Table I: Cassini flybys in 2004/2005 Details of the image processing will be described in chapter 2, Voyager maps will be shown in chapter 3, followed by Cassini image mosaics and maps in chapter 4. 2. Data Processing 2.1 Voyager All images taken by the Imaging Science Subsystems (ISS) aboard Voyager-1 and Voyager-2 are available online from the Planetary Data System (PDS) Imaging Node [http://pdsimaging.jpl.nasa.gov]. Though many images have been acquired with narrow band filters only clear filter images were used during the processing reported here. The first steps of the data processing chain are the conversion from PDS format to VICAR (Video Image Communication and Retrieval) format, followed by the radiometric and geometric calibrations using standard VICAR programs [http://rushmore.jpl.nasa.gov/vicar.html]. The next step is to convert the images to digital maps, which requires precise orbit and pointing data for each image. We used the position and pointing data by Davies and Katayama [1983a, b, c, 1984] which were derived by block adjustment techniques, and delivered electronically in 1989 [Davies et al., personal communication]. For other images improved pointing data were calculated using limb-fitting techniques and nominal pointing data as input. The inner Saturnian satellites are best described by tri-axial ellipsoids as recommended in the report of the International Astronomical Union (IAU) [Seidelmann et al., 2002]. However, to facilitate comparison and interpretation of the maps, ellipsoids were only used for the calculation of the ray intersection points, while the map projection was done onto a sphere with the mean IAU radius. All projection parameters are described in section 3. The final step of the image processing is the combination of all map projected images to a homogeneous mosaic. Special care must be taken to handle the different ground resolutions within overlapping regions and the variable illumination conditions in the different images in order to minimize the loss of high-resolution image information and contrast. Both map projection and mosaicking are carried out following procedures described in Scholten [1996] and Scholten et al. [2005] for Mars imagery. 2.2 Cassini Though the Cassini-ISS camera takes images using many different filters [Porco et al., 2004], we used only images taken with the filters CL1, CL2 or GRN, as these images show similar contrast. The processing of the Cassini images follows basically the same processing sequence as for the Voyager images. For the Cassini mission, spacecraft position and camera pointing data are available in the form of SPICE kernels [http://naif.jpl.nasa.gov]. While the orbit information is sufficiently accurate to be used directly for mapping purposes, the pointing information must be corrected using limb fits (see Figure 1 for an example). High-resolution images that do not contain the limb were registered to limb images to improve the pointing. For the Cassini maps, newly derived tri-axial ellipsoid models [Thomas, 2005] were used to calculate the surface intersection points (for new mean radii see table III). The coordinate system adopted by the Cassini mission for satellite mapping is the IAU "planetographic" system, consisting of planetographic latitude and positive west longitude, but because a spherical reference surface is used for map projections of the satellites, planetographic and planetocentric latitudes are numerically equal. Digital maps are prepared in simple cylindrical projection, a special case of equirectangular projection. The mapping cylinder is tangent to the equator of the sphere, the longitude range is 0 degrees to 360 degrees W and latitude range -90 degrees to 90 degrees [Kirk, 2004]. The prime meridian is in the center of the map. Additionally to the Voyager data processing steps, a photometric correction using the Henyey-Greenstein function [Hapke, 1993] was applied to the image data before mosaicking with function parameters adopted from Verbiscer and Veverka [1989, 1992, 1994] and Simonelli et al. [1999]. 