# Coordinate Systems Descriptions

Transformation matrices for converting vectors from the S/C coordinate system to the coordinate systems listed below are provided by Caltech as text files to the SEP instrument teams.

Spacecraft position data are also provided as text files.

**GEI: Geocentric Equatorial Inertial**. The X axis points toward the first point of Aries (i.e.
the vernal equinox), and the Z axis is aligned with the geographic north pole. When expressed
in terms of longitude and latitude, this is the well known celestial coordinate system of right
ascension and declination. This is realized with the J2000 ecliptic.

**GSE: Geocentric Solar Ecliptic**. X is the Earth-Sun line, and Z is aligned with the north pole
for the ecliptic of date.

**GSM: Geocentric Solar Magnetospheric**. X is the Earth-Sun line, and Z is the projection of
the north dipole axis.

**HCI: Heliocentric Inertial**. Z is the solar north rotational axis, and X is the solar ascending
node on the J2000 ecliptic.

**HEE: Heliocentric Earth Ecliptic**. X is the Sun-Earth line, and Z is the north pole for the
ecliptic of date.

**HEEQ: Heliocentric Earth Equatorial**. Z is the solar rotation axis, and X is in the plane
containing the Z axis and Earth, at the intersection of the solar central meridian, and the
heliographic equator. When converted to longitude and latitude, this is known as Stonyhurst
heliographic coordinates. In FITS files, this coordinate system is abbreviated as , so
that variation is also recognized by the software.

**HGRTN/RTN: Radial-Tangential-Normal**. X axis points from Sun center to the spacecraft,
and the Y axis is the cross product of the solar rotational axis and X, and lies in the solar
equatorial plane (towards the West limb). For the STEREO Ahead spacecraft, this is realized
through the dynamic coordinate frame STAHGRTN, while for STEREO Behind it is realized
through STBHGRTN. When the Sun is used as the origin, the designation is HGRTN; with
the spacecraft as origin, it is simply RTN.