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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
2. DATA PROCESSING
The image data returned from the spacecraft are distributed to
the Cassini imaging team in VICAR (Video Image
Communication and Retrieval) format [http://www-
mipl.jpl.nasa.gov/extemal/vicar.html]. The first step of the
image processing is the radiometric calibration of the images
using the ISS Team’s CISSCAL computer program (Porco et al.,
2004). The next step of the processing chain is to map project
the images to the proper scale and map format - a process that
requires detailed information about the global shape of the
satellite. The inner Saturnian satellites are best described by tri-
axial ellipsoids as derived from ISS images by Thomas et al.
(2006). However, to facilitate comparison and interpretation of
the maps, ellipsoids were used only for the calculation of the
ray intersection points, while the map projection itself was done
onto a sphere with the mean radius. The Cassini orbit and
attitude data for the calculation of the surface intersection
points are provided as SPICE kernels [http://naif.jpl.nasa.gov]
and were improved using a limb-fitting technique (Roatsch et
al., 2006). The coordinate system adopted by the Cassini
mission for satellite mapping is the IAU “planetographic”
system, consisting of planetographic latitude and positive West
longitude (Seidelmann et al., 2006). Because a spherical
reference surface is used for map projections of the satellites,
planetographic and planetocentric latitudes are numerically
equal. The Hapke photometric model (Hapke, 1993) was
applied to adjust the brightness of each map pixel so that it
represents the reflectance that would be observed at the nadir at
30-deg phase angle. Imaging data viewed at incidence and
emission angles greater than 80-deg were omitted from the map.
After photometric correction, mosaicking is the final step of the
image processing (Roatsch et al., 2006).
3-D control nets are common to correct errors in the nominal
camera pointing data. Due to the lack of appropriate stereo data,
only in the case of the Enceladus mosaic we got acceptable
results of the least- squares adjustment.
3. MOSAICS AND BASEMAPS
Imaging of the medium-sized icy satellites is ongoing and will
continue until the end of the Cassini mission, making it possible
to improve the image mosaics during the tour.
The basemaps (global mosaics) are usually produced using
images of very different resolutions. Effort was made to choose
a map resolution in pixel per degree expressible as 2 n , where n
is an integer. It was not practicable to follow this rule for all
basemaps (see Table 2) avoiding a loss of high-resolution image
information. The map projection for the basemaps is the
equidistant projection centered at 0 degrees longitude. These
basemaps cover the whole satellite from -180° till 180° West
longitude and from -90° till 90° latitude. However, some areas
of the satellites are imaged at very high resolution. These higher
resolution images were processed to separate mosaics (see
Figure 1 for an example).
At the time of writing, thousands of images from the icy
satellites are available (e.g. 3073 Enceladus images, 2553
Dione images, and 2000 Tethys images). This total data set
contains images obtained through a variety of different ISS
color filters and at spatial resolutions ranging from 3 m/pixel up
to 14 km/pixel. For our mosaic, we selected only those images
taken with the filters CL1, CL2 or GRN, as these images show
comparable albedo contrasts among different surface terrains.
For example, 83 Cassini NAC images and two Voyager image
were used to produce a 64 pixel/deg global mosaic of Dione
(Figure 2). The resolution of the selected Cassini images varies
between 0.16 and 1.72 km/pixel whereas the resolution of the
Voyager images are 1.45 km/pixel and 6 km/pixel.
4. ICY SATLLITE ATLASES
We produced a global printed map of Phoebe in a scale of
1:1,000,000 and atlases of Enceladus (1: 500,000), Dione
(1:1,000,000), and Tethys (1:1,000,000) (Table2).
Satellite
Mean radius
used for the
map
projection
[km]
Resolution
of the
digital map
[pixel/
degree]
Map scale of the
atlas
Mimas
198.8
8
-
Enceladus
252.1
40
1:500.000
Tethys
536.3
32
1:1,000,000
Dione
562.53
64
1:1,000,000
Rhea
764.0
20
-
Iapetus
736.0
16
1:3,000,000
. (in preparation)
Phoebe
106.8
8
1:1,000,000
Table 2. Resolution and scale of the maps
The atlases consist of 15 tiles each of them conform to the
quadrangle scheme proposed by Greeley and Batson (1990) and
Kirk (1997, 2002, 2003) for large satellites (Figure 3). The
scales guarantee a mapping at the highest available Cassini
resolution and results in an acceptable printing scale for the
hardcopy map of 4.5 pixel/mm. The individual tiles were
extracted from the global mosaic and reprojected, coordinate
grids were superposed as graphic vectors and the resulting
composites were converted to the common PDF-format using
the software package Planetary Image Mapper (PIMap) of the
Technical University Berlin (Gehrke et al., 2006). The
equatorial part of the map (-22° to 22° latitude) is in Mercator
projection onto a secant cylinder using standard parallels at -13°
and 13° latitude. The regions between the equator region and
the poles (-66° to -21° and 21° to 66° latitude) are projected in
Lambert conic projection with two standard parallels at -30°
and -58° (or 30° and 58°, respectively). The poles are projected
in stereographic projection (-90° to -65° latitude and 65° to 90°
latitude). The Mercator maps are 72° in longitude dimension,
the Lambert maps 90°, and the poles 360°. The individual tiles
overlap in the North-South direction by one degree, however no
overlapping region is present in the East-West direction (see
Figure 3). We have produced the maps using the same scaling
factors in overlapping regions at the matching parallels ±21.34°
and ±65.19° latitude, 1.0461 and 1.0484 respectively (Snyder,
1987). We also added a resolution map and an index map for
every individual tile, showing the image resolution, the image
numbers and the location of the images within the individual
quadrangle (Roatsch et al., 2008a and b).
The Cassini imaging team proposed new names for geological
features, in addition to the features already named by the
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