ABSOLUTE CALIBRATION OF AVHRR VISIBLE AND NEAR-INFRARED CHANNELS USING OCEAN AND CLOUD VIEWS

Methods for absolute calibration of visible and near-infrared sensors using ocean and cloud views have been developed and applied to channel s 1 (red) and 2 (near-infrared) of the Advanced Very High Resolution R adiometer (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 (12km and above) bright clouds as 'white' ta rgets. This cloud intercalibration is compared with intercalibration u sing ocean glint. The second step is an absolute calibration of channe l 1 employing ocean off-nadir view (40-70 degrees) 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 u sed 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 t he two AVHRR channels. NOAA-9 channels 1 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-11 shows no degrada tion during the 1989 mid 1991 period. This trend is similar to the cal ibration trend obtained using desert site observations, the absolute c alibration found in this work for both sensors is lower by 17 to 20 pe r cent (suggesting higher degradation) from the absolute calibration o f Abel et al. (1993, Journal of Atmospheric and Ocean Technology, 10, 493-508), that used aircraft measurements. Furthermore we show that ap plication of the calibration of Abel et al. or the present one for rem ote sensing of aerosol over Tasmania, Australia failed to predict corr ectly the aerosol optical thickness measured there. The only way to re concile all these differences is by allowing for a shift of 17 nm towa rds longer wavelengths of the AVHRR channel 1 effective wavelength. We show that with this shift, we get an agreement between the two absolu te calibration techniques (+/-3 per cent), and both of them do predict correctly the optical thickness in the two channels (+/-0.02). Recent work in preparation for publication (Vermote et al., 1995, in prepara tion) indicates that this shift is due to an out of band transmission (6 per cent at 900 nm) for AVHRR channel 1 previously unidentified.