘ white’ targets. This cloud intercalibration is compared with intercalibration using ocean glint. The second step is an absolute calibration of channel 1 employing ocean off-nadir view (40-70° ) in channels 1 and 2 and correction for the aerosol effect. In this process the satellite measurements in channel 2, corrected for water vapour absorption are used to correct channel 1 for aerosol effect. The net signal in channel 1 composed from the predictable Rayleigh scattering component is used to calibrate this channel. The result is an absolute calibration of the two AVHRR channels. NOAA-9 channels I and 2 show a degradation rate of 8-8 per cent and 6 per cent, respectively, during 1985-1988 and no further degradation during 1988-1989 period. NOAA-II shows no degradation during the 1989 mid 1991 period. This trend is similar to the calibration trend obtained using desert site observations, the absolute calibration found in this work for both sensors is lower by 17 to 20 per cent ( suggesting higher degradation) from the absolute calibration of Abel et al. ( 1993 Journal of Atmospheric and Ocean Technology,10, 493-508 that used aircraft measurements. Furthermore we show that application of the calibration of Abel et al. or the present one for remote sensing of aerosol over Tasmania, Australia failed to predict correctly the aerosol optical thickness measured there. The only way to reconcile all these differences is by allowing for a shift of 17 nm towards longer wavelengths of the AVHRR channel 1 effective wavelength. We show that with this shift, we get an agreement between the two absolute calibration techniques ( ± 3 percent), and both of them do predict correctly the optical thickness in the two channels ( + 0.02) Recent work in preparation for publication (Vermote el al, 1995, in preparation indicates that this shift is due to an out of band transmission ( 6 per cent at 900nm) for AVHRR channel 1 previously unidentified.