Satellite propagation
Inmarsat services all use direct wave (line of sight) propagation, so
your ship's Inmarsat antenna must have direct line of sight to the satellite.
Antennas have to be fitted clear of superstructure or other obstructions.
Service may be lost when alongside a dock with high buildings or when
close inshore where the terrain may shield the antenna from the satellite
(in which case the antenna is said to be in shadow).
Coverage for GMDSS purposes is considered good up to latitudes
70° North and 70° South (assuming not in shadow of terrain or buildings),
though service can often be accessed further North or South than 70
degrees. The further North/South you are from the equator, the lower
the angle of sight from the antenna to the satellite and the greater the
possibility of losing sight of the satellite when near to land or other
obstructions.
Inmarsat-C installations
The most likely piece of Inmarsat equipment that you will come
across on a GMDSS-fitted vessel is Inmarsat-C - which can be used to:
- Receive SafetyNET information (weather forecasts/warnings;
navigation warnings; SAR messages and other safety information)
through the EGC service, which is part of the Inmarsat-C system.
- Send Distress Alerts and Distress Priority Messages.
- Send messages for Medical Advice, Medical Assistance and other
Urgency and Safety type messages using special short code addressing
(a list of Urgency and Safety short codes is included in ALRS Vol. 5).
- Send and receive Routine messages - including messages to Internet
and other e-mail addresses.
All Inmarsat-C installations will include:
- A small, omnidirectional antenna.
- A transceiver unit (may be part of a combined communications unit).
A messaging terminal (may be part of a combined communications
unit and/or be a normal computer running the required Inmarsat-C
operating/messaging software) - with a printer when part of the
GMDSS equipment fit.
A Visual Display Unit (VDU) for reading messages and a keyboard
for programming and controlling the unit and for typing messages.
A DC power supply.
Depending on manufacturer, the package can be put together in
various different ways, as shown in figure 8.3.
158 THE NAUTICAL INSTITUTE
