CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 120 Astronaut Dave Williams carrying one of the four massive gyroscopes used to control the orientation of the International Space Station (ISS) during replacement operations on STS-118/ISS-13A.1 in August 2007. Williams (anchored in a foot restraint) is being moved along with the Control Moment Gyroscope (CMG) by the space station robotic arm. The CMG is being installed on External Stowage Platform-2 near the ISS airlock, where it was stored awaiting a return to Earth for refurbishment. The Motion Control System (MCS) keeps the International Space Station (ISS) “right side up” rotationally as well as maintains the ISS in the proper orbit. Without it, the ISS would simply tumble in space, eventually lose altitude, and reenter the Earth’s atmosphere. The MCS maintains the ISS in a constant attitude for day-to-day operations, maneuvers the ISS to special attitudes for visiting vehicle dockings and captures, and reboosts the ISS to counter atmospheric drag or avoid space debris. The system uses Global Positioning Satellites (GPSs), rate gyroscopes, and other sensors to allow the ISS to “know” its location as it circles the Earth. These data are also used to point solar arrays at the sun, antennas to communications satellites, and payloads to ground or other targets. The ISS MCS, as it exists today, is a shared responsibility between the US Segment and Russian Segment of the ISS. The core of the Russian Segment MCS was launched as part of the Russian Service Module (SM) on July 12, 2000. Nearly all of the Russian MCS, as it exists today, was activated when the SM was first launched. The thruster-based control system of the SM was extremely important for the early assembly of the ISS, as it was the only attitude control for the early portions. Today, the SM continues to provide thruster-based attitude control for larger maneuvers, or to assist the US Segment attitude control system when it cannot provide enough control force. Additionally, Progress cargo vehicles docked to various docking ports on the Russian Segment have thrusters that are commanded by the SM and augment its original thrusters.