title: Python coordinate transformation ECI to ECEF
body: <p>I am using sgp4 python package (developed by Brandon Rhodes); I am inputting the TLE information for ISS(Zarya). I get the position of the satellite at a certain date, in the in ECI coordinate system. However, I want to convert it to ECEF coordinate system.</p>
  
  <p>There are at least two instances of matlab functions implemented (ecitoecef, eci2ecef) and a C function (eci2ecef) implemented for the RTKLIB project.( I can't post links because I have a restriction of two links per question since this is my first question and I have reputation 1)</p>
  
  <p>I was not able to find a python function implemented. 
  However, in this previous question,
  <a href="https://stackoverflow.com/questions/15937413/python-satellite-tracking-with-spg4-pyephem-positions-not-matching">Python satellite tracking with spg4, pyephem- positions not matching</a>
  the user says "I found atleast part of the issue. spg84.propagate() returns the location in ECI, not ECEF. Quick run through eci2ecef and it lines up perfectly with the predict response."</p>
  
  <p>Is that eci2ecef referring to a python implementation? Is this available online? Is there a document online that explains exactly the step needed to be done if I were to implemented by myself?  In this question <a href="https://stackoverflow.com/questions/8233401/how-do-i-convert-eci-coordinates-to-longitude-latitude-and-altitude-to-display-o">How do I convert ECI coordinates to longitude latitude and altitude to display on a Map?</a>
  in a comment they say "They differ by the Earth's rotation rate so the transformation is just a 2-D cos sin; -sin cos type transform. ". What is the angle of the transform? Is it the Greenwhich Sideral Time? When we use sgp4, is it also the GST that we specify, or other time?</p>
  
  <p>Thank you so much for your help!</p>