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SOHO ORBIT AND ATTITUDE DATA FILES
This technical note describes the availability of the SOHO orbit data in the Heliocentric Ecliptic Coordinate System and the inclusion of the Carrington Rotation Number with the orbit data. It also describes the availability of a Full Time Resolution Attitude File and describes the additional information available in the Definitive Attitude File
I) SOHO Heliocentric Ecliptic Coordinate System
The CDHF will convert the SOHO orbit data in GCI coordinates received from the FDF to Heliocentric Ecliptic Coordinate System in addition to the GSE and GCM coordinate systems and include the results in the Orbit Data File. This conversion will give the spacecraft orbit relative to the sun in order to subtract the spacecraft velocity from the helioseismology observations.
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 Orbit Data File will also include the following information:
* Carrington Rotation (Duffett-Smith, Practical Astronomy with your Calculator, Second Edition).
II) Provision for Two SOHO Attitude Files
1) - Full Time Resolution Attitude File
The full time resolution definitive attitude file as downlinked from the spacecraft contains pitch and yaw offsets from Sun-center (10 times per second) and roll angle offsets from the projection of the Sun's North Pole (one time per second). This is called the spacecraft-body-frame coordinate system.
The CDHF will provide a full temporal resolution of the attitude telemetry without statistical computation. This will be called the Full Time Resolution Attitude File and will be transferred to the DDF for distribution together with SOHO instrument LZ data for ONLY those SOHO investigators requiring it. At present, only the SUMER and GOLF investigations have requested this high resolution attitude data which is expected to yield 864,000 sets of attitude determination per day. These data shall remain in spacecraft-body- frame coordinates in ASCII format and no further transformations will be performed.
2) - Definitive Attitude File:
The CDHF will also provide a Definitive Attitude File (DAF) which contains data which is averaged over 10-minute periods (to correspond to the Orbit files but the times may not be synchronized). The DAF shall also contain statistical information about that 10-minute duration (e.g., max and min for each value and standard deviation from the average during that 10 minutes). The CDHF will also provide the transformations of this data to the corresponding GCI, GSE, and GSM coordinates. These data will be transferred to the DDF for distribution together with SOHO instrument LZ data.