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?