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## Position Dependent Systems

For the study of the local plasma environment of a spacecraft it is common to choose an axis system which depends on the position of the spacecraft. Widely used are Radial-Tangential-Normal systems defined by the radial vector from a central body to the spacecraft and the magnetic or rotational normal axis of that body. For highest precision one should use reference systems at the epoch of date.
• Heliocentric RTN System (Burlaga, 1984)
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. Transform:

where and are the longitude and latitude of the spacecraft in the system. Cartesian coordinates of this system are commonly called Radial, Tangential, Normal (RTN) coordinates.

• Dipole Meridian System (Kivelson and Russell, 1995)
This system can be used in any dipolar field to separate radial and angular motions.
 +X-axis: vector (dipole Center-S/C). +Y-axis: cross-product of (dipole polar axis) and +X-axis. Transform:

where and are the longitude and latitude of the spacecraft in the system.

• Spacecraft solar ecliptic [F.Neubauer (personal communication)]
This system was for example used by the Helios mission.
 XY-plane: Earth mean ecliptic of date. +X-axis: projection of vector S/C-Sun on XY-plane. +Z-axis: ecliptic South pole. Transform:
• Spin axis ecliptic [NSSDC, Pioneer data pages]
This spacecraft centered system was for example used by the Pioneer missions (under the acronym PE).
 +Z-axis: spacecraft spin axis (towards Earth). +X-axis: cross-product of ecliptic polar axis of date and . Transform:

where and are the ecliptic longitude and co-latitude of the spacecraft spin axis.

• Spin axis Sun pulse
This system is a fundamental reference system for most spinning spacecraft since the S/C - Sun meridian can easily be determined on board using a narrow-slit sun sensor. Thus a spacecraft-fixed instrumental system has only a longitudinal offset with respect to linear in time.
 +Z-axis: spacecraft spin axis , right-handed orientation. +Y-axis: cross-product between +Z-axis and S/C - Sun vector . Transform:

where and are the ecliptic longitude and co-latitude of the spacecraft spin axis and is the longitude of the vector and is given in the ecliptic system.

Next: Planetary Systems Up: Description of Coordinate Systems Previous: Systems
Markus Fraenz 2017-03-13