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Earth-centered inertial coordinate frames have their origins at the center of mass of Earth and are fixed with respect to the stars. "I" in "ECI" stands for inertial , in contrast to the "Earth-centered - Earth-fixed" frames, which remains fixed with respect to Earth's surface in its rotation, and then rotates with respect to stars.
For objects in space, the equations of motion that describe orbital motion are simpler in a non-rotating frame such as ECI. The ECI frame is also useful for specifying the direction toward celestial objects:
To represent the positions and velocities of terrestrial objects, it is convenient to use ECEF coordinates or latitude, longitude, and altitude.
In a nutshell:
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