General Information Regarding Ais; What Is Ais??? - In A Nutshell - Weatherdock easyTRX2 Handbuch

User's Manual
Page 75 of 125

17. GENERAL INFORMATION REGARDING AIS

17.1. What is AIS??? – In a nutshell
Substantial investment over the last few years has resulted in the fact that
transport safety systems in coastal waters are considered to be the most
modern worldwide. The introduction of AIS technology by the
International Maritime Organisation (IMO) will contribute to the currently
available traffic information and thus improve navigation safety.
This data is exchanged automatically and in short intervals between AIS
devices via specific VHF transmitters and receivers.
The VHF transmitting unit transmits the data telegram via one of two
internationally defined AIS radio frequencies. One of the characteristics of
AIS technology compared to other radio services is the automatic
organization of several AIS devices on just one radio frequency without
disturbing each other.
This is achieved by the SOTDMA transmission method („Self Organising
Time Divison Multiple Access").
The data is transmitted within one or more time-slots via the two radio
channels which are reserved for AIS devices. Each device generates its own
transmission schedule based on the observed past data traffic and the
knowledge about future actions of other AIS devices within radio range.
In this respect, the necessary time slots needed for the transmission are
occupied. The transmitted data packets are received by every ship within
radio range, which is also equipped with such a device to ensure the
transmission of the above-stated data. The dynamic ship data is
transmitted quite frequently compared to the static and journey-related
data, which is only sent at minute intervals. The transmission of data
telegrams happens automatically, in short time intervals, depending on
the situation, i.e. speed and current maneuver situation. For example, if a
commercial ship lies at anchor it only sends a message every three
minutes, if it is moving and putting about at the same time it sends at two
second intervals.
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