                                                                              
                                                                              
  VEX Orbit Number Files                                                      
  ======================                                                      
                                                                              
  This "aareadme.txt" file describes the contents of the kernels/orbnum       
  directory of the VEX SPICE data server.  It also provides the file naming   
  conventions used for the VEX 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.                                              
                                                                              
  Contact                                                                     
                                                                              
	ESA SPICE Service       esa_spice@sciops.esa.int
                                                                              
  , or                                                                        
                                                                              
    Jose Luis Vazquez    jlvazquez@sciops.esa.int                             
                                                                              
  if you have any questions.                                                  
                                                                              
                                                                              
                                                                              
                                                                              
  References                                                                  
  ==========                                                                  
                                                                              
    1. Data Delivery Interface Document (DDID)       Appendix H - FD          
       products Issue 3.0 (VEX-ESC-IF-5003)                                   
                                                                              
                                                                              
                                                                              
  Important Note Regarding Orbit Number Files                                 
  ===========================================                                 
                                                                              
  In the Venus 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, 2006 APR 20.         
                                                                              
  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 VEX orbit number file (spacing was altered to fit the 
  page):                                                                      
                                                                              
                                                                              
   No.     Event UTC PERI       Event SCLK PERI      OP-Event UTC APO     ... 
  =====  ====================  ====================  ==================== ... 
      1  2006 APR 20 08:07:37    1/0035885257.24893  2006 APR 21 04:19:46 ... 
      2  2006 APR 22 00:32:13    1/0036030733.35058  2006 APR 22 20:44:33 ... 
      3  2006 APR 23 16:57:00    1/0036176220.25438  2006 APR 24 05:48:44 ... 
      4  2006 APR 24 18:40:36    1/0036268836.20944  2006 APR 25 07:32:26 ... 
                                                                              
                                                                              
  Given this example one can determine that:                                  
                                                                              
         --   orbit 3 pericenter time is ``2006 APR 23 16:57:00''             
                                                                              
         --   orbit 3 time boundaries are from ``2006 APR 22 20:44:33'' to    
              ``2006 APR 24 05:48:44''                                        
                                                                              
                                                                              
                                                                              
  Venus Express Orbit Number Directory Structure                              
  ==============================================                              
                                                                              
                                                                              
   aareadme.txt                    This file.                                 
                                                                              
   former_versions                 Subdirectory that contains obsolete        
                                   versions of the ORBNUM files that have been
                                   updated in the Current ORBNUM File Set.    
                                                                              
   ORVF_______________xxxxx.ORB    Contains Venus Express orbit pericenter and
                                   apocenter times, orbital elements, and     
                                   additional geometry parameters computed    
                                   from the SPK long term file named:         
                                                                              
                                      ORVF_______________xxxxx.BSP            
                                                                              
                                   where xxxxx designates the version number. 
                                   This is the predicted file that should be  
                                   used for planning.                         
                                                                              
   ORVV_MERGED_xxxxx.ORB           Contains Venus Express orbit pericenter and
                                   apocenter times, orbital elements, and     
                                   additional geometry parameters computed    
                                   from the latest set of ORVV SPK files      
                                   available in the FTP server. xxxxx         
                                   designates the version number. This file   
                                   corresponds to the reconstructed orbit of  
                                   the spacecraft, and therefore should be    
                                   used for data analysis, but not for        
                                   planning.                                  
                                                                              
                                                                              
                                                                              
-------------------                                                           
                                                                              
        This file was last modified on March 17th, 2009. (J. Vazquez)         
