19 March 2024 - Mission Day: 10336 - DOY: 079
SOHO Ancillary Data
 
 

Contents


Data sets


Conventions and Coordinate Systems

The following conventions apply to facilitate the coordination of science planning, expedite the exchange of data between different instrument teams, and enhance the overall science activities.

Spacecraft time

The SOHO On Board Time (OBT) will use the CCSDS format, level 1 (TAI reference, 1958 January 1 epoch), as discussed in section 3.3.9 of the SOHO Experiment Interface Document Part A (Issue 1). 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 will be provided in 6 bytes; the first 4 bytes are TAI seconds (2 to 2 seconds) and the last 2 bytes are fractions of a second with the resolution of the On Board Time Pulse (2 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 July 1 1993 UTC-TAI = --28sec (TBC).

The helioseismology experiments plan to center one minute observations on the TAI minute, that is where TAI modulo 60 is zero.

Ground time

Coordinated Universal Time (UTC) will be 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 will be 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 , where T is the time in years from J2000.0.

The solar rotation axis used for alignment of the SOHO spacecraft will be determined from the Carrington ephemeris elements. The Experiment Interface Document Part A (Issue 1, Rev 3) lists the longitude of the ascending node of the solar equator as 75.62 and the position of the pole of the solar equator in celestial coordinates as 286.11 right ascension and 63.85 declination. This definition is consistent with a solar rotation axis determined from the Carrington elements for a date of 1 January 1990. As mentioned in the EID Part A, this information must be updated for the actual launch date.

Please note that the spacecraft is no longer aligned with the solar rotation axis since October 29, 2010 18:20 UT. Since that date it is aligned to the Ecliptic North Pole.

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 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 etc..

Inter-instrument flag reference coordinates

The spacecraft optical axes are defined with respect to the optical alignment cube of the Fine Pointing Sun Sensor, with the optical X axis (X) nominally perpendicular to the spacecraft launcher separation plane and pointing from the separation ring through the spacecraft. The spacecraft optical Y axis (Y) is along the direction of the solar panel extension with positive Y pointing from the interior of the spacecraft towards the UVCS instrument.

The orientation of the SOHO spacecraft is planned to have the spacecraft optical X axis (X) pointing towards the photometric center of the Sun, and the spacecraft optical Z axis (Z) oriented towards the north ecliptic hemisphere such that the (X,Z) plane contains the Sun axis of rotation. As such the Y axis will be parallel to the solar equatorial plane pointing towards the east (opposite to the solar rotation direction). ESA will be responsible for achieving this orientation with the misalignment margins defined in the EID-A.

A standard coordinate system is required for joint observations between instruments on the ground (for test purposes) and in space. This system, designated (X,Y), will be defined as follows: On the ground, the Y axis is parallel to the spacecraft Z axis and the X axis is anti-parallel to the spacecraft Y axis. In space, the (Y,Z) system is however no longer accessible. We will therefore define a virtual system (Y,Z), which is nominally coincident with (Y,Z) and where Y is perfectly aligned with the solar equator and its origin is at the Sun centre, and define (X,Y) in space as above using the virtual system (Y,Z).

The inter-instrument flag system (X,Y) thus has its origin at the Sun centre, its Y axis is in the plane containing the solar rotation axis pointing north, and its X axis positive towards the west limb. Each instrument participating in the flag exchange is reponsible for determining its orientation with respect to the (X,Y) system and report the coordinates of their observations in (X,Y) coordinates in units of 2 arcsec. Off-limb observations need special treatment if X, Y>1022''.

Spacecraft orbit coordinates

The Orbit data will describe the position and motion of the spacecraft, and it will be available in several coordinate systems including: geocentric inertial (GCI) coordinates for the J2000 system; geocentric solar ecliptic (GSE); geocentric solar magnetospheric (GSM) coordinates; and Heliocentric Ecliptic coordinate system.

The GSE coordinate system is defined as follows: The origin is Earth centered, with the X axis pointing from the center of the Earth to the center of the Sun; the Y axis lies in the ecliptic plane and points in the opposite direction of the Earth's orbital motion; the Z axis completes a right-handed orthogonal coordinate system and is parallel to the ecliptic pole. The Sun position is the true ``instantaneous'' position rather than the ``apparent'' (light-time delayed or aberrated) position. The ecliptic is the true ecliptic of date.

The Heliocentric Ecliptic coordinate system is defined as follows: the origin is Sun centered, with the Z axis parallel to the ecliptic pole with positive north of the ecliptic plane; the X-Y plane lies in the ecliptic plane and the X axis points towards the first point of Aries; the Y axis completes a right-handed orthogonal coordinate system.

