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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
Figure 3. Example of a high-resolution stereopair formed by
Mars Global Surveyor MOC and Mars Express HRSC-SRC
images. Left, an HRSC color image of the caldera of
Olympus Mons showing context. Right, anaglyph
combining three 4-m/pixel SRC frames (h0037 0002
through 0004) in red with a 6-m/pixel MOC image (E10-
03979) in blue-green. Area of anaglyph is approximately
5x9 km in size and has been rotated slightly to facilitate
stereo viewing.
likely to exist. Figure 3 shows an example of a stereopair of
part of the caldera of Olympus Mons, consisting of a 6-
m/pixel MOC image and a set of three overlapping 4-m/pixel
SRC frames. The anaglyph was prepared by projecting the
two image sets in ISIS. Figure 4 shows an orthomosaic and
contour map derived from a 15 m/post DEM produced from
these images in SOCET SET.
3.3 Photoclinometry
Photoclinometry provides another means of producing
DEMs with horizontal resolutions of a few meters from SRC
images, and one that will be more widely applicable because
no high resolution stereo partner is needed. As mentioned
previously, photoclinometry must be "calibrated" by
determining how much atmospheric haze has reduced the
contrast of cach image, if quantitatively accurate results are
to be obtained for Mars. An independent estimate of the
topography is required but can be of lower resolution.
Calibration can then be accomplished either by performing
trial photoclinometry and adjusting the amount of atmo-
spheric haze subtracted from the image until feature heights
agree with the a priori data, or by simulating an image from
an a priori DEM and comparing its contrast with the real
image (Kirk et al., 2003b). The accuracy of calibration
depends on the possibility of finding adequately resolved
common features in the image and the topographic dataset.
If the only available topography comes from MOLA (which
has an effective horizontal resolution of hundreds of meters
at best), finding such features can be difficult or impossible,
but if a stereo DEM closer to the resolution of the image is
available, the photoclinometric topography can be calibrat-
ed to 10—2096 accuracy in amplitude (Kirk et al., 2003a).
Because HRSC stereo images are obtained simultaneously
with every SRC frame, nearly every SRC image should be
usable for calibrated photoclinometry, provided the surface
albedo is uniform in the area imaged. This opportunity is
also being exploited by Dorrer et al. (2004).
4. MARS EXPLORATION ROVERS ATHENA
4.1 Source Data
The Mars Exploration Rover Spirit landed in Gusev crater on
January 4 (UTC), 2004. It was followed 21 days later by the
rover Opportunity, which landed on Meridiani Planum. Each
rover carries a copy of the Athena science payload (Squyres
et al., 2003; Squyres and Athena Science Team, 2004), which
Includes two science camera systems. The topography,
morphology, and mineralogy of the scene around each rover
are revealed by the Pancam stereo camera (and also by the
837
Elevation (m)
above MOLA datum
Contour Interval 20
Figure 4. Elevation contours (20 m interval with index
contours every 100 m) derived from a DEM compiled from
the stereopair shown in Fig. 3 are overlaid on an
orthomosaic of the three HRSC-SRC frames. Simple
Cylindrical projection, north at top, ground sample distance
15 m for DEM, 4 m for orthomosaic.
Mini-TES thermal emission spectrometer) from 1.5 m above
the ground on a mast with azimuth-elevation articulation.
Filters on Pancam provide 14 color spectral bandpasses over
the spectral region from 0.4 to 1.1 mm. The angular
resolution and field of view of the camera are 0.28
mrad/pixel and 16° (Bell et al., 2003). The instrument arm
on each rover carries a Microscopic Imager (MI) that is used
to obtain high-resolution images of the same materials for
which compositional data are obtained. Its spatial resolution
is 30 mm/pixel over a 6-mm depth of field (Herkenhoff et al.,
2003). The MI has a limited two-band color imaging capa-
bility by taking exposures with its yellow-tinted dust cover
open and closed. These science cameras are supplemented
by several non-color engineering cameras (Maki et al.,
2003). Navcam, a stereo camera on the Pancam mast, has a
0.82 mrad/pixel angular resolution and 45? field of view.
Pairs of hazard avoidance cameras (Hazcams) with 124? field
of view are mounted at the front and rear of the rover.
4.2 Processing Objectives
The USGS has the primary responsibility within the Athena
team for processing of the MI images. This processing
includes deriving radiometric and geometric calibrations
and applying them to all images, creating color images by
combining either MI dust cover open/closed image pairs or
MI images and overlapping Pancam color image sets, and
making MI image mosaics and DEMs derived from MI
stereopairs. Additional DEMs and "focal section merges" are
produced by Athena team members at JPL and the NASA
Ames Research Center by a process of identifying and
combining the in-focus sections of images taken at different
distances from the target being images. The majority of
these objectives could be achieved by a simple processing
approach that considers only the MI and ignores constraints
on its motion. Because the instrument arm on which the MI
rides has only 5 angular degrees of freedom, the position
and pointing of the camera are not fully independent, but the
