116 
WmlugBi (mm) 
Figure - 6: AVIRIS Calibrated Radiance Spectrum from Swiss 1991 Data. 
This signal is the result of solar illumination, atmospheric transmission, atmospheric scattering, surface reflec 
tance and surface slope. To correct for the solar illumination and atmospheric effect a set of algorithms using the 
MODTRAN retentive transfer code is employed (Green, et al., 1990, Green, et al., 1993). These algorithms esti 
mate the water vapor, surface pressure and aerosol optical depth from the AVIRIS spectrum. An example of the 
estimation of water vapor (Green, et al., 1989 and Green, et al., 1991) is given in Figure 7. 
(mm) 
Figure - 7: AVIRIS MODTRAN Water Vapor Fit for Swiss 1991 Data. 
In this plot, an AVIRIS spectrum is fit to a MODTRAN spectrum across the 940 nm atmospheric water absorption 
band. The amount of water vapor required by MODTRAN is an output parameter of this non linear least squares 
fitting algorithm for each AVIRIS spatial element. With estimates of water vapor, pressure and aerosols each 
AVIRIS radiance spectrum is inverted to apparent surface reflectance. Figure 8 shows the inverted reflectance for 
the radiance spectrum in figure 6 . 
WavMtwiÿti (mm) 
Figure - 8: MODTRAN Derived AVIRIS Reflectance Spectrum from Swiss 1991 Radiance Data. 
The molecular absorption of chlorophyll and liquid water are present as is the cellular scattering in near infrared.