## 2 Materials and Methods  
### Technical--Organizational Details  
Technical details regarding working frequencies, bandwidths, gap between channels, etc are established by the radio regulations issued by the ITU. These regulations are updated/revised every 3-4 years at World Radiocommunication Conferences (WRCs), and their impact extends over long periods of time, between 10 and 20 years after the conference [10]. The GMDSS concept from Chapter IV of the SOLAS convention stipulates the obligation to provide ships with different communications equipment depending on the 4 maritime areas in which they can sail. These areas are defined according to the radio coverage and illustrated in Figure 1a:  
* Sea area A1 is defined as the area covered by VHF radio enabling Digital Selective Calling (DSC) and Radio Telephone (RT)--up to 20-30 nautical miles from shore;
* Sea area A2--covered by MF radio, also enabling DSC and RT--up to about 150 nautical miles from shore;
* Sea area A3 is covered by HF radio and Inmarsat geostationary satellite enabling DSC and satellite communications--global area between 70\({}^{\circ}\) N and 70\({}^{\circ}\) S;
* Sea area A4 is comprising polar regions above 70\({}^{\circ}\) N and below 70\({}^{\circ}\) S and is covered by radio communications in the HF radio range with DSC. In [11; 12; 13] there are presented the frequencies, structure, and destination of the radio channels allocated for the maritime mobile service in the MF, HF and VHF maritime bands as well as the changes that came into force starting with 1 January 2017. Thus, distress calls, distress messages, and distress traffic have absolute priority over all other types of messages/communications. Emergency communications, safety communications, and other communications follow. Moreover, all ships are required to be able to transmit distress alerts by at least two independent methods. Vessels equipped with DSC systems will transmit distress alerts through this system on one of the following frequencies in the MF/HF ranges: 2187.5 kHz, 4207.5 kHz, 6312 kHz, 8414.5 kHz, 12,577 kHz, 16,804, 5 kHz using F1B emission class (frequency modulation with digital information, without a sub-carrier for automatic reception) with a bandwidth of 300 Hz or in VHF on the frequency 156.525 MHz using G2B emission class with a bandwidth of 16 KHz. The ETSI EN 300 338-1 V1.4.2 (2017-11) standard stipulates that the integrated equipment used for communications in the MF/HF range must also be able to operate in J2B mode (\"Single Side Band SSB with digital information, with the use of a  
Figure 1: (**a**) Area of the Global Maritime Distress and Safety System (GMDSS) [14]. (**b**) The concept of the Global Satellite System for Search and Rescue [15]. modulating sub-carrier, with the carrier suppressed to at least 40 dB below peak envelope power\" [16]. According to Recommendation ITU-R M.489-2 (WRC-07), for ships not equipped with DSC systems, distress alerts will be transmitted on one of 4125 kHz; 6215 kHz; 8291 kHz; 12,290 kHz; 16,420 kHz using a J3E (single-sideband radiotelephony) emission class with a bandwidth of 3 kHz or 165.8 MHz using a G3E emission class with a bandwidth of 16 kHz [13]. Additionally, for the transmission of distress and safety messages through narrow-band direct-printing telegraphy (NBDP) are allocated the frequencies 4177.5 kHz, 6268 kHz, 8376.5 kHz, 12,520 kHz and 16,695 kHz, specifying that the transmission speed is not must exceed 100 Baud for FSK and 200 Baud for PSK. The bandwidth occupied for NBDP communications is 304 Hz. DSC equipment must comply with the standards IEC 61097-3, IEC 61097-8, IEC 62238, EN 300 338, EN 301 025, EN 301 033 and maritime radiocommunication equipment and systems must comply with IEC 60,533 (Electrical and electronic installations in ships--Electromagnetic compatibility), IEC 60945, IEC 61097-7, IEC 61162, ETS 300 162, ETS 300 225 [17]. A special category of signals transmitted by radio waves are location signals.