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