This program creates the latest PCK SPICE DB file for the Cassini
missions. The Cassini mission continually refines its approximations of
planetary constants, and so regularly releases their latest PCK with each
SPK release. There are only two PCKs that need to be considered: a
general Cassini PCK and a Jupiter PCK. Each file is versioned with a date
string.
New version of Isis 3 are capable of resolving this date string and
grabbing the latest PCK from the data area. In an effort to support
legacy versions of Isis 3, this program adds Selection groups for the
direct paths to the latest PCK files, and is called by the makedb script
within the Cassini PCK directory in order to always contain the latest
PCKs.
When initializing a file, spiceinit will grab the last PCK in the DB file
that it can use for the given coverage period. With this in mind, the
date-versioned filenames should be placed below the direct filenames such
that new versions of Isis will resolve the latest PCK at runtime. This
way, in case a user is missing the directly specified PCK, they could fall
back to whatever happens to be the latest PCK in their data area.
Unfortunately, in legacy versions of Isis 3, spiceinit only looks at the
coverage time, and does not consider whether a file exists when
determining if it is usable, thus the date-versioned filenames must be
placed above the direct filenames, and will thus not be used until legacy
support is dropped.
Once all supported versions of Isis contain the date-versioning logic,
these direct filenames should no longer be added by this program, and
removed from all future DB files.
