In MaxQ and in SPICE, the term “Reference Frame” describes a particular orientation (of coordinate system axes). This does not include the spatial position of a coordinate system’s origin, only the orientation.
A majority of the time either an inertial (non-rotating) or rotating (fixed, attached to a body) Reference Frame is used, although there are other types.
A few common reference frames:
| Reference Frame | It is |
|---|---|
| J2000 | Aligns XY in Earth’s Equatorial plane, Z: North Pole. X points to the Sun at J2000 Epoch. Y = Z x X Does not rotate with Earth. |
| ECLIPJ2000 | As above, but aligned to the Ecliptic plane. |
| IAU_EARTH | Normally ITRF93, rotates as if attached to earth |
| IAU_MOON, etc | As above, but for other bodies |
J2000 as a reference frame is related to, but different than the J2000 Ephemeris Time Epoch. J2000 as an epoch is defined as “12:00 Terrestrial Time January 1, 2000”.
J2000
J2000 as a reference frame is a coordinate system orientation. It aligns the coordinate system’s Z-axis with Earth’s North Pole and XY plane with Earth’s equator as of the J2000 time Epoch. The X axis points towards the Sun from Earth’s location at the Vernal Equinox.
J2000 diagram
The J2000 reference is convenient for objects in Earth’s orbit, such as the Hubble Space Telescope. Earth-Centered “Intertial” frames are referred to by the TLA “ECI”.
ECLIPJ2000
ELCIPJ2000 is similar to J2000, but aligns the XY plane with the Ecliptic plane.
ECLIPJ2000 diagram
ECLIPJ2000 is a convenient Reference Frame for objects orbiting the Sun, such a Space Probe traveling through the solar system on its way to Mars.
IAU_EARTH
IAU_EARTH is attached to and rotates with Earth.
IAU_EARTH rotates with the earth
IAU_EARTH is a convenient Reference Frame for objects on Earth’s Surface, such as Deep Space Network radio antenna stations. “Earth-centered, Earth-fixed” frames are often referred to as “ECEF” frames. IAU_EARTH is often defined as the ITRF93 frame through kernel FK frame assignments, and often “high precision” binary “PCK” kernels are used to obtain/predict the orientation of this frame with very high precision.
