628 
These instruments provide very different ground resolution and instantaneous field of view (IFOV). To address this 
problem, we extracted reflectances from the three data sets, on corresponding zones and averaged them to cover the same 
extent. The selected sites were a cotton field (fully covered) a leveled bare soil and a pecan orchard. 
All these measurements allow close examination of the differences in the directional behavior of all the surfaces. The 
figure 1 shows the ratio of the bidirectional reflectance in different configurations to the nadir reflectance computed for 
aircraft measurements and figure 2 show the same ratio computed for ASAS measurements. 
View zenith angle Wavelength 
Figure l.a Nadir normalized reflectance over bate soil Figure 2.a Nadir normalized ASAS reflectance over 
as a function of view zenith angle. Solar zenith=52°. bare soil as a function of view zenith angle. Solar 
- TM1 -XS1 -. XS2 ..XS3 zenith=27.5°. 
5 . 15 - 
View zenith eng le 
a function of view zenith angle. Solar zenith=52°. 
-TM1 -XS1-.XS2 ..XS3 
cotton as a function of view zenith angle. Solar 
zenith=27.5°. 
3.1 Soil 
The reflectance ratio shows variations from -20% to 50% around the nadir reflectance with varying angles, and a slight 
increase of this range with the solar zenith angle. All the spectral bands exhibit similar behavior, except for the hot spot 
always small and decreasing with increasing wavelength. 
ASAS data show different features, although the range of variation remains almost the same. The hot spot can be 
observed there in all the spectral bands available with a slight decrease with increasing wavelength. Additional data were 
acquired in perpendicular plane and in SPOT acquisition plane. They allow us to check that directional effects outside the 
principal plane are small (from -20% to 5%) and, for the perpendicular plane rather symmetrical. 
3.2 Cotton 
No hot spot configuration was available over cotton for the aircraft The reflectances exhibit variations ranging from -15% 
to 50% of nadir reflectance. All the bands show similar features, except in forward scattering (positive view zenith angle) 
The ASAS data show, in blue and red region, an increase of the reflectance in forward scattering up to 80% more than the 
nadir reflectance for a large solar angle (52°, not shown here). This behavior is seldom observed and do not appear for