Alternate Definitions for HGRTN

Heliocentric Radial Tangential Normalcoordinate system (aka RTN). Typically centered at a spacecraft. Used forIMF and plasma V vectors. The X axis (radial) is set as the primary axis,and is defined as the axis pointing from the spacecraft to the Sun. The Zaxis (tangential) is set as the secondary axis, and is defined as thatportion of the solar North rotational axis which is perpendicular to theprimary axis. The Y axis (normal) is defined as Z cross X.

This system was, for example, used by the Ulysses mission.
+X-axis: vector (Sun-S/C).
+Y-axis: cross-product of (heliographic polar axis) and +X-axis.