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Title
Mesures physiques et signatures en télédétection

near infrared (x) and
ened data in Julyr
Julyr zenith (o) and offAugust
offAugust 1986. (from
;es (full curves) and
» and (b): visible
bars stand for the
lospheric correction.
170 180 190 200
of the retrieved
). Moreover, the
n induce large
7; Fig. 1). These
to be that of the
sm which is the
signal variations
die near infrared
ace bidirectional
ional reflectance
i various sources
tal effects which
ional reflectance
' the surface, and
Tie kind of data
i unavoidable. A
iporal variations
id discuss recent
of reflectances,
t position), and a
then replaced by
time profiles of reflectance corrected from angular effects, the latter reflectance being in turn derived from the
retrieved parameters of the regression.
Data composition techniques have been applied so far to AVHRR data. One should however
emphasize that appropriate techniques of data composition have also to be prepared for the next generation of
satellite optical sensors. Even data acquired by future geostationary satellites with visible and near infrared
channels, such as Meteosat Second Generation, will need to be corrected from the effect of variations of Sun
elevation during a vegetation cycle. Future heliosynchronous sensors, such as MODIS/EOS,
MERIS/ENVISAT, VEGETATION/SPOT4, will have the same basic geometric configuration as AVHRR and
will require similar compositing as AVHRR for vegetation monitoring applications. Other future
heliosynchronous sensors, such as POLDER/ADEOS, MISR/EOS, and ATSR/ERS2 will benefit of an
additional capacity of directional measurements, resulting in fine, through the use of data composition
techniques, in an improved temporal resolution.
2 - MAXIMUM VALUE COMPOSITE AND OTHER METHODS
The data compositing method which has been most widely applied is the Maximum Value Composite (MVC)
technique (Tarpley et al., 1984), whose principle is to select, during a given period of composition, the
AVHRR data which maximizes the Normalized Differential Vegetation Index (NDVI), defined as the
difference between near infrared and visible reflectances, divided by the sum of the two. The period of
composition may vary from 1 week (Tarpley et al., 1984) to 1 month (Justice et al., 1985) depending on cloud
cover conditions. The MVC technique has been a standard routine for most uses of AVHRR in the context of
vegetation monitoring (see the references mentioned in the introduction), and has been the basis of a Global
Vegetation Index (GVI) standard product distributed by the NOAA administration. The physical rationale of
this method is that the presence of clouds, aerosols, water vapour, and the increase of atmospheric paths along
the Sun-target or target-sensor trajectories, tend to decrease the NDVI (Holben, 1986). Therefore, the selection
of the AVHRR data maximizing the NDVI is expected to minimize the influence of all these perturbing
effects.
The adequacy of the MVC technique for land surface characterization has been questioned by Thomas
and Henderson-Sellers (1987), and Gutman (1987, 1991). They first have pointed out that this method selects
an erroneous measurement if all days are cloudy during the selected compositing period. Second, Gutman
(1991) has noticed that the MVC technique selects preferentially observations in the forward scattering
direction, a conclusion supported by the data set of Roujean et al. (1992a) over the Valensole plateau in the
South-East of France. Figure 3, from Gutman (1991), shows a frequency distribution of AVHRR data viewing
angles obtained by retaining 10-day maximum of NDVI indices on a site of the US Great Plains in Kansas
during the month of July. As noted by Gutman (1991), a strong bias towards the forward scanning direction is
obvious. Moreover, extended periods of composition (20- and 30-day) do not qualitatively modify this result
(Fig. 3). It is apparent in Fig. 3 that the selection procedure does not select a single viewing angle, which
results in errors since the surface is generally characterized by significant directional effects.
Fi gure 3 : Frequency distribution of viewing zenith
angles after 10-, 20-, and 30-day compositing by
retaining the day with maximum NDVI. (from
Gutman, 1991).
Other methods have also been proposed. Duggin et al. (1982) recommend to restrict the range of
viewing geometry to about +/- 25-30 °, with a corresponding loss of temporal resolution. Viovy et al. (1992)
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