Choose the correct location of a star which is at 20� celestial latitude and along a vertical circle passing through East point of the horizon.

Altitude = -20� & Azimuth = 90�

Altitude = +20� & Azimuth = 90�

Altitude = 90� & Azimuth = +20�

Altitude = 90� & Azimuth = -20�

Correct Answer : Altitude = +20� & Azimuth = 90�

Explanation : Azimuth is defined as the angular distance measured from the North point of horizon in the horizontal direction. As the star lies on the vertical circle passing through East point, it represents the Azimuth value. East point of horizon has 90° Azimuth. So the Azimuth value of the star is 90°. Altitude is the angular distance of a point from the horizon in vertical direction. As the star is located at 20° celestial latitude, the Altitude is considered as +20°.

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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.