cameras: The
Both cameras
1odells. Due to
tching process
Kameras, die
jeide Kameras
obale digitale
hn werden die
ire stellen. Die
vältigen.
are the main
ı of the Mars
single optic
method. The
ng track triple
AOSS
.7 mm
84
.0°
4"
0°
n
240 Hz
m
.5-15 ms
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cameras are
JS) which has
.
> camera has
els. The main
listed in Table
1.3 Orbit of Mars 96 Mission
To get a very close sight to the planet Mars the spacecraft
orbit is highly eccentric. At periapsis the spacecraft
altitude will be approximately 300 km. By the true
anomaly of 90 degree the altitude will be 3000 km. The
orbit period will be about 12 - 14 h. The proposed orbit
does not guarantee constant illumination conditions. The
sun elevation will be different at overlapping image strips
and will partly be less than 30 degrees during the
mapping period.
2. MARS 96 DATA
2.1 The HRSC and WAOSS Image Data
The HRSC and WAOSS image geometry is completely
different to data geometry of other CCD sensor like the
SPOT or the TM scanner, as well as the MOM-02 three
line scanner data.
As a result of the orbit design the image pixels will have a
dynamically varying ground pixel size. There will be
across and along track effects. Due to the increasing
spacecraft altitude ground resolution will differ by the
factor of ten. The highest resolution of HRSC nadir
channel will be 12 meters at periapsis with an orbit
altitude of 300 km. By a true anomaly of 90 degree and an
altitude of 3000 km the ground pixel resolution will be 120
meter. The numerical values of the WAOSS image pixels
will be 96 and 960 m. Because of the great across track
field of view of the WAOSS camera the ground resolution
differs considerably among centre and margin pixel of a
single sensor line (Fig. 1). Due to the narrow field of view
of the HRSC sensor this effect is negligible.
true anomaly [°]
10 30 40 50. 60 70
1400
1200
E 1000 margin pixel
800 middle pix
600 center pixel
400
ground pixel size [m
200
0
0 500 1000 1500
time after periapsis passage [s]
Figure 1:WAOSS image along and across track
effects of the pixel ground size. The ground
resolution variation of the center, a middle
and the margin pixel for the nadir CCD line
is plotted.
Apart the intrastrip effects there will be interstrip effects.
Corresponding pixels of the three stereo channels will be
imaged with different pixel size due to slant range and
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
macro pixel format. The limitation of onboard mass
storage and the bottleneck of radio link communication
with the ground stations make macro pixel forming
essential. Generally the macro pixel formats of the HRSC
stereo channels will be 2 by 2 pixel or 4 by 4 pixel. The
HRSC macro pixel mode is constant during a single
scanning period. The macro pixel formats of the WAOSS
camera is more complex. Different macro pixel formats
can be joined during a scanning session to simulate
locally constant ground resolution. By that the line of
intersection between different macro pixel steps are
potential matching barriers.
Beside the macro pixel format the HRSC and WAOSS
cameras uses DCT compression to reduce the amount of
data. At high compression rates blocking structures and
artefacts will affect the image data adversely
(Schlotzhauer G. 1994, Heipke C., 1995).
Moreover the stereo image data processing will be
seriously interferred by a poor texture of large areas of the
Martian surface. Gaps, at which the correlation failed,
maybe filled by matching with a shape from shading
technique (Heipke C., et al, 1994).
Apart poor textured areas on Mars are regions with
surface discontinuities and with height differences much
larger than known from Earth.
2.2. Additional Data
For the determination of the conjugate points we need
auxiliary data in order to calculate approximate values to
reduce the search areas in image space and for the
controlling of the matching results in the object space. In
particular to handle the complicated image geometry
additional information will be required for an image data
pre-processing step.
The basic apriori information will be the navigation data of
the spacecraft: The position and pointing data together
with planetary and instrument parameters. These data are
components of the SPICE kernels (Spacecraft, Planet,
Instrument C-matrix, Event Data). The position and
pointing data will be improved by a bundle adjustment
(Ohlhof T., 1996). For this purpose an automatic and
manual tiepoint measurement will be carried out (Tang L.,
1994). These input data for the bundle adjustment
becomes also apriori information, with high priority, for the
matching process.
Existing Martian elevation models are coarse and their
accuracy and reliability are low (Ohlhof T., 1995). So we
can estimate an average terrain altitude but we will make
no further use of them.
3.1 General
Normally the determination of the conjugate points takes
place by matching the three stereo images of a single
HRSC/WAOSS mapping period.
The advantage of using only these data are:
* The scanning periods are really short. Differences of
the illumination will be very small and do not affect the
matching process. On the other hand the image data of
adjoining orbits may have completely different
illumination conditions.
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A d sl s mm
