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It is taught for navigation systems (like an airplane) on Earth we us ECEF (earth-centered, earth-fixed) and for space systems (like a satellite) we use ECI (earth-centered inertial). But why is ECEF exclusively used for navigation vehicles on Earth and ECI is exclusively used for satellites and spacecraft in space? Why can't we use ECI for an airplane to observed another perspective?

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  • $\begingroup$ You can convert between whatever systems you're interested in, but aside from some really trivial "gee-whiz" cases, there wouldn't seem to be any use for considering aircraft as flying in an ECI frame. Everything I'm flying in, around, over, and to are all pretty much "earth fixed" or close to it, so that's the useful frame of reference. $\endgroup$ – Ralph J Apr 3 at 15:05
  • $\begingroup$ "Why can't we use ECI for an airplane to observed another perspective?" What do you believe would be gained for aircraft by using ECI rather than ECEF? $\endgroup$ – a CVn Apr 3 at 15:13
  • $\begingroup$ @CVn The airplane's Inertial Navigation box could be applied to ECI. $\endgroup$ – Minimalist Apr 3 at 15:14
  • $\begingroup$ @Ralph I am just confused between ECEF and ECI. I've memorized the theory, but why "earth-fixed?" Looking for a clearer explanation. If only using a gyroscope, then ECI would work better, right? $\endgroup$ – Minimalist Apr 3 at 15:16
  • $\begingroup$ Because aircraft, though they leave the surface of the earth, are "earth fixed", while spacecraft are not (see Voyager 1 & 2, Mars rovers, etc.). While you probably could apply ECI to them, it would probably be overkill. $\endgroup$ – FreeMan Apr 3 at 15:18
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The basic reason is the goal in navigation. For an airplane it is to deliver the aircraft to a particular destination on the earth's surface, potentially also avoiding certain areas fixed to the earth's surface.

If you do all your navigation calculations in ECI, you then have to keep track of the earth's rotation to translate that to a point on the surface. If you do all the navigation in ECEF, then that translation is unnecessary.

Both methods are mathematically valid (for ECEF, you have to have some method of removing or ignoring the non-inertial frame effects like Coriolis; for ECI, you have to keep track of earth rotation). I don't have any direct evidence, but I suspect that since ECI was not an option historically, the current solutions are robust and no one is looking to create a radically different method for unknown benefit.

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  • $\begingroup$ very few ECEF/ECI experts, thanks. $\endgroup$ – Minimalist Apr 3 at 16:17
  • $\begingroup$ For ECI you not only have to deal with the rotation of the earth around its axis but its orbit around the sun (sidereal day vs. solar day). $\endgroup$ – Gerry Apr 3 at 20:29

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