## 3 Materials and Methods  
### Shipboard Data Acquisition  
The data compression method proposed in this paper needs to be experimentally validated on the actual shipboard data. Therefore, the data were collected on board the Training Ship (TS) _Fukae Maru_. This ship is operated by Kobe University, Japan, and is used for training, research, and conducting experiments. The research includes the development, implementation, and testing of various intelligent ship concepts, such as Heading Control System (HCS) and Track Control System (TCS) [39], a practical and easy-to-implement dynamic positioning (DP) system [40], and a tablet-supported control system [41]. The ship is equipped with numerous state-of-the-art electronic devices and systems that enable advanced control functions and collecting large amounts of data  
Figure 4: AIS data encoding: (**a**) Example of one record of the AIS position report message; (**b**) Obtained encoded message. from various sources [42; 43]. Figure 5a shows TS _Fukae Maru_, while Figure 5b,c show the examples of navigation screen and engine monitoring screen available in the control room, respectively. In addition, the data acquisition devices are integrated with the advanced shipboard Local Area Network (LAN) [44], whose configuration diagram is depicted in Figure 6.  
Digital data are collected by various devices and shared via LAN. These include measurements from navigation instruments, engine monitoring sensors, environmental sensors, and targets data. Navigation data consist of Global Positioning System (GPS) data and data from GPS compass, magnetic compass, and gyrocompass, while environmental data refer to measurements of various external variables, such as data from the Acoustic Doppler Current Profiler (ADCP) and anemometer. Engine-related data include sensors for monitoring engine operation, such as temperatures of various engine parts, flow rate, and pressure, whereas targets data consist of radar and AIS data. The approximate total number of data items collected and transmitted is 450, including approximately 50 measurement channels for navigation data, 50 channels for environmental data, and 350 channels for engine-related data. Navigation data are collected and transmitted every 0.2-1 s, environmental data at 1-2 s intervals, while engine-related data are transmitted every 2 s.  
Figure 7 shows an example of one line of shipboard data sent from TS _Fukae Maru_, where each line of data represents one sentence sent by the device, i.e., one data record. Figure 8 explains the structure that is characteristic for each line of data sent. As shown  
Figure 5: Examples of advanced ship systems used for data acquisition: (**a**) TS _Fukae Maru_; (**b**) Navigation screen; (**c**) Engine monitoring screen. in Figure 8, each line of data begins with a timestamp that contains, from left to right, information about the year (YYYY), month (MM), day (DD), hour (HH), minute (MM), second (SS), and millisecond (mmm). In addition, each line also contains the device code (D), which is used to identify the data source. Finally, the main information is contained within the NMEA sentence. NMEA-0183 data format protocol proposed by the National Marine Electronics Association (NMEA) represents a standard for interfacing marine electronic devices, navigation instruments, and communications equipment [45]. This protocol supports one-way serial data transmission from a single talker to one or more listeners, where talker and listener  
Figure 8: Structure of one line of transmitted shipboard data. Figure 6: Configuration of the shipboard LAN for data acquisition and recording. Figure 7: Example of one line of transmitted shipboard data. refer to a device that sends or receives data from other devices. Data are represented in printable ASCII characters form, while messages have typical lengths in the range between 11 and 79 characters [45]. Figure 9 shows the structure of the NMEA data sentence.