# UKF-Based State Estimation for Electrolytic Oxygen Generation System of Space Station
## 1 Introduction  
Electrolytic oxygen generation (EOGS) is currently recognized as the most reasonable oxygen replenishment technology for the space station [1, 2, 3, 4]. It is one of the core technologies of the physical and chemical regenerative environmental control and life support system (ECLSS) [5, 6, 7], and it is also the key technology for realizing long-term manned space flight [8]. The status of EOGS directly determines the oxygen content in the enclosed cabin with astronauts during the orbit [9]. Because the cargo spaceships between the space and earth can't be launched deliberately at any time, EOGS must guarantee the high reliability and ECLS will open the carried oxygen bottle to supply oxygen to the astronauts [10] when EOGS fails to produce oxygen. Since the oxygen carried during each launch is limited, and as the oxygen is gradually consumed, the oxygen content in the cabin will continue to decline, which not only affects the normal work of the astronaut, but also poses a serious threat to the life of the astronaut [11, 12]. The electrolytic oxygen plant of the International Space Station (ISS) developed by the United States was put into formal use after being verified in the orbit. In 2002, the main oxygen system of ISS completely stopped working after a few weeks of intermittent work, and the three astronauts on the station had to use the solid fuel oxygen tank to produce oxygen [13] before the main oxygen system was repaired. In 2004, due to the blockage of the pipeline, the main oxygen system failed again, the two astronauts inside had to rely on the oxygen in the docking cargo tank to maintain their lives, meanwhile waiting for the Russia cargo to transfer more oxygen [14]. Seen from the abovementioned events, it is necessary to monitor the working state and to diagnose the fault of the electrolysis oxygen system of the space station [15, 16, 17]. At present, different technical solutions are adopted for the EOGS system by the US and Russian space stations. The two countries are very disparate in terms of their design ideas, resource requirements, system configuration and service life. The biggest difference is that Russia adopts the solution for EOGS by flow alkaline electrolyte and membrane static water gas separator. Russia conducted flight verifications of the EOGS device at the early stage of the construction of the Mir space station, and then directly installed the station as the main oxygen supply equipment. In the initial stage of the construction of the International Space Station, the Russian electrolytic oxygen plant was used as the main oxygen supply equipment. The technical scheme of solid polymer electrolyte (SPE) used in EOGS of United States was officially put into use after on-orbit verification, meeting the oxygen demand of the International Space Station. The use of EOGS has greatly reduced the amount of ground logistics supplies for life insurance, and is one of the necessary conditions for medium and long-term manned spaceflight. Russia uses alkaline electrolyzer PEM. Russian electrolytic oxygen generator uses strong corrosive dye as the electrolyte, it has caused great difficulties in on-orbit maintenance and replacement of parts [18]. The US electrolytic oxygen generator in the ISS adopts the technical solution of solid polymer electrolyte (SPE) water electrolysis [19], and the US electrolytic oxygen generator adopts a standard cabinet-style installation method to facilitate on-orbit maintenance and replacement. The EOGS in China's future space station adopts the same method as the United States. The background of this study is China's long-term manned space mission, and the technical scheme of SPE is used in EOGS [20]. In long-term manned missions, the oxygen concentration in the cabin of the space station can be detected by the detection device in real time [21; 22]. However, the oxygen concentration in the cabin is not suitable as a standard for judging whether the EOGS system is operating normally, due to the particularity of the space station.