3. Voyager mosaics and maps The map resolutions (in pixel/deg) chosen were depending on mosaic resolution rounded to nearest integer and turn out to be similar or identical to those of the 'Standard Cartographic Products' established by the United States Geological Survey (USGS) (http://astrogeology.usgs.gov/Projects/SaturnSatellites/). Map sheets were produced to conform with the design and standards of the USGS airbrush maps and photomosaics, established by Greeley and Batson [1990], widely used in planetary cartography. The map sheets combine a Mercator map within the latitude range of -57 degrees to 57 degrees and two polar maps in stereographic projection polewards beyond +/- 55 degrees latitude. All maps include current nomenclature [http://planetarynames.wr.usgs.gov] (see Figure 2 for an example). Table II shows the resolution of all mosaics and the scale of all produced maps [Roatsch et al., 2004]. 4. Cassini mosaics and maps 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 starting points of global mosaics for any satellite are the Voyager mosaics in which areas can be replaced gradually by higher-resolution Cassini images as data become available. At some point in time new mosaics can be generated on the basis of Cassini image data, where Voyager data fill the gaps between the Cassini images. In these maps, the satellite coverage, as expected by the end of the nominal Cassini mission in July 2008, can be visualized (see Figure 3 for an example). The global mosaics are usually produced using images of a similar resolution. However, some areas of the satellites are imaged at very high resolution. These higher resolution images were processed to separate mosaics (see Figure 4 for an example). The data set of Phoebe is the only one that is complete, as no more high-resolution images are expected during the mission. Also, we will not obtain new high-resolution Enceladus images until 2008. Therefore standard maps were generated for these two satellites. Table III shows the resolution and scale of these maps. A global mosaic and a standard map sheet of Phoebe were produced in a scale of 1:1,000,000 (see Figure 5). We produced the Phoebe mosaic from 27 narrow- angle (NA) images of the Cassini ISS camera. We used the Mercator projection within the latitude range -57 degrees to +57 degrees and the stereographic projections polewards beyond +/-55 degrees, respectively. As proposed by Greeley et al. [1990] the projections are conformal, the quadrangles overlap, and the scale of the poles was chosen such that the circumference of the stereographic projection at the center of the overlap is identical to the width of the Mercator projection. The nomenclature was proposed by the Cassini imaging team and has yet to be validated by the IAU. The resolution of the mosaics is 0.233 km/pixel, although the highest resolution images have resolutions of 0.07 km/pixel. A dedicated regional orthoimage mosaic for Phoebe using true topography as reference surface was derived and is being reported in a separate paper [Giese et al., this issue]. We used 47 images of the narrow-angle and two images of the wide-angle (WA) camera to produce a 40 pixel/degrees global mosaic and a standard map for Enceladus. The map is produced in a scale of 1:500,000 consisting of a quadrangle scheme with 15 tiles, as proposed by Greeley et al. [1990] and Kirk [1998, 2002, 2003] for large satellites (see Figure 6). A map scale of 1:500,000 guarantees a mapping in the highest possible resolution. The equatorial part of the map (-21 degrees to 21 degrees latitude) is in Mercator projection onto a secant cylinder using standard parallels at -13 degrees and 13 degrees latitude. The regions between the equator region and the poles (-66 degrees to -21 degrees and 21 degrees to 66 degrees latitude) are projected in Lambert conic projection with two standard parallels at -30 degrees and -62 degrees (or 30 degrees and 62 degrees respectively). The poles are projected in stereographic projection (-90 degrees to -65 degrees latitude and 65 degrees to 90 degrees latitude). 