133
34. Natural surfaces and derived products
The operational utility of the method is evaluated using a model of natural surface reflectance. Soil bi
directional reflectance is approximated by Hapke’s functions. The bi-directional reflectance of a vegetated
surface is modelled by the SAIL radiative transfer approximation (Verhoef 1984), extended to include the hot
spot effect of finite leaf size (Jupp and Strahler 1991). Soil spectra are taken from the Purdue Soils Database
(Stoner et al. 1980) and leaf optical properties generated using the PROSPECT model (Jacquemoud and Baret
1990).
Table 3 shows results of operation on three surfaces under clear and turbid atmospheric conditions. The
sensitivity of the algorithm to violation of the assumption of Lambertian reflectance is apparent in the retrieved
albedos for vegetated surfaces. However it is apparent that the errors are consistent between clear and hazy
atmospheres. This suggests that these recovered albedos may be used in vegetation analysis. Examination of
NDVI index values derived from these reflectance shows the desired sensitivity to leaf area index but
independence from atmospheric effects. Figures 6 a and 6 b show a comparison between NDVI values derived
from top-of-atmosphere radiances and NDVI based on values corrected using this method. Further work is
currently being undertaken to evaluate the sensitivity of the derived index on canopy structure.
Surface
Channel
Surface reflectance
Retrieved parameters at
lOKm visibility.
Retrieved parameters at
40Km visibility.
Nadir
50°
albedo
visibility
albedo
visibility
Bare soil
Cl
2.53
2.56
233
9.96
2.53
36.97
C2
3.11
3.14
3.13
10.07
3.02
38.93
C3
6.96
7.05
6.99
10.00
7.00
40.07
C4
8.41
8.48
8.47
10.09
8.49
41.12
Homogenous
Cl
9.62
10.77
7.13
7.09
7.96
26.18
canopy
C2
3.26
3.59
2.49
9.05
2.77
34.75
LAI=2.0
C3
37.81
41.00
44.73
136
42.86
2.14
C4
16.43
18.15
14.91
4.15
14.75
8.02
Homogenous
Cl
10.25
1132.
7.78
7.03
8.77
26.91
canopy
C2
337
3.61
2.50
9.04
396
36.32
LAI=4.0
C3
4921
52.67
6837
2.18
69.01
2.17
C4
17.65
19.30
1635
4.06
16.06
7.86
Table 3. Results of operation on natural surface reflectance models at two visibilities
4 - DISCUSSION
A method has been presented for atmospheric correction of radiances measured by ATSR-2 optical channels.
The method uses the dual-look capability of the sensor to simultaneously retrieve atmospheric aerosol loading
and surface reflectance. Correction is performed for each channel separately. The method suggested allows the
use of accurate forward models but is operationally fast due to exploitation of pre-computation. A sensitivity
analysis has been performed using an end-to-end simulation of radiative transfer. Results indicate highly
accurate retrieval of surface albedo for a near Lambertian surface, and robustness to tropospheric aerosol
variation. The results show considerable sensitivity to deviation of the surface reflectance from Lambertian,
and that this sensitivity is implicit in the information content of two views of a surface at a single wavelength.
Initial results using a model of vegetated surface reflectance suggest indices derived from the algorithm output
are robust to variation in atmospheric turbidity.
Further work is being undertaken to explore use of the information content of multiple wavebands to allow
inversion of more sophisticated surface and atmospheric models. This necessitates assumptions about the likely
spectral variation of surface bi-directional reflectance. Realistic models of natural surface bi-directional
reflectance and exact models of atmospheric scattering are being developed for algorithm evaluation.
