121 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 The US Segment MCS was built up over several flights, starting with the United States On-orbit Segment (USOS) Destiny Laboratory, which was launched on February 10, 2001. The primary feature of the US Segment is four Control Moment Gryoscopes (CMGs), which can maintain attitude control for weeks at a time electrically without using precious rocket propellants. Because of its large size and extended lifetime, and because the MCS is shared between the US and Russian segments, the ISS has a number of unique features compared to the motion control systems of other satellites. These features include the following: n The Russian Segment carries several tons of hypergolic propellant for propulsive attitude control and reboosts. Hypergolic propellants (in this case, a fuel of hydrazine and an oxidizer of nitrogen tetroxide) react and ignite on contact with each other. The propellant is periodically replenished by Progress resupply vehicles launched from Russia (typically around four per year). In the past, propellant was also resupplied by the European Space Agency Automated Transfer Vehicle (ATV). The ISS is the only satellite for which the on-board propellant is periodically replenished for other satellites, the depletion of propellant usually marks the end of useful life. n Although the SM houses a complete set of attitude control thrusters and reboost engines, it can also control and automatically fire thrusters and reboost engines on the Progress vehicles that are normally docked to the aft of the SM or on the Docking Compartment-1 (DC-1). n Most components, including the flight computers in both the US Segment and the Russian Segment, the CMGs, rate gyro assemblies, Space Integrated Global Positioning System/Inertial Navigation System (SIGI) receivers, and antennas can and have been replaced on orbit. In many cases, failed components have been returned, serviced and refurbished, and relaunched for use as spares. As with everything else on the ISS, computers are at the core of the MCS. The work of the MCS is shared between the US Segment and the Russian Segment, extending to the computer systems at the center of the system. The MCS is built around a portion of the Command and Data Handling System, informally referred to as the “4-Box,” which includes the four-computer systems that manage and execute the motion control task. Two computers on the US Segment and two computers on the Russian Segment work together to control the ISS attitude and orbit. These computers process inputs from sensors such as GPS, star trackers, and rate sensors (discussed below) while commanding CMGs and small rockets to control the attitude and orbit. Command and Data Handling Elements The 4-Box consists of the Tier 1 Command and Control (C&C) Multiplexer/DeMultiplexer (MDM) and Russian Segment Central Computer and the Tier 2 Guidance, Navigation and Control (GNC) MDM and Russian Segment Terminal Computer. There are three C&C MDMs (Primary, Backup, and Standby) and two GNC MDMs (Primary and Backup). In both sets of computers, the Primary is performing all processing while sharing information with its powered-on and “hot” backup MDM. The Russian Central Computer and Terminal Computer are actually each a set of three independent computers that provide redundancy. See Figure 1. Command & Control Central Computer Bus Controller RS US Bus Controller Bus Controller Remote Terminal Guidance, Navigation, and Control Terminal Computer Remote Terminal Remote Terminal Bus Controller Remote Terminal Figure 1. 4-Box computer architecture of MCS. The C&C MDM and Central Computer manage the overall configuration of the system (such as which segment is in attitude