## 2 Relationship between MWL and MASS
### Key Additional Factors for MASS  
One of the critical factors is the autonomy level of MASS. Various articles and documents define the level. [PERSON] et al. [27] separated the process for MASS development into four stages; \"system decision support\", \"shore-based remote control with seafarers onboard\", \"shore-based remote control without seafarers onboard\", and \"fully autonomous\". This stage is almost the same as the degree of MASS used in the IMO [17]. These stages can be categorised into two dimensions: level of autonomy (LoA) and level of remote control (LoRC). Maritime U.K. [18] provides six autonomous levels. These categories are defined depending on to what extent the human (operator) is involved in the navigation. DNV GL's degree [19] is almost the same as Maritime U.K. Ringbom uses two key functions, LoA and manning level on board (MLoB) [28]. MLoB is classified into three levels: fully manned, periodically unmanned, and unmanned. The periodically unmanned level can apply to a wide range of situations, according to LoA. For instance, an unmanned situation onboard might be possible in the open sea and under low congestion, then a watch officer will rush to the bridge in the emergent situation by noticing a false alarm. The Norwegian Forum for Autonomous Ships (NFAS) adopts a similar approach [29]. It divides the level into two axes: \"Operational autonomy level\" and \"Bridge manning level\". Combining four levels of the operational autonomy level and three bridge manning levels, it also defines the type of autonomy (e.g., automatic ship by the combination of automatic levels, and unmanned bridge with crew onboard). From a human-centred point of view, some issues should be considered to define the autonomy level. Firstly, because multiple crews who are engaged in special duties are onboard a ship unless it is just a one-crew small boat, the manning formation and relationship should be carefully considered. From a fatigue perspective, effective management of the manning level contributes to its reduction [30]. Appropriate manning will be one of the factors to reduce stress and fatigue. \"Appropriate\" means not only the number of crews but also the tasks given to each crew. Ships provide a bridge manning matrix that describes who will be on the bridge and what tasks they should do during navigational watch. Based on the matrix, they work together as the bridge team [31]. Bridge resource management is vital for safe navigation. For instance, double manning during navigation causes ambiguous command and enhances collision risk [32]. The responsibility of seafarers is also a critical point. For instance, while the captain has a final and ultimate responsibility for the ship's safety, remote operators might be responsible for navigational watch on behalf of a captain and manage the bridge team [31]. These differences in the responsibility between seafarers are reflected in the stress. According to [PERSON] et al. [33], a captain shows the highest mental stress in the four levels of navigation staff (captain, officer on watch (OOW), steersman, and pilot), probably due to the difference in responsibility; the steersman's stress is the lowest due to them having the most straightforward tasks. The MASS related systems drastically change the bridge management team's formation including their tasks, and formation of the MASS navigation team should be elaborated. Who takes the ultimate responsibility and in which position the person is engaged in duty on the bridge or at the RCC are especially important. The second key factor is the information that operators acquire. The appropriate information positively affects MWL [6] and contributes to keeping the MWL at an optimal level. Conversely, according to [PERSON] et al. [34], improper information such as a lack, complexity, and overload of information would deteriorate air pilots' performance and face the risk. Seafarers are the same; the lack of information as well as information overload induces the issue of MWL and ship navigation in many cases.