123 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 around 410 km (255 miles) above the surface of the Earth, although the exact altitude is manipulated by reboosts and by small manipulations of the drag of the space station through solar array positioning (see Chapter 9). This manipulation of the orbit ensures the ISS is at the correct altitude and position in orbit for the numerous cargo and crew transfer vehicles that rendezvous with the ISS, as well as in the proper position to undock cargo and crew transfer vehicles that return to Earth. The ISS maintains a rotational position using attitude control while in this orbit. Unlike, for example, an airplane in orbital space, there is no naturally defined “up” or “down” on the space station. The ISS usually performs attitude control within a coordinate frame called Local Vertical, Local Horizontal (LVLH) (see Figure 5). In this reference frame, the +X axis points along the velocity vector in orbit, the +Z axis points toward the center of the Earth, and the +Y axis is perpendicular to the X-Z plane. The frame is referred to as a rotating coordinate frame since the Z axis is always pointed toward the center of the Earth. XLVLH XLVLH YLVLH YLVLH ZLVLH ZLVLH Figure 5. LVLH reference frame. Note that the reference frame rotates so that Z (and the bottom of the ISS) always points at the Earth. Pitch Yaw Roll Figure 6. YPR attitude definitions for the ISS. The exact attitude of the ISS within this frame is usually described by a Yaw, Pitch, and Roll (YPR) in degrees (Figure 6). When the ISS is precisely aligned with LVLH, it is at an attitude of Yaw=0, Pitch=0, and Roll=0—or, in shorthand, YPR 0,0,0. When at this attitude, if one were sitting atop the ISS, the Earth’s horizon would be visible in front of him or her, as if that person were in an airplane. If the person looked below, he or she would see the Earth. The ISS usually flies within a few degrees of the LVLH