The GCI coordinate system is defined as follows: Earth centered, where the X axis points from the Earth towards the first point of Aries (the position of the Sun at the vernal equinox). This direction is the intersection of the Earth's equatorial plane and the ecliptic plane --- thus the X axis lies in both planes. The Z axis is parallel to the rotation axis of the Earth and the Y axis completes a right-handed orthogonal coordinate system. As mentioned above, the X axis is the direction of the mean vernal equinox of J2000. The Z axis is also defined as being normal to the mean Earth equator of J2000.

The GSM coordinate system is defined as follows: again this system is Earth centered and has its X axis pointing from the Earth towards the Sun. The positive Z axis is perpendicular to the X axis and paralle to the projection of the negative dipole moment on a plane perpendicular to the X axis (the northern magnetic pole is in the same hemisphere as the tail of the magnetic moment vector). Again this is a right-handed orthogonal coordinate system.

Attitude files

The attitude files provide information about the pointing of the spacecraft. There are several kinds of attitude files depending on the SOHO mission phase:

Up until 2002 there are: Definitive attitude and Full Time Resolution attitude.
From October 29, 2010 onwards: Predictive attitude
During the entire life of the mission: Nominal Roll Attitude

The following graph shows the convention being used for yaw, pitch and roll:

Definitive attitude:

The SOHO Definitive Attitude files follow the ISTP standard and are produced in CDF by NASA's Central Data Handling Facility (CDHF). They contain pointing information in 10-minute intervals. The CDF files themselves have all the information to interpret the contents. Here is an overview:

Timestamp: 10-Oct-1995 09:10:00.000 Year: 1995 Day of year: 283 Ellapsed milliseconds of day: 33000000 S/C avg pitch angle ecliptic (rad): 0.00000 S/C avg roll angle ecliptic (rad): 0.00000 S/C avg yaw angle ecliptic (rad): 0.00000 S/C body axis avg pitch angle (rad): 0.00000 S/C body axis avg roll angle (rad): 0.00000 S/C body axis avg yaw angle (rad): 0.00000 GCI average pitch: 0.00000 GCI average roll: 0.00000 GCI average yaw: 0.00000 GSE average pitch: 0.00000 GSE average roll: 0.00000 GSE average yaw: 0.00000 GSM average pitch: 0.00000 GSM average roll: 0.00000 GSM average yaw: 0.00000 S/C body standard deviation pitch: 0.00000 S/C body standard deviation roll: 0.00000 S/C body standard deviation yaw: 0.00000 S/C body minimum pitch: 0.00000 S/C body minimum roll: 0.00000 S/C body minimum yaw: 0.00000 S/C body maximum pitch: 0.00000 S/C body maximum roll: 0.00000 S/C body maximum yaw: 0.00000

The SOHO Definitive Attitude files are also available in FITS format, with a binary table extension. The conversion is done at the SOHO GSFC Archive upon reception from the ECS. These FITS files contain all the columns listed above, in the same order, except the first one (the one labeled 'timestamp'). There is a row for each 10-minute entry.

Full Time Resolution Attitude:

In addition, and to fulfill the requirements from the helioseismology experiments on-board, full time resolution attitude are availble. These files contain pitch and yaw offset from Sun-center with a resolution of 10 times per second, and roll angle offsets from the projection of the Sun's North Pole one time per second. The units are degrees. This is the full time resolution of the attitude parameters as downlinked from the spacecraft.

In summary:

File Format Time Pitch Roll Yaw -------------------- ------ ---- ------- ------- ------- Definitive CDF UT radians radians radians FITS UT radians radians radians Full Time Resolution ASCII UT degrees degrees degrees

NOTE: Roll information might be incorrect at times. Please, refer to the Spacecraft Mode Files to determine whether the roll of the spacecraft was nominal (it is when in normal mode).

NOTE: Definitive and Full Time Resolution attitude files were discontinued in 2002

Nominal Roll Attitude:

Since July 8, 2003, the spacecraft is rolled 180 degrees approximately every three months. These maneuvers are listed in the nominal_roll_attitude.dat file. For a detailed description of the contents of this file see nominal_roll_attitude.info

Predictive Attitude:

Up until October 29, 2010 if the roll angle of SOHO is zero, the spacecraft +Z axis is aligned with the North Pole of the Sun ("right side up"). If the roll angle is 180, spacecraft -Z axis is aligned with the North Pole of the Sun ("upside down").

