CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 128 The Russian Segment independently determines attitude using star mappers and its own set of gyros. Star mappers mounted on the SM take images of the sky and compare the patterns made by the stars in the image to a catalog of the star patterns stored in the software. By matching images to the catalog, software in the star mapper can determine the orientation of the star mapper itself. This information is processed by the Russian Segment Terminal Computer to determine the attitude. The Russian Segment also uses a gyroscope in the SM to determine changes in attitude. This system allows the Russian Segment to determine the attitude of the ISS independently and dissimilarly from the USOS systems. Both segments share, compare, and can use the attitude information from each other. This sharing, combined with the dissimilar system designs of each system, provides a significant advantage in redundancy, since major failures (e.g., power failures) are usually localized to either the Russian Segment or the US Segment. If the US Segment loses its attitude or rate information, it can easily and automatically switch to that being provided by the Russian Segment. The Russian Segment can similarly use the navigation information computed by the US Segment. As shown in Figure 13, the USOS GNC software carries three estimates of attitude and three estimates of rate when running at full redundancy, including the estimate delivered by the Russian Segment. The software compares each of the three estimates and will vote out a single estimate that is in error. GPS 1 or Ku SELECTED ATTITUDE SELECTION RS State GPS 2 or Ku RGA 1 SELECTED RATE SELECTION RS RATE RGA 2 Figure 13. Attitude selection algorithm in the US GNC software. How does the International Space Station control its location? The ISS uses a combination of small rocket thrusters located on the SM and Progress cargo vehicles on the Russian Segment as well as non-propulsive attitude hold devices (i.e., CMGs) on the US Segment to maintain attitude. Occasionally, the orbit may need to be raised or adjusted, which is done with rocket engines on the aft of the SM or those on the aft of a docked Progress cargo vehicle. The ISS has an elegant arrangement where the duties of attitude control are shared between the US Segment and the Russian Segment. Computers to manage the systems are divided between the segments and share data, and attitude control is handed over between the segments cooperatively, depending on operational demands. Besides operational needs to position the space station (such as maneuvering to a docking attitude), the MCS counteracts small but significant forces (over time) from the low-Earth orbit environment. Those forces include: n Aerodynamic drag, primarily due to the large solar arrays. Even though the ISS is in space, its low orbit actually encounters a very thin portion of the atmosphere of the Earth. Overall drag causes the orbit to lose energy and the space station to drop to a lower altitude, requiring periodic reboosts to raise the orbit. Unequal drag on different parts of the vehicle also causes attitude torques (or rotational, twisting force) that tend to push it out of flight attitude, which needs to be constantly counteracted by the attitude control system.