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

Vernal equinox

Perihelion

Local horizon

Along the intersection of Earth�s equatorial plane and the plane of ecliptic

Correct Answer : Vernal equinox

Explanation : The X-axis of the ECI system points towards the vernal equinox. Vernal equinox marks the first day of the spring in the northern hemisphere (21 March). It is the line joining the Earth and the sun when the sun passes through earth’s equatorial plane.

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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.