An automatic identification system (AIS) receives information broadcast by other vessels and base stations that are equipped with an AIS transponder and transmits static and dynamic information from its own vessel. The system uses two data-specific channels in VHF to receive and transmit the information. There are various types of data that an AIS is capable of transmitting, these include: MMSI coding, GPS antenna positioning, speed over ground (SOG), course over ground (COG), and more. These transponders are responsible for allowing vessels to see and be seen by other marine vehicles equipped with AIS— they act as a preventative locating tool.
An AIS is categorized into two type classes. Larger vessels typically use a class A AIS unit and are only required to have the system if they weigh more than 300 gross tons. On the other hand, NON SOLAS vessels use a class B AIS unit, and are not required to do so. Both transponders communicate information received from other vessels or base stations to compatible plotters or charting systems.
When it comes to installation and mounting of either class A or B antennas, there are two options that are utilized in operation of an AIS. The first option is a dedicated AIS antenna. Its purpose is specific to the VHF needs of the system, and it does not handle any other range VHF reception. The second is a splitter, or shared antenna. In the past, this combined structure was inefficient and reduced the range of VHF reception for both purposes. Modern splitter technology now allows for “zero loss” of reception range, as it senses emerging signals before dividing them. This system can be expensive, but it is easy to install compared to two separate antenna devices.
The mounting position of both units is extremely important. An AIS VHF antenna must be placed at the highest possible point on a vessel. It is also necessary to place the antenna at least 2 meters from any other VHF systems, in order to avoid the damage that can come from excess wattage. In an emergency, an AIS can supplement a VHF antenna as long as it is equipped with an adapter. These systems are also efficient at detecting objects that radar systems can miss and are often used in addition to radar technology.
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