Generic Satellite Ephemeris Files
                     B. Semenov -- May 23, 2022
                  
This folder contains SPICE format (SPK) versions of the most recent 
natural body satellite ephemeris files produced by JPL's Solar System 
Dynamics Group. 

The naming schema for these files is usually nnnxxx.bsp, where:

  nnn      the first three letters of the planet's name  (mar, jup, 
           sat, ura, nep, plu)

  xxx      unique number, not really of any value to SPICE users
  
In some cases there is additional information included in the file name
following xxx, such as:

  -rocks   meaning the ephemeris is based on an orbital theory and not 
           on an integration

  l        meaning this is a longer version of another file having the 
           same name

  xl       meaning this is a much longer version of another file having
           the same name

  -[name]  meaning this SPK provides data for just one body specified   
  -[ID]    by its name or NAIF ID

  _part-N  meaning this SPK provide coverage for a part of the full 
           coverage span (N=1 for the first part, N=2 for the second, etc)

and so on.

For each planetary system having satellites (Mars through Pluto) there 
may be one or several files. When there is more than one file for a planet, 
each file contains ephemeris data for a unique set of satellites -- there 
is no duplication of satellites across files. (This is true in most cases
but there are some rare exceptions.)

Each satellite SPK provides the state (position and velocity) of the named 
satellites relative to the barycenter of that planetary system.

Each satellite ephemeris SPK file also contains the ephemeris for the 
central planet, what is often referred to as the location of the planet mass 
center. In this sense each planet is treated as as a satellite of its own 
barycenter, just as for the "real" satellites.  As an example, the Mars
satellite ephemeris file (name style is marxxx.bsp) contains ephemeris 
data for Phobos (ID = 401), Deimos (ID = 402) and Mars' mass center 
(ID = 499). Also included (except in SPKs with extra long coverage) are 
the locations of the earth barycenter (ID = 3), earth itself (ID = 399) 
and the sun (ID = 10).

NAIF's SPK tutorial provides more information about barycenters, and about 
how to use (read) SPK files. That tutorial is found here: 
    
    http://naif.jpl.nasa.gov/naif/tutorials.html 

Each of these SPKs contains some meta-data in the comment area. Typically 
the meta-data are those provided by the person who generated the source 
file from which the SPK was made. (The Solar System Dynamics Group uses a 
local, internal format for their ephemeris files.) You may view those
meta-data by reading the comment area, using the "commnt" utility available 
in each Toolkit. Those same comments are often available in this folder, 
in files having a "cmt" extension.

The file named "aa_summaries" provides a convenient summary of the 
contents (which satellites are included) and time coverage for every SPK 
file available in this folder.

Older versions of satellite SPK files are found one level down, in the 
"a_old_version" folder.

Be aware of one oddity: the ephemeris for the earth's moon is *NOT* 
available in a file found in this folder.  Instead, the lunar ephemeris 
is included in each SPICE planetary ephemeris (name style dexxx), found 
in the adjacent planets folder (../planets).