I'm currently working on a bit of code to calculate satellite passes for a ground station. I've found a variety of references that discuss the conversion form other coordinate reference frames to the local horizon (lat, long, radius to target) frame. Aside from other issues it would be really beneficial to have a worked step-by-step example of how to get from Earth-centered inertial (ECI - as output by SGP propagators) to the local horizon frame. Either a worked-example in an answer or a web link to a worked example would be great! I'm currently using this reference: [Transformation to local horizontal coordinates][1], I'm stuck on step 3 - conversion matrix. [1]: http://www.mssl.ucl.ac.uk/grid/iau/extra/local_copy/SP_coords/local.htm