CALIBRATION RESULTS FOR NOAA-11 AVHRR CHANNEL-1 AND CHANNEL-2 FROM CONGRUENT PATH AIRCRAFT OBSERVATIONS

A method for using congruent atmospheric path aircraft-satellite obser vations to calibrate a satellite radiometer is presented. A calibrated spectroradiometer aboard a NASA ER-2 aircraft at an altitude of 19 km above White Sands, New Mexico, was oriented to view White Sands at th e overpass time of the NOAA-11 Advanced Very High Resolution Radiomete r (AVHRR) instrument along the same view vector as the satellite instr ument. The data from six flights between November 1988 and October 199 0 were transformed into corresponding estimates of AVHRR channel radia nce at the satellite (derived from the aircraft measurements), and ave rage counts (from the AVHRR measurements), both averaged across the fo otprint of the spectroradiometer. Prelaunch measurements of the AVHRR spectral response profiles are assumed, and the radiance spectrum meas ured by the spectroradiometer was adjusted to satellite attitude using the LOWTRAN-7 computer code. Spatial misregistration of the two fligh t datasets was corrected by maximizing the correlation between them. R esults show reduced gains in both channel 1 (0.65 mum) and channel 2 ( 0.85 mum), compared to prelaunch values, with little further reduction in gain after 200 days in orbit. Results for the gain ratio (channel 1/channel 2), which is important for the calculation of the normalized vegetation index, show constant in-orbit values 5% above the prelaunc h value. Estimates of uncertainty in the results are presented, as wel l as ideas for their reduction in the analysis of future flights.