PDS_VERSION_ID          = PDS3
RECORD_TYPE             = STREAM

OBJECT                  = TEXT
   INTERCHANGE_FORMAT   = ASCII
   PUBLICATION_DATE     = 2012-03-15
   NOTE                 = "This file describes the files in the
                           extras/orbnum directory of this volume."
END_OBJECT              = TEXT

END


Orbit Number Files for the MESSENGER Mission
============================================

   This directory contains the orbit number files generated based on
   the reconstructed data part of the merged spacecraft trajectory SPK
   files for the orbital phases of the mission. 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).

   The orbit number files with the ``.orb'' extension contain data that
   follow the ``apoapsis to apoapsis'' orbit numbering scheme -- with
   the orbit number changing at apoapsis. This numbering convention was
   adopted by the MESSENGER science team based on the missions orbital
   characteristics and the needs of science planning and analysis.

   The information contained in the orbit number files includes the
   orbit number (``No.''),  orbit start UTC time (``Event UTC APO'')
   and SCLK time (``Event SCLK APO''), orbit's periapsis UTC time
   (``OP-Event UTC PERI''). It also includes a few additional items
   computed at the time of apoapsis such as planetocentric subsolar
   longitude and latitude (``SolLon'' and ``SolLat'') in the Mercury
   body-fixed frame based on the Mercury rotation constants provided in
   the latest MESSENGER PCK file at the time when the orbit number file
   was made, planetocentric subspacecraft longitude and latitude (``SC
   Lon'' and ``SC Lat'') in the Mercury body-fixed frame based on the
   same rotation constants, and spacecraft altitude above Mercury 
   (``Alt'').