IN-FLIGHT CALIBRATION OF NOAA AVHRR VISIBLE AND NEAR-IR BANDS OVER GREENLAND AND ANTARCTICA

A new method for in-flight calibration of NOAA AVHRR visible and near- IR bands is presented. The approach involves using calibrated NOAA-9 n ear-nadir reflectances over spatially and temporally uniform ice-surfa ces from Greenland and Antarctica to produce reflectance calibration c urves for AVHRR instruments in all orbits. The reflectance calibration curves consist of second order polynomial regressions of reflectance on solar zenith angle, derived from observations that are spatially un iform in all AVHRR channels over sub-regions of area 68 km by 68 km. B y comparing reflectances from uncalibrated AVHRR instruments with thes e calibration curves, new channel 1 and 2 calibration coefficients are obtained with an accuracy of approximate to 5 per cent. The main adva ntages of this calibration method are: (1) calibration targets are lar ge; (2) it can be applied over multiple years; (3) it is applicable fo r a wide range of solar zenith angles, and can therefore be used year- round. When calibration coefficients inferred from NOAA-11 (1994) and NOAA-14 (1995) observations over Greenland and Antarctica are compared with those from the formulae of Rao and Chen (1995, 1996), the two me thods are in excellent agreement in channel 1 (within approximate to 3 per cent). In channel 2, they agree to within approximate to 4 per ce nt for NOAA-14, but are significantly different for NOAA-11 (approxima te to 9 per cent). When applied to NOAA-12 AVHRR observations for 1994 -95, channel 1 and 2 calibration coefficients are approximate to 20 pe r cent and approximate to 35 per cent larger than prelaunch values, re spectively.