164 useful during early construction when full solar ar ray capability was unavailable, but due to other  constraints, it was not desirable to operat e at the higher temperature X-POP attitude. 

Since assembly Flight 12A in September 2006, the ±YVV and ±X-POP attitudes are no longer used,  although in future unique situations, they may be used as contingency attitudes.   GN&C Operational CMG Usage

Prior to CMG activation, the ISS attitude control wa s accomplished using thrusters. Thrusters are now   used as a backup for CMG control, for large a ttitude maneuvers, or for translational control.   Nominal quiescent operations are handled by the CMGs using a concept known as momentum  management. A Momentum Manager controller is designed to perform via the following  two methods:  (1) seeking the TEA by finding the attitude having the lowest momentum, and (2) maintaining a particular attitude by expending the appropriate amount of moment um to bias that attitude. Usually, controllers  combine a TEA-seeker in one ISS body axis (pitch) with an attitude bias for the other two axes (yaw and roll).  GN&C software maintains a given attitude by commandi ng the CMG inner and outer gimbals to specific  locations. This positions the individual momentum contributions from the CMG flywheel spin axes to  provide a total momentum vector for vehicle control.  External torque disturbances on the vehicle can be  countered by moving the vector as appropriate.  If a disturbance torque imparts a momentum that is  greater than the total combined control momentum,  the CMGs ‘saturate’, having expended all available mo mentum to counter the di sturbance. The controller  commands thruster firings to ‘desaturate’ the CMGs, or unload the momentum to a specified value. The  CMG gimbals are then re-positioned to attain a lower total momentum. A CMG desaturation happens quickly, and may result in rates of gimbal rotati on that are substantially higher than those needed for  quiescent operations having low or non-existent di sturbance torques. The CMG desaturation function can  be disabled, and attitude control is then handed completely over to the thrusters in the case of CMG momentum saturation. 

CMG Operational History 

The original four CMGs were launched on the Z-1 Truss, Mission STS-92, ISS Flight 3A, on October 11, 2000. They remained dormant and unpowered for a few months until being activated on February 12,  2001 during Space Station Flight 5A. On June 8, 2002, CMG1 developed a problem and failed resulting in  3 operational CMGs. Operations using a 3-CMG control configuration was requi red until a spare replaced  the failed CMG1 on August 1, 2005 during Space Shuttl e Flight LF-1. The ISS then continued to operate  under 4-CMG control until CMG3 exhibited signs of distress. The initial CMG1 Failure Root Cause Investigation together with the CMG3 experience led to the development of on-orbit monitoring criteria  intended to identify problems and prevent a hard failu re. An additional benefit of implementing these  criteria was to preserve the hardware for re-use and subsequent forensic analysis. On October 10, 2006 CMG3 was shut down because the unbalance sensor had reached the shut-down criterion. It was then taken out of service but remained on the Z1 Truss wi th the Electrical Assemb ly (EA) and heaters enabled,  until it was removed on August 13, 2007. A 3-CMG control configuration was again implemented for  attitude control. During CMG3’s non-operational time period, various diagnostic tests were performed to  support the joint L-3 S&N, Boeing, and NASA decision to remove and replace CMG3. This “soft failed”  CMG was replaced by another ground spare on August  12, 2007 during Space Shuttle Flight 13A-1 and  the ISS was restored to a 4-CMG control system.   The CMG1 investigation identified high gimbal rates as a contributor to the failure. During the earlier  stages of ISS assembly, CMG gimbal angular velocity was restricted by CMG firmware to a maximum rate of 0.054 rad/s (3.1 deg/s). Following the two CMG failures, GN&C engineers limited all CMG gimbal  NASA/CP-2010-216272