Where is the origin of Earth-centered inertial coordinate system situated?

Center of the mass of the Sun

Center of mass of the Earth

At the fixed point on the Earth�s surface

At the center of gravity of an aircraft/spacecraft

Correct Answer : Center of mass of the Earth

Explanation : The origin of ECI system is located at the mass center of the earth but does not rotate with the Earth. Thus, this is fixed with the relative stars and is hence inertial.

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How many rotations are required to transform the ECI coordinate system to the perifocal system?

Correct Answer : Option (D) : 3

Explanation : The coordinate transformation from XYZ frame to X’Y’Z’ frame is carried out by a single rotation about one axis. The other two axes remain in the same plane. But to transform from the ECI coordinate system to the perifocal system, there have to be successive 3 rotations. The first rotation is about Z axis, second is through X’ axis and the final rotation is bout Z’’ axis.