Th`ese Marouan BOUALI - Sites personnels de TELECOM ParisTech

112 5. Application : Restoration of Aqua MODIS Band 6
Figure 5.1 – (Left) Image from Aqua MODIS band 6 showing 4 non functional detectors
(Right) Image from Aqua band 6 after removal of missing lines : functional detectors are
contaminated with stripe noise
approaches related to the analysis of snow cover use the Normalized Difference Snow Index
(NDSI). This index exploits snow high reflectivity in the visible (0.5-0.7 µm) and its low
reflectance in the SWIR (1-4 µm). Defined as a spectral band ratio, the NDSI allows a
robust separation of snow from clouds and also reduces the influence of atmospheric effects
and viewing geometry [Salomonson and Appel]. On MODIS, the NDSI can be expressed
as the difference of reflectances ρ measured in the visible band 4 (0.555 µm) and a SWIR
band such as band 6 divided by the sum of the two reflectances :
NDSI 16 = ρ 4 − ρ 6
ρ 4 + ρ 6
(5.1)
The evaluation of the NDSI index on the Aqua sensor is problematic due to the non
functional detectors. Nevertheless, to ensure complementary observations to Terra MODIS
band 6 and provide continuous monitoring and mapping of snow coverage, scientists have
relied on Aqua MODIS band 7 (2.105-2.155 µm). This is a reasonnable alternative, given
that snow has similar reflectance properties over bands 6 and 7, which happen to display a
high correlation over land surfaces. As a result, Aqua-based snow coverage is determined
using a secondary NDSI defined as :
NDSI 17 = ρ 4 − ρ 7
ρ 4 + ρ 7
(5.2)
It was however pointed out in [Hall et al.], that the reflectance of snow is slightly weaker
in band 7 compared to band 6. Consequently, values of NDSI 17 are higher than NDSI 16
and the estimation of snow coverage is compromised. Given the importance of retrieving