Python coordinate transformation ECI to ECEF

First I studied the paper AIAA 2006-6753 which is online at CelesTrak, and which formed the basis for making the sgp4 library available in Python in the first place:

https://celestrak.org/publications/AIAA/2006-6753/

Based on its recommendations — and in particular on the information in its Appendix — I am attempting to integrate Earth satellites into my new Skyfield astronomy library. A new release with this work should be out soon, but in the meantime you can study the transformations that I am using (based on the above paper) at:

https://github.com/brandon-rhodes/python-skyfield/blob/master/skyfield/sgp4lib.py

I am not sure that I have the transforms correct, but will be checking the results against as many official sources of positions as I can find to determine whether I have made any systematic error.

The ECI to ECEF transformation is also implemented in satellite-js which is derived from PyEphem.

You may have to check it's correctness.