# How to get the coordinates from NASA JPL Ephemerides DE430 ( ECRS) to ECI?

I'm trying to make a software that determines the orbit of a satellite which is affected by the Sun and Moon. The coordinates [x y z] that I have for the satellite are in Eci. For the position of the Sun and Moon I'm using DE430 from jpl. My script takes the UTC value and transforms it into JD. For now I'm using the UTC to find the position and it is pretty accurate ,but I've read that I need to transform UTC to TDB and after that to perform this operation. The problem is that all of these coordinates are given with respect to ICRS. I need the coordinates to be reffered to ECI in order to obtain the correct values. This is a bit confussing for me , because I do not understand how should I do that or what rotational matrix should I use. There is an online a document from SOFA that reffers to that, but since I'm trying to learn by myself is a bit chaotic for me to understand which steps should I follow. http://www.iausofa.org/2018_0130_F/sofa/sofa_lib.pdf

• Use the SPICE toolkit. It will handle all of your coordinate frame and time system transformations, as well as interpolating data from DE430. – Mark Adler Apr 1 '18 at 17:46
• Thank you for the info , but I was trying to implement the algorithms by myself (in Matlab). Using SPICE would fix my currently problem but won't make me understand how this works. – Alexandru Lapusneanu Apr 1 '18 at 18:49
• Then I would recommend that you at least use SPICE to verify that your solutions, and therefore your understanding, is correct. – Mark Adler Apr 1 '18 at 19:38
• @AlexandruLapusneanu This is an interesting question, but a proper answer to all of your current question might fill several chapters of a book. The best way to use Stack Exchange questions in this case would be to break this up into a few segments, try to do some reading, search this site for potentially helpful answers, then summarize what you know so far and ask a specific questions on where you might be stuck – uhoh Apr 2 '18 at 3:58