## 2 Maritime Communications Systems  
### Overview of Maritime Communications  
Wireless communication is the only viable solution for ships at sea to exchange information with other ships and land-based stations. Before the development of modern wireless communications systems, ship communication was limited to communication within a visible distance using signals with various lights and flags. Ships began to be equipped with wireless communication devices with the invention of radiotelegraphs. However, there were no system regulations for distress communications until 1914, when the first version of the International Convention for Safety of Life at Sea (SOLAS) was adopted in response to the RMS _Titanic_ disaster [24]. Maritime communications have been used for three primary purposes: distress and safety communications, communications in support of navigation, and general communications. Distress and safety communications include distress calls and communication during search and rescue (SAR) operations for vessels in distress. In contrast, communications supporting navigation refer to exchanging information with nearby vessels and port managers during a voyage. Finally, general-purpose communications include various public communication services that serve similar purposes as on land. Maritime communications have recently seen an increased transition from analog to digital communications, as well as a significant increase in the need to exchange larger amounts of data in general. With the implementation of the Global Maritime Distress and Safety System (GMDSS) [25; 26; 27] in 1999, it became possible to send digital distress signals automatically via satellite communications, instead of the already obsolete methods of sending SOS or Mayday messages via telegraph and telephone, respectively. Communications in support of navigation have also evolved with the introduction of digital radio communications systems, such as AIS [19], which allows the exchange of navigational information with nearby vessels and coast stations via terrestrial Very High Frequency (VHF) communications. General public communications include using various Internet services by ship's crew and passengers, which is achieved using maritime communications satellites. The maritime communication system GMDSS is based on the requirements of the SOLAS Convention and the International Convention on Maritime Search and Rescue (SAR) [28]. The GMDSS enables the automatic transmission of digital distress signals in the form of digital selective calls (DSC). This form of low-speed digital communication replaced voice communication over the VHF band, which was mainly used for these purposes. Thus, the GMDSS removes the requirements for specific qualifications, such as the use of Morse code, in favor of the efficient utilization of simple distress buttons and switches. Therefore, any crew member can respond in the event of an emergency. In addition, the GMDSS relies on satellite communications to ensure stable communications and requires maintenance of radio equipment to ensure reliable operation while the vessel is underway. The overview of the GMDSS is shown in Figure 2.  
With their activation in the second half of the twentieth century, communications satellites became widely used for long-distance maritime communications, which until then had been limited to the use of radio waves in the Medium Frequency/High Frequency (MF/HF) band. Today, mobile communications via Immarsat satellites, among others, are used, offering different services, such as Fleet Xpress, FleetBroadband, and Immarsat C [29; 30]. In addition to the initially used voice communication, data transmission via satellite communications is becoming more and more developed. However, satellite communications still use a narrow band, which leads to a lower communication speed.