that the coefficient A(0°) is now equal to 1.00 and does not depend of the viewing angle. This coefficient A is
close to the result given by Justice et al. [1991] for a Sahelian environment.
Figure 3: (a) Plot of Uh 2 o from Sunphotometer versus (T4-T5) over land for 9v equal to 0°
(b) Angular dependency of A(0) determined
3 - SURFACE REFLECTANCE IN CHANNEL 3
3.1. Formula of the surface reflectance.
The radiance in the channel 3 includes both the emissive part R[j and reflective part of the surface and of the
atmosphere. For a cloud free atmosphere the signal can be written as follow:
R3 = Rj + R]j (02)
with
R 3 = ^ P 3 E s cos(0 s ) T 3 (0 V ) X 3 (0 S ) (03)
where 0 S et 0 V are the solar and view zenith angles, X the transmittance, p 3 the reflectivity of the surface at
3.75|im, E s the incident solar radiance.
Using the Planck function, we can define the temperatures T 3 and T^ from R 3 and R^, then the equation (02)
becomes:
R 3 = B 3 (T 3 ) = B 3 (Tp + Rj (04)
In order to evaluate R]j, we need to determine the temperature due to emission T To minimize the effects of
variable atmosphere we use the relationship between T^-T 4 and T 4 -T 5 . Using the 36 atmospheres described
above, we compute T|, T4 and T5 for view zenith angle from 0° to 50° and for 20 emissivities ranging 1.00 to
0.80 for channels 3, 4 and 5.
T 4 (£,=1.00) - T, (£,=1.00)
T 4 (£ 4 =0.96) - T,(E,=0.%)
Figures 4: Comparisons between T 3 -T 4 with T 4 -T 5 from MODTRAN simulations for several case of £3 and e 45
(a) Typical case of Sunglint e 3 =0.80 and 0.94, £ 45 = 1.00, (b) Case of vegetation £3=0.90 and 0.94, £ 45 = 0.96
