Assessing MODIS LWIR band calibration accuracy

Abstract

Five cases using NASA ER-2 aircraft based SHIS and MAS radiances have been used to assess the L1B radiometric performance of Terra and Aqua MODIS Collection 5 radiances for LWIR bands 31-36. The composite results of these cases show that the split window bands 31 (11 μm) and 32 (12 μm) have performed well within the 0.5% radiometric specification over their lifetime. This is in agreement with results from other ground based and satellite based comparisons that are discussed in the paper. However, the LWIR CO₂-sensitive bands 34-36 appear to be performing outside of their 1% accuracy specification, especially for Terra MODIS. This is also observed in global Aqua AIRS-MODIS comparisons. Possible causes for this behavior are under investigation, with the most likely contributors being spectral characterization error, OOB influences due to spectral filter leaks, or possibly scan mirror characterization. It seems that an optical leak from Terra MODIS band 31 into bands 32-36 is probably not a significant contributor to the large residuals of bands 34-36, owing to an effective radiometric correction. Calibration coefficient error is probably only a small contributor since, after adjustments in 2002, the on-orbit calibration now closely follows that of the pre-launch calibration.