135 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS CHAPTER 7 and tested together. Flight directors and motion control flight controllers at each center jointly manage, over voice circuits, procedures and authorizations for commands. Assembling the Motion Control System The ISS MCS has gone through several evolutions throughout the assembly sequence. Initial capability was provided with the launch of the Functional Cargo Block (FGB) with its basic propulsive control system used for attitude control from launch, by the addition of the Node 1 on Dec 6, 1998, and until arrival of the SM on July 26, 2000. Upon arrival of the SM, the FGB control system was permanently shut down and converted to propellant storage. During the next 7 months, the SM provided attitude determination and attitude control using thrusters, with orbit determination being done via ground-based radar. The USOS CMGs arrived with the Z1 truss during Space Transportation System (STS)-92/ISS-3A in October, although the CMGs were inactive. The CMGs were activated and the first attitude control handover was performed to the USOS MCS following the arrival of the USOS Destiny Laboratory on February 10, 2001, along with the necessary flight computers and software. The USOS MCS and CMGs have performed normal day-to-day attitude control since this first activation, with Russian thrusters engaged only every few weeks for larger attitude maneuvers, reboosts, or docking/capture of cargo or crew transport vehicles. The arrival of the last major assembly—the S0 truss on STS-110/ ISS-8A in April 2002—completed the ISS control system. The S0 truss mounted the two RGA attitude rate sensor packages and four GPS antennas (with the SIGI GPS receivers launched earlier in the USOS Laboratory). This upgraded equipment, along with a software update to the GNC and C&C MDMs, allowed the US Segment to fully determine attitude, attitude rate, and orbits independent of the Russian Segment, thus greatly extending the redundancy of the ISS MCS. This completed the initial system assembly. Although assembly and reconfiguration of the system continues in some respects, each Progress vehicle is used for auxiliary propulsive elements, primarily to provide reboost engines and augment roll control with thrusters. Additionally, software continues to be incrementally upgraded to take advantage of operational experience, such as using the Ku-band antenna to help determine attitude along with the GPS receivers. Control Moment Gyroscope Failures The CMGs were the subject of a considerable engineering and test effort while under development because of their criticality, and the fact these mechanical devices must spin at a high speed for decades. The first CMGs used in space were developed for the Skylab Program, which used three CMGs. During the relatively short Skylab operational mission, one CMG suffered a spin bearing failure and was shut down, and a second was near failure. The ISS CMGs were direct descendants of the 1973 Skylab CMGs. NASA made improvements in the bearing design to increase the operational lifetime. Despite this effort, problems with the CMGs continued early in the ISS Program. The CMGs were activated on February 12, 2001, during STS-98/ISS-5A, after being launched late in 2000 and stored with only survival heaters active on the Z1 truss. These CMGs were responsible for nearly all of the ISS attitude control after that time. On June 8, 2002, controllers in MCC-H noticed that, after little more than a year of operations, CMG-1 vibrated as it spun. Over the next several hours, the vibrations worsened until one of the two mechanical spin bearings failed. It took more than an hour for the energy in the spinning wheel to dissipate, at which time one side of the bearing assembly became so hot it melted the ball bearings inside. The crew reported a sound, which astronaut Carl Walz described as “a pretty loud, audible noise. A kind of growling noise in the Node.” The CMGs are mounted in the Z1 truss, which in turn is mounted to the zenith port of the Unity Node. As with all key elements of the ISS, the CMGs can be replaced (Figure 17). After a great deal of concern over the health of the remaining three CMGs, especially during the stand down following the Space Shuttle Columbia accident in 2003, NASA replaced CMG-1 during STS-114/ ISS-LF1 in 2005 and returned the gyroscope to Earth. CMG-3 exhibited similar issues shortly after CMG-1 was returned. CMG-3 was eventually shut down and