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 near-nadir reflectances over spatially and temporally uniform ice-surfaces from Greenland and Antarctica to produce reflectance calibration curves 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 uniform in all AVHRR channels over sub-regions of area 68 km by 68 km. By comparing reflectances from uncalibrated AVHRR instruments with these calibration curves, new channel 1 and 2 calibration coefficients are obtained with an accuracy of 5 per cent. The main advantages of this calibration method are: (1) calibration targets are large; (2) it can be applied over multiple years; (3) it is applicable for 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 methods are in excellent agreement in channel 1 (within 3 per cent). In channel 2, they agree to within 4 per cent for NOAA-14, but are significantly different for NOAA-11 ( 9 per cent). When applied to NOAA-12 AVHRR observations for 1994-95, channel 1 and 2 calibration coefficients are 20 per cent and 35 per cent larger than prelaunch values, respectively.