## 2 Maritime Communications Systems
### Overview of Maritime Communications
In addition to the lower bandwidth, the fees for using satellite communication are still relatively high, so that this form of communication is not an optimal solution for transmitting larger volumes of maritime data over longer distances. VHF Data Exchange System (VDES) is a maritime radio communication system that uses the Maritime mobile VHF band for relatively low-speed communication between ships, coast stations, and low Earth orbit (LEO) satellites [31]. VDES consists of the following components: AIS, Application Specific Messages (ASM), and VHF Data Exchange (VDE). AIS [19] enables digital communication between the ship and the receivers within the range, including those on other ships or coast stations. A regulation [32] effective since 2004 made AIS mandatory equipment for ships of 300 gross tonnage (GT) or more on international voyages, cargo ships of 500 GT or more not in international waters, and all passenger ships [20]. As a technology that is the subject of many research studies and is being used in the development of various new applications [21; 33; 34; 35; 36; 37], AIS is discussed in more detail in the following subsection. be roughly divided into three main categories: dynamic information, static information, and voyage-related information. Dynamic information represents the ship's position report and is transmitted every 2-10 s depending on the ship's speed while the ship is underway or every 3 min when the ship is at anchor. The dynamic information includes the following data: ship Maritime Mobile Service Identity (MMSI) in the form of a nine-digit identification number, navigation status, rate of turn, speed over ground (SOG), position of the ship and the corresponding position accuracy, course over ground (COG), true heading, and timestamp in Coordinated Universal Time (UTC) seconds. The AIS transceiver transmits static and voyage-related information every 6 min. The information transmitted includes the following data: seven-digit IMO ship identification number, radio call sign, name of the ship, type of the ship and the cargo, dimensions of the ship, location of the positioning system's antenna, type of the positioning system, draught of the ship, destination, and estimated time of arrival (ETA) in UTC. Data encoding used for transmitting the AIS data utilizes a fixed, predetermined number of bits for each data item. However, the number of bits varies between data items: 6 bits for the message ID, 2 bits for the indicator of repeating a message, 30 bits for the MMSI, 4 bits for the navigation status, 8 bits for the rate of turn, 10 bits for the SOG, 1 bit for the indicator of the position accuracy, 28 bits for the longitude, 27 bits for the latitude, 12 bits for the COG, 9 bits for the heading, 6 bits for the UTC seconds, 4 bits for the regional use, 1 bit for the Receiver Autonomous Integrity Monitoring (RAIM) flag, and 19 bits for the communication status. Each data item is here represented by an integer value. Data items containing real-numbered values have a predetermined number of digits \(n\) after the decimal point assigned to them. The integer values are, thus, obtained by multiplying the original real-valued number by \(10^{n}\). The integer value of each data item is then converted to the binary format, thus forming the 168-bit binary data sequence. The binary sequence is divided into groups of six bits, and encoding is performed on these groups, i.e., each six-bit binary number is assigned with the corresponding valid character from the American Standard Code for Information Interchange (ASCII) table. The ASCII code of the corresponding alphanumeric characters and symbols is finally transmitted. Figure 4 depicts the method used for the AIS data encoding, showing an example of one record of the dynamic information message within the AIS position report, with a total of 168 bits divided into the six-bit sections. Bit groups are indicated in Figure 4a, along with the AIS data items they represent within the message.