Investigation of the Crew Induced Disturbance to the ISS GN& C System

Due to the complexity of the International Space Station (ISS), interactions between seemingly isolated systems occur which require interdisciplinary investigations to determine the cause and effect relationship of what would otherwise seem to be random events. An example of these complex relationships is apparent when studying the ISS guidance, navigation and control (GN&C) system and the activities that resulted when the system began responding to unexplained very low-frequency torque disturbance events at seemingly random times. Investigations revealed that the GN&C system was responding to a near-1-Hz ISS structural resonance. The GN&C control software was aliasing a 1-Hz attitude rate signal, so that it appeared as a very low frequency rigid body disturbance. While the source of the GN&C response was quickly identified, the cause of the 1-Hz structural resonance events remained undiscovered. ISS engineering personnel logged the event occurrences, seeking correlations with concurrent operations. After several weeks of observation, payload microgravity accelerometers showed an emerging pattern suggesting that the disturbances were correlated to crew exercise periods. Subsequent efforts to isolate the cause correlated with crew use of the resistive exercise device. The frequency and magnitude of the events were also affected by changes in the vehicle configuration, which altered the station´s structural response. Discovery of this system interaction led the GN&C team to redesign the anti-aliasing filter to attenuate all structural resonances in this range within the control system. While this solution prevents the GN&C system from responding to this ISS flex event, the interaction of crew exercise and the ISS structural flex continues to be investigated by the ISS structures team. The interaction of these seemingly isolated systems has highlighted the complex nature of the ISS and is an excellent example of the engineering investigat- - ions that must occur on such a large vehicle