==============================================================================
MEX Orbit Number Files
==============================================================================

  Last update:  11 MAY 2006  

  This "aareadme.txt" file describes the contents of the KERNELS/ORBNUM
  directory of the MEX SPICE data server.  It also provides the file 
  naming conventions used for the MEX Orbit Number files, and it provides
  identification of the most current version of each kind of ORBNUM file.
  These are created from the SPK files converted from the ESOC's DDID
  Orbit Files.

  Orbit Number files (*.ORB) contained in this directory are unix text
  files. These may be used in a non-compatible text environment by using
  the ASCII mode of FTP to port the files.  [Ref 2a]

  Contact Jorge Diaz del Rio (jdiaz@rssd.esa.int) or Joe Zender 
  (Joe.Zender@esa.int) if you have any questions regarding this file.


==============================================================================
References
==============================================================================
    1. Data Delivery Interface Document (DDID)       Appendix H - FD 
       products Issue 3.0 (MEX-ESC-IF-5003)

    2. Selected SPICE Tutorials from the NAIF server:  
ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/office/individual_docs/

       a) Porting Kernels Tutorial      09_porting_kernels.ppt
       b) Derived Quantities Tutorial   21_derived_quant.ppt


==============================================================================
Important Note Regarding Orbit Number Files
==============================================================================
 
     In the Mars Express 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 project accepted 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.

     Orbit number 1 corresponds to the first mapping orbit,2004 JAN 13
 
     All derived quantities provided on each record -- sub-solar lon/lat,
     sub-spacecraft lon/lat/alt, and orbital elements -- are computed for
     the time of pericenter.

------------------------------------------------------------------------------
Example
------------------------------------------------------------------------------
 
     This is an example of MEX orbit number file (spacing was altered to
     fit the page):
 
        No Event UTC PERI       Event SCLK PERI  OP-Event UTC APO     ...
        == ==================== ================ ==================== ...
         1 2003 DEC 30 14:31:08 1/031415532.6386 2003 DEC 30 18:18:32 ...
         2 2003 DEC 30 22:05:53 1/031442817.0170 2003 DEC 31 01:53:20 ...
         3 2003 DEC 31 05:40:46 1/031470110.0775 2003 DEC 31 09:28:11 ...
         4 2003 DEC 31 13:15:33 1/031497397.5425 2003 DEC 31 17:03:01 ...
        ...
 
     Given this example one can determine that:
 
         --   orbit 3 pericenter time is ``2003 DEC 31 05:40:46''
 
         --   orbit 3 time boundaries are from ``2003 DEC 31 01:53:20'' to
              ``2003 DEC 31 09:28:11''
   
==============================================================================
Mars Express Orbit Number Directory Structure
==============================================================================
 aareadme.txt       Text file describing the directory structure, the naming
                    conventions for the ORBNUMs, and the current version(s)
                    of each file.

 former_versions    Subdirectory that contains obsolete versions of the 
                    ORBNUM files that have been updated in the Current
                    ORBNUM File Set.


==============================================================================
MEX Orbit Number Naming Covention               ffff_______________vvvvv.ORB
                                                xxxx_MERGED_vvvvv.ORB
==============================================================================


          token           description
          ------------    --------------------------------------------------
          ffff            Source Product Type Identifier: 
                             ORMF -- S/C Orbit, Mars centric. Long 
                                     Term Planning
          xxxx            Source Product Type Identifier:
                             ORMM -- S/C Orbit, operational, Mars
                                     centric.
          vvvvv           version number


==============================================================================
Current Orbit Number File Set
==============================================================================

 Name                            Comments
 -----------------------------   --------------------------------------------
 ORMF_______________xxxxx.ORB    Contains Mars Express orbit pericenter and 
                                 apocenter times, orbital elements, and
                                 additional geometry parameters computed from
                                 the SPK file named:
                                 ORMF_______________xxxxx.BSP
                                 where xxxxx designates the version number.

 ORMM_MERGED_xxxxx.ORB           Contains Mars Express orbit pericenter and 
                                 apocenter times, orbital elements, and
                                 additional geometry parameters computed from
                                 the latest set of ORMM SPK files available in
                                 the FTP server. xxxxx designates the version
                                 number.