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The signal acquisition system operates in two channels called
CCDI and CCD2. The first one generates images corresponding
to. B2, B3 and B4 while the second generates images
corresponding to the bands Bl, B3 and B5. In each channel
(channel CI and channel C2), three CCD chips per band were
combined to generate about 6000 pixels per row.
3. PSF ESTIMATION METHODOLOGY
Basically, there exist three ways to determinate the PSF. The
first one uses images with targets that must have well-defined
shape and size as airport runway, bridges, etc or artificial target.
The second method utilizes images acquired by higher
resolution sensor, which are compared with the image under
study. Finally, the third one uses the system design
specifications and the system analytic model (Fonseca, 1987;
Fonseca and Mascarenhas, 1987).
The first two approaches have the advantage of estimating the
imaging system PSF by using in-flight images acquired by the
system. In this work, the first approach was implemented and
on-orbit images of bridges were chosen as target to estimate the
spatial resolution in along-track and across-track directions.
3.1 Target Images
The Rio-Niteroi Bridge over Guanabara Bay (Figure 1 and
Figure 2) was chosen as target to estimate the spatial resolution
in the along-track direction. This bridge is 13.29-km long with
only one deck and its width is 26.6 meters. On the other side,
the Causeway Bridge over the Lake Pontchartrain (Figure 3 and
Rio de Janeiro / Büzios
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Fig.1 Map of Rio-Niteroi bridge in
Guanabara bay.
Fig.2 Aerial image of Rio-Niteroi bridge in
Guanabara bay.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
Figure 4) was used as target to estimate the spatial resolution in
across-track direction. The bridge is constituted of two decks
and a gap between them. The target is a 38.62-km long double
deck bridge where each deck is 10.0 meters width and the gap is
24.4 meters width. The two decks of the bridge were
constructed at different times (1956 and 1969) and exhibit
slightly different reflectance. In addition, the water background
is reasonably uniform.
3.2 Data preparation
The Rio-Niteroi Bridge and Lake Pontchartrain Causeway
Bridge images were acquired by CBERS-1 CCD sensor on
December 02, 2001 and October 06, 2002, respectively. Figure
5-top and Figure 6-left show, respectively, Rio-Niteoi bridge
and Causeway bridge images of band 3. In order to facilitate the
visualization, the images were enhanced by histogram contrast
and zoomed up.
The images acquired by CBERS-1 system, before any kind of
processing (raw data), present a striping effect as shown in the
Figures 5 (top) and 6 (left). Odd columns are brighter than even
columns. This is due to the non-uniform detector gains, since
each detector is responsible for one column in the images. The
processing procedure to remove the stripping effect has been
described in (Bensebaa, et al. 2003).
Figure 5 (bottom) and Figure 6 (right) display the processed
images of Rio-Niteroi Bridge and Causeway Bridge,
respectively. One can observe that the striping effect has been
completely eliminated without removing the target information.
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Fig.3 Map of Causeway bridge over
Pontchartrain lake.
Fig.4 Aerial image of Causeway bridge over
Pontchartrain lake.
