JPLEPHREAD

   Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
   craigm@lheamail.gsfc.nasa.gov
   UPDATED VERSIONs can be found on my WEB PAGE:
      http://cow.physics.wisc.edu/~craigm/idl/idl.html

   Open and read JPL DE200 or DE405 Ephemeride FITS File

   Planetary Orbits, Interpolation

   JPLEPHREAD, FILENAME, INFO, RAWDATA, JDLIMITS, STATUS=, ERRMSG=

   JPLEPHREAD opens and reads the JPL DE200 or DE405 planetary
   ephemerides, as available in FITS format.  The user must have the
   IDL Astronomy Library installed to use this routine.
   This routine is the initialization stage of a two-stage process to
   interpolate the JPL ephemeris.  In this first stage, the file is
   opened, and the relevant portions of the table are read and stored
   into the two variables INFO and RAWDATA.  In the second stage, the
   user actually interpolates the ephemeris for the desired bodies
   and to the desired ephemeris time using JPLEPHINTERP.
   Users must decide ahead of time the approximate dates of interest,
   and pass this range in the JDLIMITS parameter.  Any date covered
   by the ephemeris is valid.
   JPLEPHREAD is able to read files of the following format:
     DE200 - Chebyshev - FITS format - Note 1
     DE405 - Chebyshev - FITS format - Note 1
     DE200 - Taylor    - FITS format - Note 2
   Note 1 - Chebyshev formatted FITS files are available in the
            AXBARY package by Arnold Rots, found here:
              ftp://heasarc.gsfc.nasa.gov/xte/calib_data/clock/bary/
            or at the Markwardt FTP site:
              ftp://cow.physics.wisc.edu/pub/craigm/bary/
   Note 2 - Taylor-series based ephemerides have been available for
            years in the FTOOLS / LHEASOFT package produced by NASA's
            Goddard Space Flight Center.  The original file is
            de200_new.fits, which covers the years 1959-2000,
            inclusive.  A newer file is named
            de200_1950-2050_v2.fits, and covers the years 1959-2050.
            See Markwardt FTP site for these files.

   FILENAME - name of ephemeris file (scalar string).
   INFO - upon completion, information about the ephemeris data is
          returned in this parameter in the form of a structure.
          Users must not modify INFO, although several fields are
          useful and may be accessed read-only:
              TSTART/TSTOP (start and stop time of data in Julian
                            days);
              C (speed of light in km/s);
              DENUM (development ephemeris number [200 or 405])
              AU (1 astronomical unit, in units of light-seconds)
   RAWDATA - upon completion, raw ephemeris data is returned in this
             parameter.  Users are not meant to access this data
             directly, but rather to pass it to JPLEPHINTERP.
   JDLIMITS - a two-element vector (optional), describing the desired
              time range of interest.  The vector should have the
              form [TSTART, TSTOP], where TSTART and TSTOP are the
              beginning and ending times of the range, expressed in
              Julian days.
              Default: entire table is read (note, this can be
              several megabytes)

   STATUS - upon completion, a value of 1 indicates success, and 0
            indicates failure.
   ERRMSG - upon completion, an error message is returned in this
            keyword.  If there were no errors, then the returned
            value is the empty string, ''.

   Find position of earth at ephemeris time 2451544.5 JD.  Units are
   in Astronomical Units.
   JPLEPHREAD, 'JPLEPH.200', pinfo, pdata, [2451544D, 2451545D]
   JPLEPHINTERP, pinfo, pdata, 2451544.5D, xearth, yearth, zearth, $
                 /EARTH, posunits='AU'

   AXBARY, Arnold Rots.
      ftp://heasarc.gsfc.nasa.gov/xte/calib_data/clock/bary/
   HORIZONS, JPL Web-based ephermis calculator (Ephemeris DE406)
      http://ssd.jpl.nasa.gov/horizons.html
   JPL Export Ephemeris FTP Site
      ftp://navigator.jpl.nasa.gov/pub/ephem/export/
      (ephemeris files are available here, however, they must be
      converted to FITS format using the "bin2eph" utility found in
      AXBARY)
   JPL Export Ephemeris CD-ROM - Ordering Information
      http://www.willbell.com/software/jpl.htm
   Standish, E.M. 1982, "Orientation of the JPL Ephemerides,
      DE200/LE200, to the Dynamical Equinox of J2000", Astronomy &
      Astrophysics, vol. 114, pp. 297-302.
   Standish, E.M.: 1990, "The Observational Basis for JPL's DE200,
      the planetary ephemeris of the Astronomical Almanac", Astronomy
      & Astrophysics, vol. 233, pp. 252-271.

   JPLEPHREAD, JPLEPHINTERP, JPLEPHTEST

     FXBCLOSE, FXBOPEN, FXPAR(),

   Written and Documented, CM, Jun 2001
   Use GETTOK() instead of STR_SEP()  W. Landsman  July 2002
  $Id: jplephread.pro,v 1.6 2001/07/01 03:32:02 craigm Exp $

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