The document discusses frequency ranges used in the Global Maritime Distress and Safety System (GMDSS). It explains that the radio spectrum is divided into nine frequency bands allocated by the International Telecommunication Union (ITU) for different radiocommunication services. The 156.4875-156.5625 MHz band is allocated globally to maritime mobile service on a primary basis for distress and safety. Frequency assignments are authorized by administrations according to the ITU Radio Regulations to prevent interference between services and regions. Propagation of radio waves is influenced by the external environment and sea area, with different distress alerting methods used depending on available coast radio and satellite coverage.

1. Lecture
Topic: GMDSS frequencies. frequency ranges
Objective: to uncover the nature of the GMDSS frequencies.
frequency ranges
We consider the following questions:
1. Distribution of radio waves of various ranges
2. The influence of external environment on radio wave
propagation.
1. Distribution of radio waves of various ranges
The radio spectrum is subdivided into nine frequency bands
as shown in Table 1. The unit of frequency is the hertz (Hz).
Table 1. Frequency bands
Subdivisions of these bands are allocated for their use by one or
more terrestrial or space radiocommunication services under
specified conditions by an entry in the Table of Frequency
Allocations which is contained in the Radio Regulations, Article 5.
An example of a service is the maritime mobile service.

2. Frequency bands may be allocated globally or to an ITU
Radiocommunication Region as shown in Fig. 1. The services may be
given primary or secondary categories where a secondary service
means that its stations shall not cause interference to stations of the
primary service and cannot claim protection. An example of a band
allocated globally to the maritime mobile service on a primary basis
is 156.4875-156.5625 MHz which is VHF channel 16 and its two
guard band channels 75 and 76.
Other bands used by ships may be in mobile bands, such as the
mobile-satellite band used by Inmarsat, in which case they are shared
with other mobile services. Alternately, they may not be available for
use in all regions or may be a secondary allocation. Special provisions
and additional application of the allocations are described by
footnotes to the Table of Frequency Allocations.
Frequencies are assigned to a user through authorization by an
administration for a radio station to use the frequency. The
procedures for the assignment of frequencies are given in the Radio
Regulations, Article 4. Basically, Member States of the ITU are
required to assign frequencies in accordance with the Table of
Frequency Allocations and the provisions of the Radio Regulations in
order to prevent interference.

3. Figure 1
The three regions for the allocation of frequencies
2. The influence of external environment on radio wave
propagation.
The method of distress alerting depends on the sea area in which the
ship is sailing and on the equipment carried. The likely methods of
initiating a distress alert in the four sea areas defined according to the
coverage of MF, HF and VHF coast radio services and satellite
services are shown in Table 2.
Table 2
Frequencies and methods for distress alerting