Dale Ireland asked about the Shuttle orientation when Roll, Pitch and Yaw are given. As nobody has replied yet here is what I learned many years ago while working on a Spacelab mission. The Shuttle Body-Axis coordinate system is defined as +X (roll axis) toward the nose; +Y (pitch axis) toward the right; +Z (yaw axis) toward the bottom of the Orbiter all taken from the center of mass. To specify the Shuttle attitude Roll, Pitch and Yaw are given in that sequence (for historical reasons) although the actually performed sequence to get from a reference system toward the specified attitude would be first Pitch, then Yaw and then Roll. Two reference-axis systems are in use: LVLH and M50. In LVLH (Local Vertical Local Horizontal) the +Z axis is parallel to the radius vector and points from the Shuttle's center of mass toward the center of the Earth. The +Y axis is pointed in the direction of the negative angular momentum vector. If that sounds complicated what it actually means is that in a circular orbit (which the Shuttle approximates most of the time) the +X direction would point into the direction of flight and +Y toward the right. So 0 Pitch, Yaw, Roll in that system would be like an airplane flying. Pitch 180, Yaw and Roll 0 would be Payload Bay Down, Tail Forward and Pitch 90, Yaw 0 and Roll 90 would be Nose Up Left Wing Forward etc. The M50 system refers to inertial attitudes. It is the equatorial system but referred to the epoch Mean of 1950 rather than the more modern J2000.0 which is normally used in astronomy these days. If no particular reference system is specified for the Shuttle attitude they probably mean LVLH as a default. An attitude deadband is specified for the various phases of the mission. 0.5 degrees in each axis would be typical if no special pointing requirements are present. So the numbers Roll 0.3, Pitch 179.5 and Yaw 0.5 would be quite normal on a real time display for the Shuttle attitude which was probably be defined as Pitch 180, Roll, Yaw 0. Gerhard HOLTKAMP Darmstadt, Germany ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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