Automatic Identification System

Inland AIS - Represents the automatic identification and river data - ship-to-ship, ship-to-shore and shore-to-ship. Installed on board AIS transponder emits static and dynamic data, data related to the trip. AIS base stations located on the banks of the river receive information in radio range and can automatically detect, identify and track vessels equipped with AIS transponder. There is opportunity to replay the history of movement of vessels in a given period of time.

The major function of the AIS subsystem is to receive information from all vessels within its range (the Bulgarian stretch of the Danube river). By means of microwave link and the network area the data from the vessel’s AIS stations are transmitted to the RIS Center, where they are recorded in databases. Specialized software enables the visualization of the database information on vector maps.

Architecture of AIS subsystem

BULRIS AIS subsystem represents AIS network covering eight base stations, based in Silistra, Tutrakan, Ruse, Svishtov, Somovit, Oryahovo, Lom and Florentin. The data collected from these base stations is stored in centralized database. The system has tools for configuration and diagnostics. The system complies with the standard of Inland AIS and Recommendation ITU-R 1371-3.
The presented AIS subsystem with similar functionality is in the exploitation on rivers in France, Croatia, Serbia and Slovakia.

Physical Architecture

Fig. 1 Physical Architecture of the AIS subsystem

The physical architecture of the AIS subsystem is part of the infrastructure of RIS.

Base Stations

Main components of the base stations are the transmitters of the base station and the base station controller. The base station controller (BSC) is implemented as a software package running on a standard PC. The software of the BSC is used for interfacing to the transmitter of the base station, controls the transmitter of the base station and the base station associated with AIS center. BSC can be connected with two transmitters at the same time, but only one transmitter is active and the other one is in standby mode. If the active transmitter fails, the backup one is activated immediately.
BSC handles all sent and received through the interface of the base station transmitter AIS messages. Only messages coming from the intended area of ​​the base station are processed further. Each controller adds to the AIS message the UTC offset, which is used throughout the AIS system. The MMSI number of the transmiting base station is also added message.  

After receiving the full AIS message, it is forwarded to AIS subsystem and stored on a local disk on BSC. In the absence of a link between the BSC and the AIS center, the data is stored only locally. After reconecting with AIS center, BSC begins synchronization of all unsent data. During the synchronization of the old data, CBS also sends current messages that have priority. Synchronized data remains stored on the local disk on BSC until the disk is full.

AIS base station controller is connected via TCP / IP connection to the central segment using a network router. Data security connection can be SSL encrypted, although it is recommended that all the AIS network to be in the VPN.

Radio coverage

Fig.2 Radio coverage of AIS base stations

AIS router

The AIS router is based on the AIS subsystem. It manages the connections between base stations, AIS database and the users. AIS router connects:
-Base Station Controller
-AIS web server
-AIS database
-Other AIS routers
AIS router uses TCP / IP connection, but any such connection must be authenticated with username and password. Therefore, the AIS router has interface for administration of user rights, which can be implemented as Active Directory or an SQL database. SSL encryption is also available. AIS router supports comprehensive protocol for communication with customers used to receive AIS data in many different formats and also to manipulate the router operations. 

Connections between the base stations and the router

This connection is established between the base station controller and the AIS router. They use a special command protocol for data exchange and control commands sent from the router to the base station controller.
BSC sends to the router AIS data, DGPS data and important archives. AIS router sends to BSC control commands, which include:
-DGPS data
-Instructions for sending AIS messages (eg safety, binary, interrogators)
-Control AtoN
-Control configuration of base stations and BSC
DGPS routing data
Source of the DGPS data is a reference station, which is connected to a base station controller. BSC uses this data to generate their own message, but also transmits DGPS data to the AIS router. The AIS router can be configured to relay the data to an unlimited number of other BSC. These data controller base stations use them to generate their own messages. This structure allows one source of DGPS data to be used in more than one base stations to generate DGPS correction AIS messages.

Real and historical mode
After connecting to the AIS router (regardless of whether the connection is direct or through the AIS web server) user by default starts working in Real mode. He can go in historical mode using appropriate commands or actions. While the user is in the historic mode, all AIS data (ship locations, ship static data safety messages, etc.) to which it has access can be replayed back in time up to a certain point.

Virtual buoys (AtoN)
The router allows the authorized users to create virtual buoys (Aids-to-Navigation - Helpers navigation). The buoys are only virtual, which means that they are not physically there, but by AIS messages sent by the base stations, buoys appear to the TTI of the vessels. The user can define virtual MMSI, name, location, size and other parameters for the virtual buoys. After the creation of the virtual buoys, the router instructs the BSC to activate sending AtoN AIS messages from the base station. Also after the deletion of a virtual buoy, BSC is instructed to stop sending AtoN messages.

Data export to database
The router allows users to get AIS data from the AIS database by using appropriate commands. If the user is connected directly to the router, the data will be in XML format. AIS users running web server can seek and receive data through the user interface.

AIS database
Through AIS router, all AIS data is finally stored in the AIS database. It keeps all AIS NMEA messages in the original format, along with a time interval, the time of entry into the base ship decoded MMSI, MMSI-and the base station, the type of message and a reference to decrypt data. There is a separate table for each AIS message, which holds the decrypted data. These tables have been extended with data from the vessel that were available at the time of actualization of the records in the database.

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