# Convert ECI to ECEF

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1. Oct 23, 2015

### Rawls

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.

2. Oct 25, 2015

### ogg

Well, ECEF is NOT an inertial frame. This has implications in terms of whether it makes sense to think of the conversion as a matrix multiplication. So, your first step is to understand the difference between an inertial frame of reference and one which is undergoing a (non-linear) acceleration. Step Two is to NOT use jargon such as ECEF and the possibly obsolete ECI. Third is to post this in the appropriate section. Why would you think a question about orbital mechanics belongs in linear and abstract algebra? Sorry, if I could, I would give more specifics - but that isn't in my area of interest/competence. Sorry if my reply isn't helpful - I referred to ESA's navipedia for the little information I used.

3. Oct 29, 2015

### Rawls

Why did you waste your time responding? I don't recall mentioning anything about ECEF being an inertial frame so step 1 is irrelevant. ECI is not obsolete and if it were it doesn't make a difference. If you don't understand the jargon then clearly this post isn't relevant to you, move onto another. I posted this in this section because this process requires matrix algebra to complete and its relevant. I don't need to understand navigational coordinate systems I need to do math. SOME ADVICE FORE YOU, IF YOU DON'T KNOW WHAT YOU ARE TALKING ABOUT DON'T WASTE YOUR TIME (OR OTHERS') REPLYING WITH SNIDE REMARKS. It isn't helpful or necessary. I solved the issue and this thread is done.