8.10. Cassini ISS NAC

This is a proof of concept showing the strength of building the Stereo Pipeline on top of ISIS. Support for processing ISS NAC stereo pairs was not a goal during our design of the software, but the fact that a camera model exists in ISIS means that it too can be processed by the Stereo Pipeline.

Identifying stereo pairs from spacecraft that do not orbit their target is a challenge. We have found that one usually has to settle with images that are not ideal: different lighting, little perspective change, and little or no stereo parallax. So far we have had little success with Cassini’s data, but nonetheless we provide this example as a potential starting point.

8.10.1. Rhea

Rhea is the second largest moon of Saturn and is roughly a third the size of our own Moon. This example shows, at the top right of both images, a giant impact basin named Tirawa that is 220 miles across. The bright white area south of Tirawa is ejecta from a new crater. The lack of texture in this area poses a challenge for our correlator. The results are just barely useful: the Tirawa impact can barely be made out in the 3D data while the new crater and ejecta become only noise.

../_images/cassini_rhea_quad.png

Fig. 8.8 Example output of what is possible with Cassini’s ISS NAC. Upper left: original left image. Upper right: original right image. Lower left: mapprojected left image. Lower right: 3D Rendering of the point cloud.

8.10.1.1. Commands

Download the N1511700120_1.IMG and W1567133629_1.IMG images and their label (.LBL) files from the PDS.

ISIS> ciss2isis f=N1511700120_1.LBL t=N1511700120_1.cub
ISIS> ciss2isis f=W1567133629_1.LBL t=W1567133629_1.cub
ISIS> cisscal from=N1511700120_1.cub to=N1511700120_1.lev1.cub
ISIS> cisscal from=W1567133629_1.cub to=W1567133629_1.lev1.cub
ISIS> fillgap from=W1567133629_1.lev1.cub to=W1567133629_1.fill.cub

(Note the optional fillgap command above.)

ISIS> cubenorm from=N1511700120_1.lev1.cub to=N1511700120_1.norm.cub
ISIS> cubenorm from=W1567133629_1.fill.cub to=W1567133629_1.norm.cub
ISIS> spiceinit from=N1511700120_1.norm.cub
ISIS> spiceinit from=W1567133629_1.norm.cub
ISIS> cam2map from=N1511700120_1.norm.cub to=N1511700120_1.map.cub
ISIS> cam2map from=W1567133629_1.norm.cub map=N1511700120_1.map.cub \
ISIS>   to=W1567133629_1.map.cub matchmap=true
ISIS> parallel_stereo N1511700120_1.map.equ.cub                     \
        W1567133629_1.map.equ.cub result/rhea

See Section 6 for a discussion about various speed-vs-quality choices. Also consider the following modifications to stereo.default:

### PREPROCESSING
alignment-method none
force-use-entire-range
individually-normalize

### CORRELATION
prefilter-mode 2
prefilter-kernel-width 1.5

cost-mode 2

corr-kernel 25 25
corr-search -55 -2 -5 10

subpixel-mode 3
subpixel-kernel 21 21

### FILTERING
rm-half-kernel 5 5
rm-min-matches 60 # Units = percent
rm-threshold 3
rm-cleanup-passes 1