133 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 would eventually saturate the CMG system and require thruster firings. By slightly changing the attitude of the ISS, however, the attitude control software can push the momentum state of the CMG system lower by manipulating the small aerodynamic and gravity gradient torques acting on the system. This software control mode is called “momentum management.” The controller keeps the momentum variable at the lowest while loosening up on the attitude and attitude rate constraints. While in momentum management, the ISS attitude will gently rock by several degrees over the course of an orbit as the software works to push the momentum state of the CMGs to zero. The advantage of momentum management control is that thruster firings are never needed unless a significant unexpected force such as a vent acts on the system. This saves propellant and preserves the microgravity environment for many of the ISS payloads. The disadvantage is that the attitude wobbles by several degrees, which is unsuitable for precision alignment of the ISS attitude required for vehicle dockings. Momentum management is also unsuitable for rejecting thruster plume disturbances from nearby vehicles, which is why it is not used for visiting vehicle capture operations performed with the SSRMS robotic arm. Finally, momentum management only works near certain attitudes in which external forces are balanced. These attitudes are called Torque Equilibrium Attitudes (TEAs). The most typically flown TEA is one that is usually within a few degrees of the ISS LVLH (0,0,0). A different control logic—referred to as “attitude hold”—is used for precision attitude alignment and disturbance rejection, or for attitudes that are not at the TEA. In attitude hold, the CMG system maintains the attitude precisely (within a few tenths of a degree) by prioritizing controlling the attitude and attitude rate control over keeping the total momentum constant. This greater stability allows the system to reject strong disturbances, thus making attitude hold suitable for vehicle dockings and robotic capture operations. CMG momentum can build rapidly in this mode since the attitude control software does not attempt to optimize the ISS attitude to control gravity gradient or aerodynamic torques, and the stack may not be at a TEA. In these cases, the system may saturate in minutes, and would require frequent desaturation firings of Russian Segment jets. A variation of the attitude hold logic is called the USOS Thruster Only (USTO). In USTO, the USOS software bypasses the CMG system and commands thruster firings of the Russian Segment directly by manipulating the software logic used for CMG desaturation firings. These attitude control concepts are implemented in the software in the form of controllers, which are loaded by the ground as mission needs dictate. These three types of controllers map directly to the above attitude control concepts: n Momentum management controllers for use during quiescent orbit operations. n Attitude hold controllers for fine control. n Attitude hold controllers (USTO logic implemented) for direct USOS control of Russian Segment thruster firings. Russian Segment Attitude Control The Russian Segment performs attitude control using thrusters spread throughout the Russian Segment. The SM contains the original Russian Segment thruster package that is still in use today. Additionally, Progress vehicles docked to the aft port of the SM and the Nadir port of the DC-1 docking compartment have thrusters that are usually placed under control of the SM, when present. Although no longer in use, the European Automated Transfer Vehicle, when docked to the SM aft port, was also controlled by the SM. The Progress and ATV thrusters are generally preferred to the SM thrusters because the distance between the thrusters and the center of mass of the ISS, or the moment arm, is large. As when using a lever, the longer the moment arm, the greater the mechanical advantage and the less a thruster needs to fire. Additionally, unlike the SM, the Progress and ATV are not permanently attached therefore, they are unconstrained by lifetime usage limitations that affect the Zvezda thrusters, which have been in use since 2000.