Since October 29, 2010 the reference for the nominal roll attitude of the spacecraft no longer is the solar North Pole, but the Ecliptic North Pole. Therefore, the roll angle with respect of the solar rotation axis goes from -7.25 degrees in June to 7.25 degrees in December. The files in http://sohowww.nascom.nasa.gov/data/ancillary/attitude/predictive contain information about the roll attitude every 10 minutes. The format is identical to that of the definitive attitude files described above. The roll angle of SOHO with respect to solar north is also provided in the daily files SO_AT_ROL_yyyymmdd_Vxx.DAT (or FITS) [10 min resolution] and SO_AT_R1H_yyyymmdd_Vxx.DAT (or FITS) [1-hour resolution] in above directory. Note that the angles in those files are in degrees, instead of the radians in the predictive attitude files (SO_AT_PRE_yyyymmdd_Vxx.DAT and .FITS).

Orbit files

The orbit files (predictive) provide information about the position and velocity of the spacecraft relative to Earth and Sun.

Until September 1997 a set of definitive orbit files was produced but this data set is identical to and has been superseeded by the orbit files (predictive). FDF did not produce any more definitive orbit files after September 1997.

CDF orbit files:

This files are originated at NASA's Flight Dynamics Facility (FDF) and converted into CDF by CDHF. As with the attitude CDF files, all the required information to interpret the contents is included within the CDF format. In overview, each ten minutes we have:

Timestamp: 10-Oct-1995 09:10:00.000 Year: 1995 Day of year: 283 Ellapsed milliseconds of day: 33000000 GCI S/C position (km): x, y, z GCI S/C velocity (km/s): vx, vy, vz GSE S/C position (km): x, y, z GSE S/C velocity (km/s): vx, vy, vz GSM S/C position (km): x, y, z GSM S/C velocity (km/s): vx, vy, vz GCI Sun vector (km): x, y, z HEC S/C position (km): x, y, z HEC S/C velocity (km/s): vx, vy, vz Carrington rotation (Earth): 1901 Heliographic longitud (Earth, rad): 0.000 Heliographic latitude (Earth, rad): 0.000 Carrington rotation (S/C): 1901 Heliographic longitude (S/C, rad): 0.000 Heliographic latitude (S/C, rad): 0.000

FITS orbit files:

The SOHO Orbit files are available in FITS format, as a binary table extension. The conversion is done at the SOHO GSFC Archive upon reception from the ECS. These FITS files contain all the columns listed above, in the same order, except the first one (the one labeled 'timestamp'). Each of the cartesian coordinates of the position and velocity vectors is a separate column (there are 36 in total). There is a row for each 10-minute entry.

SPICE kernels:

The SPICE kernels are in a format suitable for use by the SPICE software available from the NASA/JPL Navigation and Ancillary Information Facility. SPICE is available in SolarSoft as part of the STEREO package. The kernels are generated from the GCI data in the CDF ephemerides, but can be converted into any other coordinate system using SPICE. You can find the kernels here. There is a mission-long kernel as one kernel per year, except for 1998 which is split into two periods before (soho_1998a.bsp) and after (soho_1998b.bsp) the temporary loss of contact.

Time Correlation Log

The Time Correlation Log is an ASCII file that contains all the changes to the on-board clock since the beginning of the mission. Is a cumulative file (i.e. the new information is appended to it). It is generated by the Flight Operations Team (FOT).

Each entry has two lines. The first one describes the change to the clock and the second one specifies the command used to do it. Same samples:

1995-264-12:04:00.6 OBT COARSE SETTING: -131 seconds +0 ticks KNK0F000 FFFFFF7D0000 1995-265-01:08:23.5 OBT FINE PHASE SETTING: +2622 steps (+0.160 secs) KNK10000 0A3E 1995-265-01:12:38.4 OBT FREQ ADJUST: Add 1 tick every 200 cycles (+5.200 secs) KNK11000 000000C8

Project Data Base

The Project Data Base is a collection of files used by the Mission Operations and Data Systems Directorate (MO&DSD) ground system elements for SOHO telemetry and telecommand processing. For further information see the description document in the PDB directory.

Spacecraft Mode

The Spacecraft Mode files describe in which mode was the spacecraft at a given time. These are ASCII files with five columns:

Spacecraft mode files Columns are: 1. Date (from spacecraft OBT AYLOBTTS) 2. Time (from spacecraft OBT AYLOBTTS) 2. Currently active spacecraft mode (AKMDSACT) 3. Currently commanded mode (AKMDSCOM) 4. Mode transition in progress (AKMDSTRP) Modes are: 0 Inactive 1 Standby 2 ISA 3 FSA 4 RMW 5 CRP 6 CSP 7 Normal mode Mode transition: NOTE: This parameter is set to 1 since the gyroless software update on 27-Aug-1999 14:58:54 0 No mode transition in progress 1 Mode transition is in progress

 
 

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Last modification: July 27, 2020

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