Coordinate Systems used for spacecraft trajectory description:

Heliographic Inertial Coordinate System (HGI):
The HGI coordinates are Sun centered and inertially fixed with respect  to an X-axis directed along the intersection line of the ecliptic and solar equatorial planes. The solar equator plane is inclined at 7.25 degrees from the ecliptic. This direction was towards ecliptic longitude of 74.367 degrees on 1 January 1900 at 1200 UT ;because of precession of the celestial equator, this longitude increases by 1.4 degrees/century. The Z axis is directed perpendicular and northward from the solar equator, and the Y-axis completes the right-handed set. This system differs from the usual heliographic coordinates (e.g. Carrington longitudes) which are fixed in the frame of the rotating Sun.

 Solar Ecliptic Coordinate System (SE)
The SE is a heliocentric coordinate system with the Z-axis normal to and northward from the ecliptic plane. The X-axis extends toward the first point of Aries (Vernal Equinox, i.e. to the Sun from Earth in the first day of Spring). The Y-axis completes the right handed set. The Vernal Equinox direction changes slowly; commonly invoked equinox epochs are (1) B-1950, (2) Mean-of-(current) Date, and (3) J-2000. The ecliptic longitude SE_LONG increases from zero in the x-direction towards Y-direction; the latitude, SE_LAT increases to +90 deg towards north ecliptic pole and to -90 deg towards south pole.

Cometocentric (solar) Ecliptic Coordinate System (CSE)
The FLYBY and BEFOREFLY files of the magnetic field data contains data for intervals of the closest approach of the VEGA spacecrafts to Haley comet.Files contains the following data: the position of the spacecraft in cometocentric (solar) ecliptic coordinates (CSE), magnetic field data in a coordinate system centered at the spacecraft with axes parallel to the CSE-system.
   CSE coordinate system for the SC POSITION: axis origin at comet center,  X-axis to the SUN, Z-axis to ecliptic north pole, (X,Y)-plane is parallel to the ecliptic plane.
   CSE coordinate system for the MAGNETIC FIELD DATA:  spacecraft centered, axes parallel to the CSE system for the position.
   This coordinate system is correct only for VERY SHORT TIME, about 4 hours around
the closest approach of the SC to the Comet, since for a larger angular distance
of the SC from the Comet, the direction to the Sun from both objects will be
different.