## 2 Materials and Methods
### Multiple Remote Control System Architecture  
The multiple remote control system is compatible with a variety of terminal devices, enabling users to operate computers, smartphones, smartwatches, and multi-site central control centers, as well as any devices with web browsing capabilities. Through these devices, users can access web services on the cloud server, view real-time underwater environmental sensing data, and perform remote operations of the underwater ROV. To achieve multi-platform remote operation, this research developed a web-based control interface called the Smart-Fish Control System. This interface serves as the core component of the multi-remote control system, deployed on a cloud server and accessible via web browsers on computers, smartphones, and smartwatches. The control interface displays real-time video from the underwater ROV, along with real-time sensor readings, such as temperature and pH levels. Users can send control commands to the underwater ROV through this interface, allowing them to manipulate its movement direction, among other functions.  
Communication between the user interface and the underwater ROV utilizes a publish/subscribe model based on the MQTT (Message Queuing Telemetry Transport) protocol. In this system, the cloud server operates as an MQTT message broker. The Smart-Fish Control System functions as an MQTT client, publishing relevant control messages--targeting various control topics, such as movement commands--to the broker when users interact with the interface. The Raspberry Pi (Raspberry Pi Foundation, Cambridge, UK) on the buoy also acts as an MQTT client, subscribing to these control topics. When the user interface publishes a control command (such as forward) to the MQTT broker, the message is promptly transmitted to the Raspberry Pi on the buoy. This publish/subscribe architectural design is both lightweight and reliable, ensuring command delivery even in situations  
Figure 2: Cloud and software architecture for underwater vehicle communication. (**a**) Overall software architecture diagram; (**b**) Software component connection architecture diagram.  
where mobile communication networks may occasionally experience delays. The system supports a variety of control devices, enabling truly diverse remote control options:  
1. Smartphones and Tablets: Users can access the Smart-Fish Control System via web browsers on smartphones or tablets, allowing for the comprehensive control and monitoring of underwater ROV. The touch interface of mobile devices is optimized to ensure intuitive and precise operation, making it particularly suitable for outdoor environments at aquaculture farms.
2. Smartwatches: A specialized, simplified control interface was developed for smartwatches, enabling users to send straightforward commands (such as instructing the underwater ROV to surface). This feature offers aquaculture operators an exceptionally convenient emergency control option.
3. Multi-Site Central Control Center: Professional monitoring stations established at aquaculture headquarters or research institutions, equipped with large display screens, can simultaneously oversee multiple underwater ROV systems and data from aquaculture areas, providing a comprehensive monitoring perspective. Figure 3 shows the central control room setup at National Taiwan Ocean University, demonstrating both operator monitoring via multi-screen stations and collaborative real-time data analysis. The system design enables the simultaneous monitoring and management of multiple aquaculture farms across different regions from the central control center, significantly enhancing management efficiency and response capabilities.