For all central bodies except Earth, where the central body shall be defined via an accompanying “CENTER_NAME”. An inertial frame obtained by evaluating the central body’s fixed (rotating) frame at some specified epoch, rather than evolving in time.

For the Earth system only, these inertial axes are associated with the FK4 star catalog and its theory modeling the mean equator and mean equinox. The epoch is the beginning of the Besselian year 1950, corresponding to 31 Dec 1949 22:09:46.866 or JD 2433282.4234591. The B1950 axes are realized by a constant rotation offset from the J2000 axes, using a formula available from the Explanatory Supplement to the Astronomical Almanac.

The DTRFyyyy is the inertial realization of the ITRS computed at DGFI-TUM (Deutsches Geodätisches Forschungsinstitut/Technische Universität München). Only two other VLBI centers compute these realizations, the others being IGN in Paris and JPL in Pasadena. The DTRF considering corrections for non-tidal atmospheric and hydrological loading, as of year “yyyy” (e.g. 2000).

The quasi-inertial frame Earth Mean Equator and Mean Equinox of the J2000 epoch (JD 2451545.0 TDB which is 1 Jan 2000 12:00:00.000 TDB). The J2000 frame is realized by the transformational algorithm (also known as the FK5 IAU76 theory) between it and the Earth Fixed frame. The algorithm uses the 1976 IAU Theory of Precession, the 1980 Nutation model, and the Greenwich Mean apparent Sidereal Time (expressed as a function of time in UT1), updated by IERS Technical Note No. 21 to include an adjustment to the equation of the equinoxes.

The Earth's rotating fixed frame, realized via the transformational algorithm and parameters of IAU 1976/1980, IAU 2000A, and/or the International Terrestrial Reference Frame solutions as of year "yyyy" (e.g. 1993, 1997, 2000).

The Greenwich True-of-Date (GTOD) rotating coordinate system. This is realized as a rotating, right-handed, Cartesian system with the origin at the center of the Earth. The orientation of this system is specified with the xy plane in the Earth’s true of date Equator, the z axis directed along the Earth’s true of date rotational axis and is positive north, the positive x axis directed toward the prime meridian, and the y axis completing the right-handed system.

The International Celestial Reference Frame is the realization of the International Celestial Reference System per IERS conventions 2003 (IERS Technical Note TN-32, ICRF1) by McCarthy and Petit and IERS 2010 conventions (TN-36, ICRF2) by Petit and Luzum. ICRF is the standard Barycentric reference system. The ICRF is periodically reevaluated, such that each realization must be annotated (i.e., ICRFn, where the ‘n’ character is an integer starting from 1). The ICRF axes are defined as the inertial (i.e., kinematically non-rotating) axes associated with a general relativity frame centered at the solar system barycenter (often called the BCRF). The IAU (International Astronomical Union) is the authority for the definition of the ICRF. The ICRF frame is realized by the transformational algorithm between it and the Earth Fixed frame.
Note that the term ‘ICRF coordinate system’ is not restricted to the system whose origin is at the solar system barycenter--- rather, the term describes a coordinate system whose origin is determined from context (i.e., for a central body, its center of mass location) whose axes are aligned with the axes of the BCRF. In fact, the IAU uses the term GCRF to refer to the system with origin at the geocenter (i.e Earth’s center of mass location) with axes parallel to the BCRF. [Note that ‘aligned’ here refers to directions in Euclidean space – not in a curved space governed by general relativity.]

Latest version of the ITRFyyyy reference frame

The quasi-inertial frame Mean Equator and Mean Equinox of the J2000 epoch (JD 2451545.0 TDB which is 1 Jan 2000 12:00:00.000 TDB). The J2000 frame is realized by the transformational algorithm (also known as the FK5 IAU76 theory) between it and the Earth Fixed frame. The algorithm uses the 1976 IAU Theory of Precession, the 1980 Nutation model, and the Greenwich Mean apparent Sidereal Time (expressed as a function of time in UT1), updated by IERS Technical Note No. 21 to include an adjustment to the equation of the equinoxes.
Note that the term ‘J2000 coordinate system’ is not restricted to the system whose origin is at Earth’s center--- rather, the term describes a coordinate system whose origin is determined from context (i.e., for a central body, its center of mass location) whose axes are parallel to the axes of the J2000 system defined at the Earth.

The quasi-inertial frame mean ecliptic system evaluated at the J2000 epoch. The mean ecliptic plane is defined as the rotation of the J2000 XY plane about the J2000 X axis by the mean obliquity defined using FK5 IAU76 theory.

Mean of Date quasi-inertial frame definition for the Earth. Mean Equator and Mean Equinox of date. The transformation between J2000 and MeanOfDate is computed using a sequence of Euler rotations. Rotation angles are computed using cubic polynomials of time past the J2000 epoch in JED according to the 1976 IAU Theory of Precession angles and rates, as found in the US Naval Observatory circular No. 163. The MeanOfDate Z axis is the Earth’s mean spin axis; the MeanOfDate X axis defines the mean vernal equinox

Mean of Epoch quasi-inertial frame definition for all central bodies except Earth, where the central body shall be defined via an accompanying “CENTER_NAME”. The MeanOfDate system evaluated at some specified epoch, rather than evolving in time. This frame does not rotate with respect to the Inertial frame.

Moon Mean Earth (ME) rotating frame. This is the preferred lunar frame for associating lunar topography. It is defined as a constant rotation from the Principal Axes frame associated with a particular instantiation of the Jet Propulsion Laboratory Development Ephemeris (JPL/DE). Typically, the X axis pointed along the mean direction to the center of the Earth and the Z axis pointing to the mean direction of rotation. The ME frame is typically used to specify the location of objects on the Moon.