5. Outlook The Cassini spacecraft will continue its imaging campaign through the Saturnian system. Satellite close flybys are scheduled for Tethys, Dione, Rhea, Iapetus, and Enceladus within the nominal mission ending in 2008. These data will be used to further improve the existing semi-controlled mosaics and maps and also to update or calculate initial global geodetic control networks of the Saturnian satellites for controlled orthophoto mosaics. Acknowledgements: The authors gratefully acknowledge helpful discussions with R. Kirk (USGS) about the cartographic standards to use for the Cassini mapping activities and for reviewing the Voyager and Cassini maps. References Davies, M. E. and Katayama, F. Y., 1983a, The control networks of Mimas and Enceladus, Icarus 53, 332-340. Davies, M. E. and Katayama, F. Y., 1983b, The control networks of Tethys and Dione, J. Geophysical Research, 88, 8729-8735. Davies, M. E. and Katayama, F. Y., 1983c, The control network of Rhea, Icarus 56, 603-610. Davies, M. E. and Katayama, F. Y., 1984, The control network of Iapetus, Icarus 59, 199-204. Hapke, B., 1993, Theory of Reflectance Spectroscopy, (Topics in Remote Sensing; 3), Cambridge University Press 1993, p. 272. Greeley, R. and Batson, G., 1990, Planetary Mapping, Cambridge University Press. Kirk, R.L., Becker, T.L., Rosanova, T., Soderblom, L.A., Davies, M.E. and Colvin, T.R., 1998, Digital Maps of the Saturnian Satellites- First Steps in Cartographic Support of the Cassini Mission, Jupiter after Galileo, Saturn before Cassini Conference, Nantes, France. Kirk, R., 1997, 2002, 2003, presentations to Cassini Surfaces Working Group. Porco, C. C. and 19 co-authors, 2004, Cassini Imaging Science: Instrument Characteristics and Anticipated Scientific Investigations at Saturn, Space Science Review 115, 363-497. Porco, C. C. and 34 co-authors, 2005a, Cassini Imaging Science: Initial Results on Phoebe and Iapetus, Science 307, 1237-1242. Porco, C. C. et al., 2005b, Enceladus, submitted to Science. Roatsch, T., Oberst J. , Giese B. , Wahlisch M. , Winkler V. , Matz K.-D., Jaumann R. , Neukum G., 2004. Cartography of The Icy Saturnian Satellites, International Archives of Photogrammetry and Remote Sensing, Vol. XXXV, Part B4, p. 879-884, Istanbul. Scholten, F., 1996, Automated Generation of Coloured Orthoimages and Image Mosaics Using HRSC and WAOSS Image Data of the Mars96 Mission, International Archives of Photogrammetry and Remote Sensing, Vol. XXXI, Part B2, p.351-356, Vienna. Scholten, F., Gwinner, K., Roatsch, T., Matz, K.-D., Wahlisch, M., Giese, B., Oberst, J. , Jaumann, R. and Neukum, G., 2005. Mars Express HRSC Data Processing - Methods and Operational Aspects, Photogrammetric Engineering & Remote Sensing, 71, 1143-1152. Seidelmann, P.K., Abalakin, V.K., Bursa, M., Davies, M.E., de Bergh, C., Lieske, J.H., Oberst, J., Simon, J.L., Standish, E.M., Stooke, P., Thomas, P.C., 2002, Report of the IAU/IAG/COSPAR Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2001. Simonelli, D. P., Kay, J., Adinolfi, D., Veverka, J., Thomas, P. C., and Helfenstein, P., 1999, Phoebe: Albedo Map and Photometric Properties, Icarus 138, 249-258 Smith, B. A., and 29 co-authors, 1982, A new look at the Saturn system: The Voyager 2 images, Science 215, 504-537. Thomas, P., 2005, Presentation at the Cassini-ISS Team Meeting, Florence, Italy. Verbiscer, A. and Veverka, J., 1992, Mimas: Photometric roughness and albedo map, Icarus 99, 63-69. Verbiscer, A. and Veverka, J., 1994, A Photometric Study of Enceladus, Icarus 110, 155-164. Verbiscer, A. and Veverka, J., 1989, Albedo dichotomy of Rhea: Hapke analysis of Voyager photometry, Icarus 82, 336-353.
Figure 1: Limb position for satellite Dione, predicted from the SPICE kernel (top) and after interactive limb fitting (bottom)
Figure 2: Voyager map of Rhea at scale 1:5,000,000. The nominal scale is that of the Mercator map at the Equator; the scales of the polar maps are somewhat larger.
Figure 3a: Iapetus base map generated from Voyager data.
Figure 3b: Iapetus base map generated from Cassini and Voyager data, produced following the Iapetus flyby in December 2004.
Figure 3c: Total Iapetus coverage planned through the end of Cassini's nominal miss
Figure 4: High-resolution mosaics of Phoebe (top) with a map scale of 10 m/pixel, in sinusoidal projection and in a latitude /longitude range of 8.2 degrees N to 17.5 degrees S and 13.4 degrees W to 305.4 degrees W. Mimas (bottom) with a map scale of 370 m/pixel, in orthographic projection and in a latitude /longitude range of 71 degrees N to 69.8 degrees S and 313 degrees W to 180 degrees W.
Figure 5: Phoebe map at 1:1,000,000 scale
Figure 6: Quadrangle scheme adopted for regional mapping, beginning with high-resolution maps of Enceladus.
Table II: Resolution and scale of the Voyager maps. Note, that there is not map of Iapetus due to the low image resolution.
Table III: Resolution and scale of the Cassini maps.