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The term ‘self-organized’ refers to the manner in which time slots are assigned to users. With the SOTDMA method, only the first free slot available for transmission has to be found. The next five slots are always automatically reserved. Per-minute, there are 2,250 slots available for the transmission of AIS data telegrams.
- They use both spare time slots and, to utilise the AIS slot map as efficiently as possible, use any available space in slots reserved by class A devices. Class B units contain the same amount of receivers and transmitters as Class A, but they are used slightly differently.
- There is an inherent issue within the AIS standard; the TDMA radio access scheme defined in the AIS standard creates 4,500 available time-slots in each minute but this can be easily overwhelmed by the large satellite reception footprints and the increasing numbers of AIS transceivers, resulting in message collisions, which the satellite.
The ship AIS equipmentworking principles:
After the ship is equipped with AIS equipment, the equipment needs to send out relevant information of the ship on the one hand, and also receive information of other ships within the effective range of VHF.
one hand ,To receive information, using the method of text representation, on the other hand can vividly expressed in radar map, all 'delta', said with a triangle symbol visually shows the relative position of the vessel, and movement direction, on the electronic chart, can use the vector line shows the speed of the vessel, when it is necessary to use wake line of navigation marks, and differential GPS position data from GPS, the precision is high.If a target is selected on the ship AIS equipment or a click of the mouse is used from the ship sign in the electronic chart, the corresponding ship name, call sign, MMSI registration number and important navigation information such as course, speed, CPA and TCPA can be displayed instantaneously. After knowing these information, the driver can easily judge the surroundings.
The movement of other ships to ensure the safety of navigation, while communicating with each other can call their names, information exchange is very convenient.
AIS works in the VHF navigation frequency band. Two VHF frequencies, 161.975mhz (87B channel) and 162.025mhz (88B channel), were designated as AIS working channels by the international telecommunication union radio conference in 1997.In terms of complete communication, one radio channel is sufficient, but in order to prevent interference and communication loss caused by channel conversion, each AIS station USES two channels for sending and receiving.
Except for manual intervention, AIS transponders all work in autonomous continuous mode, and the transmitting mode is 9.6Kb GMSK FM bandwidth of 25KHz Or 12.5KHz data using HDL protocol.According to the actual conditions of ship-to-ship communication, AIS USES the core technology of self-organizing time-division multiple access (SOTDMA).According to the requirements of the IMO AIS performance standards for the capacity of the ship reporting requirements, the system should have 2000 slots per minute, but in fact, the design of the system is 4500 slots per minute, each frame 60 seconds, namely every 60 seconds to build 2250 slots, each time slot is about 26.67 ms, can transmit 256 bits of information, each AIS station report automatically according to the capacity of information select one to three time slot, one frame and frame transmitting or receiving ship AIS equipment information.
System real-time dynamically adjust the channel allocation in the specific work, in an AIS station began to use of the channel state before you send to observe a period of time, find out the usage of time slot, then you can choose not to take up the time slot, indicate the need to take up the number of frames, and then send the data, the AIS station continuously, can avoid sending time overlap, new AIS station will not conflict.When the data link load exceeds 90% of the theoretical value, the newly added station can occupy the time gap of the farthest station, thus ensuring the system has a great overload capacity.
Marine AIS AtoN Station
The self-organizing time-sharing multiple access technology can automatically solve the competition problem between this station and other stations, even if the system is overloaded and the communication is still intact;The system can process more than 2000 reports per minute, and the data received by the ship can be updated every 2 seconds.AIS is downward compatible with DSC, so the shore-based GMDSS system can identify, track and control ships equipped with AIS equipment .
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