networks in real time to a center where the data are analyzed. GOES and METEOSAT 
geostationary satellites are presently used. Data collected in Brazil (Holben et al., 1994) 
showed the high potential of this new generation of instruments. There are plans to expand 
aerosol measurements using such networks, as part of the international research and 
monitoring activities and as part of the support and validation of analysis of remote sensing 
from future satellite systems (The Earth Observing System, EOS of NASA and ADEOS 
mission of NASDA). 
4. INDIRECT METHODS FOR CORRECTING THE AEROSOL EFFECT 
An indirect correction is pursued by defining functions that describe the surface 
properties and that are less sensitive to the aerosol effect. Atmospherically resistant 
vegetation index, ARVI, was proposed and developed to be used for remote sensing of 
vegetation from the Earth Observing System MODIS sensor, scheduled to be launched in 
1988. The same index can be used for remote sensing from Landsat TM, and the SeaWiFS 
ocean sensor to be launched in 1994. Due to the introduction of bi-linear gain the SeaWiFS 
data are expected to be useful over the land as well. The index takes advantage of the 
presence of the blue channel (0.47±0.01 pm) in the MODIS sensor, in addition to the red 
(0.6610.025 gm) and the near IR (0.86510.02 pm) channels that compose the present 
normalized difference vegetation index - NDVI. The resistance of ARVI to atmospheric 
effects (in comparison to the NDVI) is accomplished by a self correction process for the 
atmospheric effect in the red channel, using the difference in the radiance between the blue 
and the red channels. 
P Qf, — P fift 
NDVI definition: NDVI =— — 
P S6 + P66 
POf. — P DD 
ARVI definition: ARVI = — — , p Tjn =(2p„-p._) 
n i n r RB r 66 r 47 / 
^86 ^RB 
where p. is the surface reflectance at i/100 pm. This atmospherically resistant vegetation 
index is based on the assumption that for most surfaces the information in the red channel 
is similar to that in the blue channel, and therefore the information content of p is 
similar to that of p^. But since on average the atmospheric effect in the blue channel is 
twice as large as in the red channel, the atmospheric effect on p R0 is much smaller than that 
on p 66 - The definition of ARVI is also based on the knowledge that the reflectance in the 
near IR (0.86 pm) is usually high (0.2-0.4) and therefore the atmospheric effect in this 
channel is small. 
Simulations using radiative transfer computations on analytic and natural surface 
spectra, for various atmospheric conditions, show that ARVI has a similar dynamic range to 
the NDVI (as a function of vegetation cover), but is on average 4 times less sensitive to 
atmospheric effects than the NDVI (see Fig. 7). The improvement is much better for 
vegetated surfaces than for soils. It is much better for moderate to small size aerosol 
particles (e.g. continental, urban or smoke aerosol) than for large particle size (e.g. maritime 
aerosol or dust). Note also that while the atmospheric effect on the red channel is a bias 
(almost always an increase of the reflectance), the definition of ARVI was chosen so that on 
average the atmospheric effect on ARVI will be random, sometimes positive and sometime 
negative. Therefore, ARVI not only significantly reduces the atmospheric effect but also 
transfers it into a random error. Due to the excellent atmospheric resistance of ARVI, it is 
expected that remo 
will include molec 
except for dust con 
Fig. 7: The orig 
index, NDVI (den 
the new vegetatio 
(solids lines), as a 
fraction of the sin 
vegetation for for 
alfalfa. Compu 
performed using t 
code (Tanre et al 
zenith angle is 60° 
(After Kaufman and 
5. Polarization 
Polarization m 
potential of polari 
planets with large 
1974; Santer and He 
atmosphere is ver} 
system were provic 
Shuttle missions ( 
performed from ba. 
What can we 
the use of polarizi 
polarization genera 
an experiment was 
al., 1993) with the ] 
in the south part o 
with different kind 
cover was theref 
contribution to the 
0.7 pm, the measun 
pattern suggesting 
A.=850nm, the ima; 
surface contributior 
in the blue parts c 
satellite signal can 
measurements frorr 
al., 1994) aboard thi 
of NASA in 2003. 
16