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the NA</title></head><body>
<div>Dear SPICE Users-</div>
<div><br></div>
<div>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:</div>
<div><br></div>
<div> <font face="Lucida Grande" size="-4" color="#000000">
ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/</font></div>
<div><br></div>
<div>An "aareadme" in this directory provides many of the
details. But below we provide those details, and a bit more
information.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>Best regards,</div>
<div>Chuck Acton, for the NAIF Team</div>
<div><br></div>
<div><br></div>
<div><br></div>
<div>I. Organization</div>
<div>-------------------</div>
<div>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. </div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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).</div>
<div><br></div>
<div>2) <font color="#000000"> In each top level and each
"a_old_versions" directory we have provided three<br>
files that provide some summary information, as follows.<br>
<br>
aa_summaries.txt:<br>
<br>
contains SPK content and coverage
information for every .bsp<br>
file in that directory. These are
produced using the Toolkit's<br>
"brief" utility program, with
the -t option.<br>
<br>
aa_spk_production_dates_by-alpha.txt: <br>
<br>
provides a simple alphabetically
organized directory listing of<br>
all the *.bsp files in the
directory.<br>
<br>
aa_spk_production_dates_by-date.txt: <br>
<br>
provides a simple time ordered
directory listing of all the *.bsp<br>
files in the directory.<br>
<br>
CAUTION: the date associated with each file is the date the SPK
file<br>
was produced on the NAIF machine. This is *NOT* the same as the
date<br>
the ephemeris solution was originally produced by the author of
the<br>
source product; in some cases there are many months or even years
of<br>
difference!<br>
</font></div>
<div><font color="#000000">In most cases more and metadata about a
given kernel are provided within</font></div>
<div><font color="#000000">the so-called "comment area"
inside the binary kernel. You can examine<br>
this metadata using the "commnt" or "spacit"
utility program found in<br>
each SPICE Toolkit. NAIF strongly recommends that a potential
user of<br>
a given kernel first examine these metadata before making a
decision<br>
about whether or not to use the kernel.<br>
</font><br>
<font color="#000000"></font></div>
<div>II. Updates</div>
<div>----------------</div>
<div>The updating has been done only in the area of SPK files for
natural satellites and for some asteroids.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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).</div>
<div><font face="Lucida Grande" size="-4"
color="#000000"
>ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/asteroids/</font
></div>
<div><br></div>
<div><br></div>
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