CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 124 attitude, a typical torque equilibrium attitude (which is described later) as the space station coasts is YPR -4,-3,0. Some special attitudes— e.g., a pitch up of 90 degrees for some Russian vehicle undockings—are used for short periods of time, usually for a few hours, at most. By flying in an LVLH reference frame, the “bottom side” of the ISS faces the Earth as the space station travels around it. This provides some advantages. Scientific packages intended for Earth observation (such as cameras) can be mounted in a fixed position on the underside of the ISS, whereas communications and other antennas are afforded a clear view of space on the top. Thermal protection can be specifically designed for the Earth-facing or space-facing side of the ISS. One disadvantage is the sun will appear to be constantly in motion as the ISS passes beneath it thus, articulating solar arrays were designed to track the sun and provide maximum power generation. A final feature of the ISS orbit is the geometry of the ISS orbit with respect to the sun. As the space station orbits, the sun rises and sets every orbit (16 times a day). When the sun is highest in the sky, it is also referred to noon (as on Earth)—or, more specifically, orbit noon, since noon happens once per orbit. A line drawn from the center of the Earth to the spot on the orbit where orbit noon occurs is called the orbit noon vector. A line drawn from the center of the Earth directly to the sun is called the sun vector. Both of these can be visualized on Figure 7. Sun Orbit Noon β Angle Figure 7. Definition of beta angle (β). Orbit Noon ZXPOP XXPOP YXPOP ZXPOP XXPOP YXPOP Figure 8. XPOP reference frame. The sun can be almost directly overhead at noon or it may be well off to the left or right side of the orbit, depending on the orientation of the