I have done a search online to understand how to convert ECI to ECEF. I wanted to learn what the conversion is so I could just create a matlab script to make it happen. My problem is that most of the information I find assumes you already have some knowledge about ECI and ECEF. If I had more time I am confident I could learn about each system, make sense of the knowledge found all over the web and construct my own script. My ultimate goal is to go from ECI to ENU. There are matlab toolboxes that can make this easy however I do not have access to those tool boxes so I must create my own. Step 1: Convert ECI to ECEF. I want to take Matrix A which is in ECI and convert it to matrix A' which is in ECEF. So A'=eci2ecef(A). Having limited knowledge I would like to know the direct formula (that is explained step by step if possible). Step 2: Convert ECEF to ENU. Matrix A''=ecef2enu(A'). Maybe there is a way to go from ECI directly to ENU, that would be great but from what I have been reading it does not appear to be the case. Any help would greatly be appreciated. If you require more information from me please just ask. I cannot yet think of how I can make this question better for those who come across it.