Cynthia Vaskis

SLM521 Spring 2004

Dropin #3

4/6/04

File: dp3quatv.htm

Quaternions define the desired amount of rotation of an object in a simple straight forward manner through an angular rotation about a vector defined in the original vector space.

See MathWorld’s description of quaternion. When you want to use the values of the quaternion to rotate the axes of the space object, look at the Euler’s parameters.

A quaternion vector’s definition is a vector that if you hung onto it (looking down it toward the origin) and rotated it clockwise, the old coordinate system would be rotated into the new coordinate system position. Here we are using it to rotate the old STFOVCU vector into the NSU vector by rotating the angle ALPHA about the quaternion vector since the quaternion is perpendicular to the plane of the two vector’s that were used to create it. Thus, the Star Tracker’s FOV center vector (STFOVCU) ends up pointing directly at the new Star (NSU). If the new Star is within the field of regard, (the farthest amount the Star Tracker can move in any direction to see stars without moving the space object itself), then a slewing movement is performed within the Star Tracker itself and no maneuver has to happen for the whole Space Object.