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Figure 3: Detail from an ET3 image (nadir channel)
showing the summit of the Kronplatz in the Swiss Alps.
The blocky appearance (blow-up in the upper left
corner) is due to the effects of DCT data compression.
Note that although the diameters of the cables
(assiciated with cable cars going to the Kronplatz) are
below the nominal resolution of the camera, they are
clearly visible in the image. The distance from the
camera location to the summit was about 40km
—
,
S nsgeb uut
© 2 ©
3 <= S
N & E =
zn g
A.
$i.
3 2.32 a
Fk
frequency [%]
t2
0 50 100 150 200 250
DN
Figure 4: Histograms of panchromatic channels nadir,
stereo 1, and photometry 1 and 2, respectively.
3.3. Performance of the Flat-Field Correction
During the analysis of data from HRSC Extended Test 3 all
images were corrected for the different responsitivity of
sensor elements (Fig. 5). These "flat-field errors" are
visible in the uncorrected images as characteristic vertical
stripes, which, after the flat-field correction, are effectively
1.2 r- = ; . fr
TT
& Viris] di
S 1.0 : sacs
2 EE
5 0.9 | : | | |
0.8 LL 1 i | L | 1 I Li Hd ] LL il | 11-1 ] 1
0 1000 2000 3000 4000 5000
pixel number
Figure 5: Flat-field correction factors for sensor 1N
(nadir) as determined during the radiometric calibration.
351
Figure 6: Detail of an ET3 imaging sequence (nadir
channel) before (left) and after (right) flat field
correction. Note that the contrast in the images is
enhanced. The difference between adjacent pixels is
typically less than 2-3 DN values, i.e. only about 1% of
the total DN range. The horizontal line through the
image shows the location of the profile plotted in Fig.
Ta
removed. The performance of this correction has been
verified plotting the DN values along a profile in the
direction of the CCD sensor line before and after the flat-
field correction. To minimize the effects of texture, a profile
was selected which contains a relatively homogeneous sky
region (Figs. 6 and 7). The profiles show that much of the
high-frequency noise in the image is removed.
| ET3 Imaging
195 f Sequence 149 —
nadir channel
S 190
>
Z
A 185
180 F d
L1 p d ad L3 LL eel
0 50 100 150 20
pixel (sample) number
Figure 7: Brightness profile through the sky (see Fig. 6)
in the nadir image: The pixel (sample) number is plotted
against the respective DN values before and after the flat-
field correction. It is evident that the high-frequent noise in
the uncorrected image (dotted line) is effectively removed in
the corrected image (solid line).
3.4. Simulation of Imaging Conditions in Mars
Orbit
During selected imaging sequences, the rotational speed of
the turntable was gradually increased in order to simulate the
changing spacecraft speed as expected in the highly
elliptical Mars orbit. The scan rate of the camera was
changed accordingly in several steps, resulting in
increasingly smaller exposure times for the scan lines and a
decrease in image brightness.
To verify this type of camera operation, profiles of
greyvalues (DN values) through homogeneous sky regions
were sampled in flight direction (Fig. 8). The profiles show
indeed a correspondence between exposure times and
observed brightness. Additionally, this test could be used to
verify the correct operation of the software which computes
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996