[Spice_announce] Improvements and Updates to Generic Kernels area on the NAIF Server

[Charles H. Acton]

Dear SPICE Users-

NAIF has done some work to update data in, and better organize, 
portions of the SPK area within the generic_kernels tree on the NAIF 
server:

   ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/

An "aareadme" in this directory provides many of the details. But 
below we provide those details, and a bit more information.

We hope you find these long-past-due changes helpful and 
comprehensible. As always we solicit your feedback on further 
improvements, problems created, or suspected errors.

We know still more clean-up and documentation of the generic_kernels 
area is needed, and that provision of more/better tools to help you 
pick a SPICE file or files for some particular endeavor would be 
appreciated by many.

Best regards,
Chuck Acton, for the NAIF Team



I.  Organization
-------------------
We have somewhat modified the directory structure of the SPK trees, 
adding an "/a_old_versions" sub-directory wherever one did not exist, 
and moving old versions of the various kinds of SPK files to that new 
sub-directory.

1) The most significant change is that the ../spk/satellites 
directory is now rather smaller than it had been (although still not 
really "small"). All older versions of natural satellite SPKs have 
been moved to the a_old_versions folder; what remains in the primary 
folder are just the latest files for each natural satellite.

A similar cleanup was done for the planets, comets, and asteroids SPK 
directories. (One result is there are currently no comet SPK files in 
the primary comet directory:  .../spk/comets).

2)  In each top level and each "a_old_versions" directory we have 
provided three
files that provide some summary information, as follows.

    aa_summaries.txt:

       contains SPK content and coverage information for every .bsp
       file in that directory. These are produced using the Toolkit's
       "brief" utility program, with the -t option.

    aa_spk_production_dates_by-alpha.txt:  

       provides a simple alphabetically organized directory listing of
       all the *.bsp files in the directory.

    aa_spk_production_dates_by-date.txt:  

       provides a simple time ordered directory listing of all the *.bsp
       files in the directory.

CAUTION:  the date associated with each file is the date the SPK file
was produced on the NAIF machine. This is *NOT* the same as the date
the ephemeris solution was originally produced by the author of the
source product; in some cases there are many months or even years of
difference!

In most cases more and metadata about a given kernel are provided within
the so-called "comment area" inside the binary kernel. You can examine
this metadata using the "commnt" or "spacit" utility program found in
each SPICE Toolkit.  NAIF strongly recommends that a potential user of
a given kernel first examine these metadata before making a decision
about whether or not to use the kernel.


II.  Updates
----------------
The updating has been done only in the area of SPK files for natural 
satellites and for some asteroids.

All current natural satellite ephemerides available from JPL's Solar 
System Dynamics (SSD) Group (R. Jacobson in particular) have been 
moved to the NAIF server and made into the usual style of SPK files 
for such bodies. By "usual style" we mean that to the natural 
satellites and the mass center of the parent planet that are received 
on any given SSD satellite ephemeris file we add from an SSD planet 
ephemeris file (one of the so-called "DE" files): the barycenter of 
the planet system, the earth mass center and earth-moon barycenter, 
and the sun.

There are a few rather new Satellites for which SSD either does not 
yet have the data needed to make an ephemeris, or, does not yet have 
permission to publish the ephemeris. As new ephemerides do become 
available to NAIF we will add them to the generic_kernels collection 
and update the pertinent summary files described above.

As a result of some work done for a flight project a new SPK file 
containing ephemerides for a collection of 300 asteroids has been 
generated: this is named "codes_300ast_20061020.bsp." the data cover 
the period 1799 to 2200.  See the extensive comment file 
(codes_300ast_20061020.cmt) for a complete description and pedigree 
(source data provided by Jim Baer).
ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/asteroids/

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