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From: (John Prussing)
Subject: Re: Reference for roll,pitch,yaw ?
Date: 28 Dec 1999 01:47:34 GMT

"swimboy" <> writes:


>Does anyone how the roll, yaw and pitch are defined once the orbiter is
>orbiting the eath ?



The standard convention for spacecraft attitude axes is that the
roll-pitch-yaw axes are orbital axes (not vehicle body axes). For a
circular orbit the roll axis is along the orbital velocity vector, the
pitch axis is normal to the orbit plane, and the yaw axis is along the
radius vector from the center of the planet to the vehicle.

An easy way to remember it is to picture the shuttle landing.  If you
suddenly translate it along the local vertical up to orbital altitude,
the roll axis of the shuttle (from the center of mass out the nose)
defines the orbital roll axis, the pitch axis of the shuttle (out the
wings) defines the orbital pitch axis, and the yaw axis of the shuttle
(normal to the roll-pitch axes) defines the orbital yaw axis.

But if the shuttle is orbiting in an attitude where the nose points
down toward the center of the earth, the orbital yaw axis is (for that
attitude) the shuttle roll axis, etc.

But the main point is that the orbital roll-pitch-yaw axes are defined
independent of the attitude of the vehicle.
John E. Prussing
Department of Aeronautical & Astronautical Engineering
University of Illinois at Urbana-Champaign

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