## 2 Unmanned Ship Positioning System and Its Multi-Sensor Data Fusion Framework  
### Construction of Unmanned Ship Positioning System  
In order to realize the information complementarity among the multi-sensor measurement data, reduce the interference of the surrounding environment on the working state of the positioning system, improve the reliability of the multi-sensor data samples, and then improve the positioning accuracy of an unmanned ship for water quality detection, a ZigBee-GPS/BDS multi-sensor combined positioning system, as shown in Figure 1, was constructed. The entire positioning system includes a ZigBee multi-node positioning system, GPS positioning system, BDS positioning system, PC communication system, etc.  
A GPS/BDS positioning system is mainly composed of space, ground monitoring, and user receiver. First, the ground monitoring part monitors and controls the operation of each satellite through the main control station and monitoring station, traverses the navigation message, and maintains the system time. Then, the GPS and BD satellites in space continuously send their own ephemeris and time information to the GPS/BDS dual module installed in the unmanned ship. Finally, the GPS/BDS dual module calculates the latitude and longitude coordinates of the unmanned ship in real time by analyzing the signal messages sent by the respective satellites [12].  
A ZigBee multi-node positioning system is mainly composed of blind nodes, reference nodes, and wireless gateway nodes. First, the communication environment of the full grid network is constructed according to the reference nodes with known position coordinates. Then, a blind node installed on the unmanned ship is placed in a communication network  
Figure 1: ZigBee-GPS/BDS multi-sensor combined positioning system for an unmanned ship.  
composed of reference nodes [13]. After that, the wireless gateway node is connected to the PC through the serial port. On the one hand, it receives the configuration data of each reference node and mobile node from the monitoring software and forwards them to the corresponding nodes. On the other hand, it receives the valid data fed back by each node and transmits them to the monitoring software. Finally, the RSSI technology is used to determine the distance or direction from the unknown node to the beacon node, and the maximum likelihood estimation method is used to calculate the position of the blind node on the unmanned ship during navigation in the river.  
Figure 2 shows the self-developed unmanned ship for water quality testing. The unmanned ship is equipped with a detector with multiple water quality detection sensors connected to the launch and recovery device, as well as a positioning system including Zigbee, GPS, and BDS modules. The controller of the unmanned ship is an industrial computer with a Linux system, which is responsible for receiving and processing the data of each sensor, issuing control instructions, and transmitting signals with the PC.  
The lidar of Silan A3 is responsible for collecting the environmental information around the unmanned ship. The ATK1218-GPS-BDS dual-positioning module is selected in the GPS/BDS positioning system to obtain the latitude and longitude information of the unmanned ship. The CC2530 wireless module is selected in the ZigBee positioning system to obtain the plane coordinate information of the unmanned ship. The Hikvision DS-2 camera is selected in the vision system to be responsible for the collection of environmental information of the unmanned-ship channel.