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For an interplanetary mission design problem, I am using the JPL Horizons Ephermeris to obtain the state vectors of Venus and Earth. I have selected the Coordinate origin as the body centre of the Sun. But in the ephemeris table, it is showing reference frame as ICRF and Coordinate system as Mean equinox and Ecliptic of Reference Epoch.

Can someone help me to understand the ephemeris table/how can I know actually from which frame the state vectors are being measured? For reference, I am attaching the text document.JPL Ephemeris for Venus

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  • $\begingroup$ There are two things to think about with coordinate systems; where is the origin and which directions are the axes pointing? I usually use the solar system barycenter as the origin. It's close to the body center of the sun but it's more "standard". It doesn't matter too much for most calculations since you are probably going to be subtracting one position from another. Have a look at this answer first and then message me with more questions, or just update your question. $\endgroup$ – uhoh Dec 19 '18 at 12:44
  • $\begingroup$ You can also look at Need help getting the true ICRF coordinates of SOHO using Horizons $\endgroup$ – uhoh Dec 19 '18 at 12:48
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    $\begingroup$ I understood the basics of the ICRF frame. Now, I am bit stuck with obtaining the position vector of the spacecraft from the ICRF frame. I have the velocity vectors as I got from the Robust Lambert solver designed by Dr.Izzo. I need some help with determining the position vectors. Thank you. $\endgroup$ – Suraj Dec 25 '18 at 3:25
  • $\begingroup$ To answer your posted question, the Horizions output is using x, y, z directions discussed in detail here with the understanding that you can tilt (rotate around the x direction) by about 23 degrees to get ecliptic as the reference plane instead of the xy plane. You still have the flexibility of choosing an origin yourself, and in this case it's the Sun instead of the solar-system barycenter. I don't understand what question you have about the Horizions output beyond that. $\endgroup$ – uhoh Dec 25 '18 at 3:35
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    $\begingroup$ Thanks for the inputs! I will post it as a different question. $\endgroup$ – Suraj Dec 25 '18 at 3:50

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