On-Orbit Performance of the Major Constituent Analyzer

The Major Constituent Analyzer (MCA) was activated on-orbit on 2/13/01 and provided essentially continuous readings of partial pressures for oxygen, nitrogen, carbon dioxide, methane, hydrogen and water in the ISS atmosphere. The MCA plays a crucial role in the operation of the Laboratory ECLSS and EVA operations from the airlock.

This paper discusses the performance of the MCA as compared to specified accuracy requirements. The MCA has an on-board self-calibration capability and the frequency of this calibration could be relaxed with the level of instrument stability observed on-orbit.

This paper also discusses anomalies the MCA experienced during the first year of on-orbit operation. Extensive Built In Test (BIT) and fault isolation capabilities proved to be invaluable in isolating the causes of anomalies. The process of fault isolation is discussed along with development of workaround solutions and implementation of permanent on-orbit corrections. During the anomaly resolution process, the philosophy of on-board diagnostics was re-examined and the level of hardware protection had to weighed against nuisance alarms and instrument availability. Effects of the microgravity environment were determined to be a factor in the frequency of on-orbit error code occurrences. Radiation-induced upsets are compared to predictions. The resolution of anomalies involved the removal and replacement (R & R) of Orbital Replaceable Units (ORUs). R & R procedures were developed and refined with the guidance and participation of representatives from the Astronaut Office of the NASA-JSC Flight Crew Operations Directorate.