Body Axis Coordinate System

https://gyazo.com/a8d2243043ac3ba139f72d470a51ca8b

Finally, in case of space vehicles like the Space Shuttle etc., a modification of the latter convention is used, where the vehicle's (body's) positive y- or pitch axis again always points to its right, and its positive z- or yaw axis always points down, but “down” now may have two different meanings: If a so-called local frame is used as external reference, its positive z-axis points “down” to the center of the Earth as it does in case of the earlier mentioned NED-system, but if the inertial frame is used as reference, its positive z-axis will point now to the north celestial pole, and its positive x-axis to the Vernal Equinox or some other reference meridian.

最後特に意味がわからない

The Space Shuttle Body Axis Coordinate System (BACS) is a rotating, right-handed, Cartesian system. The origin of the Space Shuttle BACS is the center of mass of the orbiter. The three axes, X, Y, and Z, are defined as follows:

The X axis is parallel to the orbiter structural body X-orbiter axis, and is positive toward the nose.

The Z axis is parallel to the orbiter plane of symmetry, is perpendicular to the X axis, and is positive down with respect to the orbiter fuselage.

The Y axis completes the right-handed orthogonal system.

The Euler sequence that is commonly associated with this system is a yaw, pitch, roll sequence. This attitude sequence is yaw, pitch, and roll around the Z, Y, and X axes, respectively.

ZはXに垂直らしい　なんか調べてたらXに垂直じゃなくて常に地面の方向を差す　みたいな感じの図もちょくちょくあるんだけどそれは何？