mMHhhmmmbmh
HM
We selected 7 steady sites (Table 1), expected to be unvarying seasonally as well as from year to year.
Table 1 - Positions of the targets
Latitude
Longitude
Surface type
Argentina
37IFS
66.U W
Frame
Australia
20.0 S
132.0 E
Sand desert
Brazil
0.0
59.0 W
Forest
China
40.0 N
85.0 E
Sand desert
Kalahari
23.6 S
24.0 E
Stone desert
Libya
24.3 N
13.0 E
Sand desert
USÄ
32.5 N
106.2 W
Sand desert
The signal extracted from the original GVI product was calibrated in visible and near infrared reflectances, using the first
post-launch available coefficients (from Kaufman and Holben, 1993) and correction for atmospheric effects where
applied using the SMAC method (Rahman and Dedieu, 1994) with climatological values for water vapor (Oort, 1983)
and ozone content (London et al., 1976) and a constant aerosol optical depth of 0.05 at 550 nm. Surface temperature was
derived through a split window formula (Kerr et al., 1993). The mean and standard deviation of those three values were
also computed over 3x3 pixel windows around selected target. These last quantities were used to filter the signal
according to a method described in Cabot and Dedieu 1993a.
It can be seen on figure 1 that the signal we obtain after all these preprocessing is much smoother than the original signal
but still shows variations of about 10%, partly due to bidirectional effects.
METEOSAT measurements were extracted from the so-called ISCCP B2 archive, which provide visible and thermal
infrared channel digital counts from the METEOSAT satellites, subsampled to a spatial resolution of 30 km. One image
is available every 3 hours. We focused here on METE0SAT4, which entered in operation in June 1989 and is still in
operation. Digital counts were extracted from this data set over the libyan desert, for two hours (11.30 and 14.30 UT),
calibrated in reflectances using an fixed value of the calibration factor (Kriebel and Amann, 1991), corrected for
atmospheric effects, in the same way as NOAA/AVHRR measurements, and filtered for cloud and large optical depth
contamination. The filtering procedure is described in Dedieu 1993. Filtered reflectances, plotted on figure 2, still show a
large dispersion, but are principally organized in two main clusters corresponding to the two hours of acquisition.
0 200 400 600 800 1000 1200
Day tinea launch
Figure 1. Surface reflectance over Australian site.
VIS: unfiltered - filtered, NIR:.. unfiltered -- filtered of the number of day since launch, over Libyan desert
3. METHOD
3.1 Gain drift monitoring
After accounting for atmospheric contamination, one has to consider bidirectional effects. In this study, Rahman et al
(1993) model was used. The basic assumption is that, if we are able to normalize the reflectances acquired throughout the
whole period to a reference geometry, since this reflectance should be constant we should see in this normalized signal
only an evolution due to the gain drift of the instrument.
In order to achieve this normalization, we retrieved, through a least square computation, parameters of the model,
performing the inversion on a sliding window. With these retrieved parameters, we computed reflectances for a reference
geometry (Sim at zenith and nadir viewing). Since the reflectance of the surface is assumed to be constant, and according
to the following expression for the reflectance:
na(DN-DN 0 )
( 1 )
E o cos0 s (d 0 /d) :
42
where p is the surface r
atmospheric solar irradi
distance, a the gain os
assumptions that i) atrr
variations of the calibral
If we compute a reflects
we then can write:
where (0) represents th
procedure to the two ser
We assumed that the off:
GVI data. Kaufman and
1990.
3.2 Inter calibration
To achieve intercalibra
reflectances. This is doi
1991, in a METEOSA1
simulated digital counts
where a,, and a M rep
and a 2 are the weights ti
Eq,, Eo 2 and Eq are the ex
are the computed digital
4. RESULTS AND DIS
In what follows, we com
value for the calibration
of the ratio of the calibr;
the factor of the day of i
used a calibrated spectn
operated in order to get
transformed in satellite 1
year a desertic area in N
geometric conditions, ii
measurements, correctec
sensor with calibration p
Arizona (UAZ).
4.1 Gain drift monitorinj
The different sites, i
present in almost all
authors. Second, son
a remaining noise d
atmosphere over the
can see on the graph
shortly after tire eru
beginning of the peri
Comparison between
with the highest refl<
they show very simi
