PDS_VERSION_ID          = PDS3
RECORD_TYPE             = STREAM

OBJECT                  = TEXT
   INTERCHANGE_FORMAT   = ASCII
   PUBLICATION_DATE     = 2008-11-11
   NOTE                 = "This file describes the files in the
                           extras/orbnum directory of this volume."
END_OBJECT              = TEXT

END


Orbit Number Files for Viking Orbiters
======================================

   This directory contains the orbit number files generated at the
   archive preparation time using the NAIF's ORBNUM utility program for
   some of the reconstructed spacecraft trajectory SPK files included in
   the archive. The orbit number files have the same names as the
   corresponding SPK files but with the extension ``.bsp'' replaced by
   ``.orb''.

   The orbit number files are plain ASCII text files that have lines
   terminated with a line feed character, LF (ASCII 10).

   In the Viking orbit number files produced by NAIF's ORBNUM utility
   program each of the records contains the time of THAT orbit's
   pericenter and the time of the NEXT orbit's apocenter. For example,
   the line for orbit ``3'' provide UTC and SCLK of orbit ``3''
   pericenter and UTC of orbit ``4'' apocenter.

   Since the Viking orbit numbering scheme dictates the orbit number
   change to occur at apocenter, the time boundaries for a given orbit
   can be determined by taking the apocenter time from the PREVIOUS
   orbit record and the apocenter time from THIS orbit record.

   The information contained in the orbit number files includes the
   orbit number (``No.''), pericenter UTC time (``Event UTC PERI'') and
   SCLK time (``Event SCLK PERI''), apocenter UTC time (``OP-Event UTC
   APO''). It also includes a few additional items computed at the time
   of pericenter such as planetocentric subsolar longitude and latitude
   (``SolLon'' and ``SolLat'') in the Mars body-fixed frame based on
   the IAU 1994 Mars rotation model, planetocentric subspacecraft
   longitude and latitude (``SC Lon'' and ``SC Lat'') in the Mars
   body-fixed frame based on the IAU 1994 Mars rotation model,
   spacecraft altitude above Mars (``Alt'') relative to the IAU 1994
   Mars ellipsoid, and Sun-Mars distance (``Sol Dist'').

   Since some SPK files used to generate these orbit number files did
   not provide continuous coverage, the corresponding orbit number
   files may contain ``blank records'' that provide only the orbit
   number in the first column with all times and additional item
   columns set to blanks.


Contact Information
===================

   Regarding Viking project specific information provided in this data
   set and for general information about SPICE system, contact:

      Boris Semenov
      (818)-354-8136
      Boris.Semenov@jpl.nasa.gov

   address:

      Navigation and Ancillary Information Facility (NAIF)
      Jet Propulsion Laboratory
      MS 301-125L
      4800 Oak Grove Drive
      Pasadena CA 91109 USA