Calibration Results for NOAA-11 AVHRR Channels 1 and 2 from Congruent Path Aircraft Observations

Abstract

A method for using congruent atmospheric path aircraft-satellite observations 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 the overpass time of the NOAA-11 Advanced Very High Resolution Radiometer (AVHRR) instrument along the same view vector as the satellite instrument. The data from six flights between November 1988 and October 1990 were transformed into corresponding estimates of AVHRR channel radiance at the satellite (derived from the aircraft measurements), and average counts (from the AVHRR measurements), both averaged across the footprint of the spectroradiometer. Prelaunch measurements of the AVHRR spectral response profiles are assumed, and the radiance spectrum measured by the spectroradiometer was adjusted to satellite altitude using the LOWTRAN-7 computer code. Spatial misregistration of the two flight datasets was corrected by maximizing the correlation between them. Results show reduced gains in both channel 1 (0.65 µm) and channel 2 (0.85 µm), 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 prelaunch value. Estimates of uncertainty in the results are presented, as well as ideas for their reduction in the analysis of future flights.