The SOHO Ancillary Data                                                       
------------------------------------------------------------------------      
                                                                              
Spacecraft time                                                               
                                                                              
The SOHO On Board Time (OBT) uses the CCSDS format, level 1 (TAI              
reference, 1958 January 1 epoch), The SOHO OBT is an unsegmented time         
code with a basic time equal to 1 second and a value representing the         
number of seconds from 1 January 1958 based on International Atomic           
Time. The OBT Pulse is adjusted to maintain the OBT within +/-20ms of the     
ground TAI.                                                                   
                                                                              
The SOHO OBT is used to time tag the data packets sent to the EOF and to      
the Data Distribution Facility (DDF). The time tags for the spacecraft        
and instrument housekeeping packets are generated by the spacecraft           
on-board data handling system. The time tags for the instrument science       
data packets are inserted by the instruments generating the science           
data. The time tags are provided in 6 bytes; the first 4 bytes are            
TAI seconds (2^0 to 2^31 seconds) and the last 2 bytes are fractions of a     
second with the resolution of the On Board Time Pulse (2^-11 seconds).        
                                                                              
The SOHO Daily Pulse is generated every 86,400 seconds, and is                
synchronized to the TAI with an accuracy better than 100ms. The Daily         
Pulse will correspond to the beginning of a TAI ``day'', that is the          
Daily Pulse will occur at the zeros of TAI modulo 86,400. As of 1             
January 1993, the difference between TAI midnight and 00:00 UTC was 27        
seconds. Since 1 July 1993 UTC-TAI = --28sec (TBC).                           
                                                                              
                                                                              
Ground time                                                                   
                                                                              
Coordinated Universal Time (UTC) is used as the operational time              
reference in the Experiment Operations Facility. The ``SOHO operations        
day'' is defined to begin at 00:00 UTC and the computer systems in the        
SMOCC and EOF will be synchronized to run on UTC.                             
                                                                              
                                                                              
Solar rotation axis                                                           
                                                                              
The solar rotation axis is calculated using the Carrington ephemeris          
elements. These elements define the inclination of the solar equator to       
the ecliptic as 7.25 degrees, and the longitude of the ascending node of      
the solar equator on the ecliptic as (75.76 + 0.01397*T) degrees, where T     
is the time in years from J2000.0.                                            
                                                                              
Heliographic longitudes on the surface of the Sun are measured from the       
ascending node of the solar equator on the ecliptic on 1 January 1854,        
Greenwich mean noon, and are reckoned from 0 to 360 degrees in the            
direction of rotation. Carrington rotations are reckoned from 9 November      
1853, 00:00 UT with a mean sidereal period of 25.38 days, and are             
designated as CR^1903 etc..                                                   
                                                                              
------------------------------------------------------------------------      
Downloaded: ftp://sohoftp.nascom.nasa.gov/pub/data/ancillary/index.html       
            7 Sept 2004                                                       
Modified:   M. Knight                                                         
            14 Sept 2004                                                      
