Background of Hyperspectral Imagery
What is AVIRIS Data?
The NASA AVIRIS spectrometer obtained several frames of data over the Kennedy Space Center, Florida on March 23, 1996. AVIRIS acquires data in 224 bands of 10 nm width in the reflected visible and near infrared spectrum (400 - 2500 nm) at a spatial resolution of 18 m. The effects of atmospheric water vapor can often be removed effectively by radiative transfer models which utilize either concurrent field measurements or the relative differences in absorption exhibited by the various channels. Unfortunately, concurrent measurements are often not available, and even then only represent local conditions at a fixed location in a flightline. Thus, it is common to utilize the spectral responses measured by the sensor to compute the correction.
False color composite of AVIRIS frame over Kennedy Space Center
Radiance Correction
The KSC AVIRIS data were converted to apparent surface reflectances using the ATREM (Atmosphere Removal Program) [4] developed by the Center for the Study of Earth from Space at the University of Colorado, Boulder, Co. The ATREM software was developed to determine the scaled surface reflectance from hyperspectral imagery from both AVIRIS and HYDICE sensors. The atmospheric scattering used by ATREM is modeled after the MODTRAN 5S radiative transfer code. The surface is assumed to be horizontal and have a Lambertian reflectance. If topography is known, then the scaled surface reflectance can be converted into real surface reflectance.
Columnar water vapor image removed by ATREM.
The ATREM model is a good approximation to a true radiometric correction of the imagery. A by-product from the ATREM software is an image of the columnar water vapor which was removed from the input hyperspectral data. The two figures above show one of the March 23, 1996 KSC AVIRIS scenes prior to the ATREM correction and a water vapor scene representing columnar water vapor estimated by the program. The images show a significant amount of water vapor, which causes attenuation of the upwelling radiance, to have been removed from the imagery.
Comparison of raw AVIRIS data and the results from ATREM for a given pixel location.
The above figure illustrates the results of the ATREM processing for all bands at a given pixel location. The collected radiance values of the AVIRIS is converted into surface reflectance using the ATREM program. The above data values were multiplied by log10 in order to plot them on the same scale. It shows that ATREM software is able to correct the visible bands which are most affected by atmospheric scattering. The ATREM software "over-compensates" the correction for the bands in the water absorption region of the spectrum (Bands 102-116 and Bands 150-172). This over-compensation is attributed to a low signal-to-noise ratio by the AVIRIS sensor has at these wavelengths; thus these bands are not considered in the subsequent analysis.