RECORD_TYPE           = "STREAM"                                              
OBJECT                = TEXT                                                  
  PUBLICATION_DATE      = 2003-10-01                                          
  NOTE                  = "Converting IMAGE_TIME Document."                   
END_OBJECT            = TEXT                                                  
END                                                                           
                                                                              
Converting IMAGE_TIME to START_TIME and STOP_TIME (UTC)                       
=======================================================                       
**Note: IMAGE_TIME keyword has been replaced by NATIVE_START_TIME**           
                                                                              
DS1 had an onboard counter that kept ephemeris time.  However,                
the counter drifted from the true ephemeris time by as many                   
as tens of seconds.  During the course of the mission, the                    
counter was occassionally sychronized with the true time.                     
The JPL NAIF team mentioned the counter had most drift during                 
the early part of the mission.                                                
                                                                              
To ensure accurate image times for the encounter with comet                   
Borrelly, the navigation team sychronized the onboard ephemeris               
time counter immediately before the event.  The JPL NAIF team                 
noted that before this reset, the counter was probably off by                 
several seconds.  Therefore,  all PDS label values for START_TIME             
and STOP_TIME prior to 2001-09-22 UTC could vary from the true UTC            
by several seconds.                                                           
                                                                              
In the MICAS VISCCD labels, IMAGE_TIME is the value of the                    
onboard ephemeris counter at the start of an image exposure.                  
This counter represents ephemeris time seconds past the                       
reference epoch of 1998 Jan 01 12:00:00 ET.  To convert                       
IMAGE_TIME to START_TIME (UTC):                                               
                                                                              
  1) Convert '1998 JAN 01 12:00:00 TDB' to ET.                                
  2) Add IMAGE_TIME to the result of step 1).**                               
  3) Convert the result of step 2) to UTC for START_TIME.                     
                                                                              
To convert IMAGE_TIME and EXPOSURE_DURATION to STOP_TIME (UTC):               
                                                                              
  1) Convert '1998 JAN 01 12:00:00 TDB' to ET.                                
  2) Add IMAGE_TIME and EXPOSURE_DURATION to the result of step 1).**         
  3) Convert the result of step 2) to UTC for STOP_TIME.                      
                                                                              
** If the onboard ephemeris counter was off and the drift is                  
   known is seconds, add this value in step 2).                               
                                                                              
Below is a section of the FORTRAN code that PDS SBN used                      
to convert IMAGE_TIME and EXPOSURE_DURATION to UTC                            
START_TIME and STOP_TIME in labels.  The code uses                            
NAIF/SPICE routines and assumes the SPICE kernels for                         
the DS1 spacecraft clock and the leapseconds are loaded:                      
                                                                              
  CALL STR2ET ( '1998 JAN 01 12:00:00 TDB', ETBASE )                          
  ET = ETBASE + ETIMG + 0.D0                                                  
  CALL TIMOUT( ET, 'YYYY-MM-DDTHR:MN:SC.###', UTCSTART )                      
  ET = ET + EXPOSR                                                            
  CALL TIMOUT( ET, 'YYYY-MM-DDTHR:MN:SC.###', UTCSTOP )                       
                                                                              
where ETBASE   = reference epoch converted to ET.                             
      ETIMG    = value of IMAGE_TIME from the PDS label                       
      0.D0     = known drift (s) for the onboard ephemeris counter;           
                 Zero is valid for encounter because the onboard              
                 counter was reset for the event;  Prior to                   
                 encounter, the drift is undetermined and could               
                 be non-zero.                                                 
      EXPOSR   = value of EXPOSURE_DURATION from the PDS label                
      UTCSTART = UTC start time of the image                                  
      UTCSTOP  = UTC stop time of the image