In which direction does the Y-axis of ECI system point to?

Vernal equinox

Normal to the equatorial plane

Points 90 degrees to the west of the x-axis in equatorial plane

Points 90 degrees to the east of the X-axis in equatorial plane

Correct Answer : Points 90 degrees to the east of the X-axis in equatorial plane

Explanation : The Y-axis of the ECI system points to 90 degrees to the east of the X-axis in the equatorial plane. Y satisfies the cross product of Z and X thus completing the right-handed coordinate system.

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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.