I'm not sure this answers your question, but ICRF is a non-precessing
On J2000 (2000-01-01 12:00:00 UTC), the z axis of the ICRF frame was
exactly parallel to a line connecting the Earth's center to the
Earth's north pole.
Since then, the Earth's north pole has precessed, so the ICRF z axis
and the geocenter-north pole line are no longer parallel.
To see this, you can visit http://ssd.jpl.nasa.gov/horizons.cgi and
use these settings:
(this is actually looking from the north pole "down" to the Earth's
center, so we would expect a negative z value).
The interesting part of the results:
2445701.000000000, A.D. 1984-Jan-01 12:00:00.0000, 1.007696415090285E+01, -3.712508526111991E-02, -6.356744326939433E+03, -2.994380889331686E-07, -2.570088696418019E-07, -4.731801778858530E-10, 2.120384334099952E-02, 6.356752314245179E+03, 5.329972950848752E-25,
2451545.000000000, A.D. 2000-Jan-01 12:00:00.0000, 1.601081935638483E-01, 1.746805655485580E-01, -6.356752309828780E+03, 2.230286001476848E-07, -8.078442785945724E-07, -1.658173608778433E-11, 2.120384334099953E-02, 6.356752314245181E+03, -5.549834335071262E-23,
2457389.000000000, A.D. 2016-Jan-01 12:00:00.0000, -9.877267476902714E+00, 3.153970319756332E-01, -6.356744632653835E+03, 1.794887917287106E-07, -5.409368308792473E-07, -3.057333255094828E-10, 2.120384334099952E-02, 6.356752314245179E+03, -5.736383388350969E-23,
It's a little hard to notice because of the scientific notation (and
the embarrassing fact that x and y aren't exactly zero at J2000), but
the x and y values at J2000 (the middle entry) are nearly zero.
The y value is nearly zero for all three entries, but the x value
changes. In 1984, the x value is about +10m, and, in 2016, the x value
is almost -10m, showing the effects of precession.
One benefit of a non-precessing frame like ICRF is that star positions
are relatively constant (excluding proper motion and the solar system's own
motion). For example, the z axis will always point (very roughly) to
Polaris, even after precession has changed the north star to, for
By default, NASA now uses ICRF coordinates to indicate where planets
are, so, if you're looking to map planets on a J2000 starchart, you
should use the ICRF coordinates.
The only problem you'll have using ICRF/J2000 coordinates (instead of
J2016 coordinates) is that any calculations you make for
stellar/planetary azimuth and altitude will be off slightly, not
enough to really notice unless you are doing precision work.