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The user can choose this frame in order to define the initial orbit parameters.
5.1.3. ICRF
The ICRF (International Celestial Reference Frame) is defined by the IERS conventions (see §References 4),
and realizes an ideal reference system, by precise equatorial coordinates of extragalactic radio sources
observed in Very Long Baseline Interferometry (VLBI) programmes.
The user can choose this frame in order to define the initial orbit parameters.
5.1.4. CIRF
The CIRF (Celestial Intermediate Reference Frame) is defined by the IERS conventions. It is deduced from
the ICRF by taking into account the precession-nutation model.
This frame is used by STELA for the integration of the spacecraft motion (see §STELA fundamentals
for more information).
The CIRF is the integration frame for GTO dynamical model only.
The user can choose this frame in order to define the initial orbit parameters.
5.1.5. Rotating TIRF
The TIRF (Terrestrial Intermediate Reference Frame) if defined by the IERS conventions as follows :
O the Earth mass center,
The z-axis aligned with the true Earth's spin axis at date and north-oriented (it is aligned with the
Celestial Intermediate Pole),
(x,y) : true equator of date,
The x-axis aligned with the intersection between true equator and Greenwich meridian,
The y-axis completing the direct orthonormal trihedron.
The TIRF is deduced from the CIRF by a rotation of angle "ERA" (Earth Rotation Angle depending on UT1)
around z-axis. It can also be deduced from the ITRF (International Terrestrial Reference Frame) by rotations
describing the «polar motion».
The user can choose this frame in order to define the initial orbit parameters.
5.1.6. Terrestrial Frozen at Epoch
The Terrestrial Frozen at Epoch frame (TFE) is a TIRF-like frame, but frozen at a reference date, and whose
x-axis is oriented along a reference longitude. It can be deduced from the CIRF (at reference date) by two
consecutive rotations along the z-axis :
First rotation of angle "ERA"
Second rotation of angle "reference longitude"
The user can choose this frame in order to define the initial orbit parameters.
Note that when using this frame in the statistics mode and dispersing the initial date, the difference frozen
epoch - initial date is kept.