Help with frame conversion between ITRF2014 and J2000

[jeongyunl]

Hello,

I want to transform ITRF2014 ephemeris to J2000, and vice versa.
There seems all the bits necessary but couldn't figure out how to put them together.

What would Itrf2014ToJ2000() function look like?
j2000_state= Itrf2014ToJ2000(epoch, itrf_state)

Here's what I tried so far: I modified perturbed_satellite_orbit.py like this:
(then I compared body fixed cartesian positions from dependent_variable_history with STK converted ITRF states, the difference was ~26km)

dependent_variables_to_save = [
 ...
    propagation_setup.dependent_variable.central_body_fixed_cartesian_position( # Added this
        "Delfi-C3, "Earth"
    )
]
...
environment_setup.add_rotation_model(bodies, "Earth",
    environment_setup.rotation_model.gcrs_to_itrs(
        environment_setup.rotation_model.IAUConventions.iau_2006,
        global_frame_orientation))

[DominicDirkx]

If you want to save the spacecraft's body-fixed state, that dependent variable is indeed the way to go. If you want the ITRF model for the Earth, then use this Earth rotation model:

https://py.api.tudat.space/en/latest/dynamics/environment_setup/rotation_model.html#tudatpy.dynamics.environment_setup.rotation_model.gcrs_to_itrs

[jeongyunl]

I actually used that. I made a mistake when I posted python code snippet.

I'll come up with an example code that demonstrates ITRF coordinates I get from tudat don't match what I got from different sources.

[jeongyunl]

I found the issue.
I was passing UTC time to DateTime object and used epoch the DateTime object returned.
Instead I switched to convert_date_string_to_ephemeris_time( tudat::spice_interface::convertDateStringToEphemerisTime())
Problem solved.