# What is the difference between GCRS and J2000 frames?

What is the difference between GCRS and J2000 frames? Which is commonly used worldwide in satellite communications?

I've read a statement here, that GCRS coordinates are based on even more precise axes than those of the old J2000 system. However, the JPL Horizons shows results in J2000 frame.

About the difference between GCRS and ITRS: is GCRS an ECI frame, and ITRS is an ECEF frame?

Second question first:

About the difference between GCRS and ITRS: is GCRS an ECI frame, and ITRS is an ECEF frame?

Correct.

What is the difference between GCRS and J2000 frames? Which is commonly used worldwide in satellite communications?

The difference is small. The GCRF frame is essentially the ICRF frame, but with a slightly different relativistic time scale. The JPL ephemerides (the underlying basis of JPL Horizons and JPL's SPICE system) treat the J2000 frame and the ICRF frames as equivalent. The ephemerides aren't precise enough to see the difference between the hundredths of arc second difference between the J2000 and ICRF frames. With regard to satellite communications, the beamwidths of the antennae used for deep space communication are over ten times the small discrepancy between the J2000 and ICRF frames; even the uncertainty in antenna pointing exceeds this small discrepancy. For near Earth satellites (where the GCRF is more applicable than the ICRF), beamwidths are even wider.

• Thanks! So, GCRS is different from J2000, and both are different from ICRS? :) Why GCRS is more applicable than ICRF and J2000? – Leeloo Mar 28 '18 at 15:12
• In theory, all reference frames are equally valid; you could use Neptune-centered inertial frame to model the behavior of a satellite in low Earth orbit or Earth weather. In practice, some reference frames are better than others, depending on the application. An Earth-centered inertial frame is more applicable to satellites orbiting the Earth than is a solar system barycenter-based frame frame (or a Neptune-centered inertial frame), just as an Earth-centered, Earth-fixed frame is better than an Earth-centered inertial frame for modeling what happens near the surface of or inside of the Earth. – David Hammen Mar 28 '18 at 17:44
• I mean ICRS, GCRS and J2000 all are Earth Centered Inertial frames. Is GCRS more applicable for Earth satellites? Is it widely used in practice? And why is it 'better'? – Leeloo Mar 28 '18 at 23:01
• @uhoh -=One of my personal favorite tests as a key developer of a dynamics software package used widely across JSC was to model the orbit of an object about the Moon in Moon-centered vs Earth-centered vs Earth-Moon barycenter vs Neptune inertial vs solar system barycenter inertial coordinates. In theory, all of those frames are equally valid. In practice, only one did a good job of approximating reality -- and that one (Moon-centered inertial, duh) required a good solar system barycenter model of the orbits of the potentially disrturbing bodies. – David Hammen Mar 29 '18 at 2:42
• @uhoh I implemented in J2000 and in GCRS. Got a little difference in z-axis. However, I didn't ask which is better. I asked, which is widely used, which is standart for Earth satellite communications? As David stated, I could also use a Neptune inertial frame, however it would be bizarre if my soft by default calculates in this frame ;) – Leeloo Mar 29 '18 at 7:00