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Explanation of Coordinate Systems
The coordinate systems used on the
"Where is STEREO?"
page follow the definitions given in the following references
 Fränz, M. and Harper, D. (2002). Heliospheric coordinate
systems. Planetary and Space Science, 50, 217239.
 Hapgood, M. A. (1992). Space physics coordinate transformation: A user
guide. Planetary and Space Science, 40, 711717.
 Russell, C. T. (1971). Geophysical coordinate transformations. Cosmic
Electrodynamics, 2, 184196.
Below, each coordinate system is summarized.
Note that the various ecliptic coordinate systems can differ in how precession
is handled. Ecliptic coordinates can either be referenced to the J2000 epoch,
or precessed to the epoch of date. We follow the conventions of Fränz and
Harper (2002) in deciding which to apply for each coordinate system.
In the table below, the notation "J2000" means that the coordinates are
referenced to the J2000 epoch, while "D" means that the coordinates were
precessed to the mean ecliptic of date.
For each coordinate system, two axes are given. The third axis in each case
completes the righthanded system.
Geocentric coordinate systems 
GEI  Geocentric Equatorial Inertial 
X=First point of Aries
Z=Geographic north pole 
J2000 
GEO  Geographic 
X=Intersection of Greenwich meridian and geographic equator
Z=Geographic north pole 

GSE  Geocentric Solar Ecliptic 
X=EarthSun line
Z=Ecliptic north pole 
D 
Heliocentric coordinate systems 
HCI  Heliocentric Inertial 
Z=Solar rotational axis
X=Solar ascending node on ecliptic 
J2000 
HAE  Heliocentric Aries Ecliptic 
X=First point of Aries
Z=Ecliptic north pole 
J2000 
HEE  Heliocentric Earth Ecliptic 
X=SunEarth line
Z=Ecliptic north pole 
D 
HEEQ  Heliocentric Earth Equatorial 
Z=Solar rotation axis
X=intersection of solar equator and solar central meridian as seen
from Earth 

The calculations are based on the JPL/NAIF
SPICE
package, using IDL
software which is available as part of the
Solar Software Library.
A user's guide is available in both
PDF and
Postscript formats.
Roll angles for the two spacecraft are defined such that the nominal
orientation of each spacecraft is close to zero degrees. This is done by
subtracting 180 degrees from the Behind values reported by the spacecraft.
After the two spacecraft pass behind the Sun in 2015, the definition of
"nominal" orientation will need to be changedthis is not currently handled
by the software.
Last Revised: Thursday, 22Apr2010 21:10:37 UTC
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