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Recommendation ITU-R M.1371-5(02/2014) - Technical characteristics for an automatic identification system using time division multiple access in the VHF maritime mobile frequency band. Within territorial waters enforcement of AIS standards, the accuracy of transmissions, and any special local requirements for AIS fitment and use is the. 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. This timing is determined by the reference time UTC (obtained in the MAC), common to all AIS receiving stations integrated into a region of coverage. Within the AIS standard, each slot has a duration of 26.67ms. If each frame has a time duration of AIS of 60s, the total number of slots in a frame AIS is N ¼ 60 Δt slot ¼ 60 slots (1).

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.

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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.

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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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Universal Shipborne Automatic Identification Syste

What is an AIS?
Picture a shipboard radar display, with overlaid electronic chart data, that includes a mark for every significant ship within radio range, each as desired with a velocity vector (indicating speed and heading). Each ship 'mark' could reflect the actual size of the ship, with position to GPS or differential GPS accuracy. By 'clicking' on a ship mark, you could learn the ship name, course and speed, classification, call sign, registration number, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available. Display information previously available only to modern vessel traffic service operations centers could now be available to every AIS-equipped ship.
With this information, you could call any ship over VHF radiotelephone by name, rather than by 'ship off my port bow' or some other imprecise means. Or you could dial it up directly using GMDSS equipment. Or you could send to the ship, or receive from it, short safety-related email messages.
The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation.

How does it work?

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a standard marine electronic communications link (IEC 61162/NMEA 0183) to shipboard display and sensor systems. Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters. Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections. Heading information and course and speed over ground would normally be provided by all AIS-equipped ships. Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided.
The AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. Transmissions use 9.6 kb GMSK FM modulation over 25 or 12.5 kHz channels using HDLC packet protocols. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.
Each station determines its own transmission schedule (slot), based upon data link traffic history and knowledge of future actions by other stations. A position report from one AIS station fits into one of 2250 time slots established every 60 seconds. AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions. Slot selection by an AIS station is randomized within a defined interval, and tagged with a random timeout of between 0 and 8 frames. When a station changes its slot assignment, it pre-announces both the new location and the timeout for that location. In this way new stations, including those stations which suddenly come within radio range close to other vessels, will always be received by those vessels.
The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.
The system coverage range is similar to other VHF applications, essentially depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends and behind islands if the land masses are not too high. A typical value to be expected at sea is nominally 20 nautical miles. With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.
The system is backwards compatible with digital selective calling systems, allowing shore-based GMDSS systems to inexpensively establish AIS operating channels and identify and track AIS-equipped vessels, and is intended to fully replace existing DSC-based transponder systems.

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What is broadcast by AIS

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A Class A AIS unit broadcasts the following information every 2 to 10 seconds while underway, and every 3 minutes while at anchor at a power level of 12.5 watts. The information broadcast includes:
* MMSI number - unique referenceable identification
* Navigation status - not only are 'at anchor' and 'under way using engine' currently defined, but 'not under command' is also currently defined.
* Rate of turn - right or left, 0 to 720 degrees per minute
* Speed over ground - 1/10 knot resolution from 0 to 102 knots.
* Position accuracy - differential GPS or other and an indication if RAIM processing is being used
* Longitude - to 1/10000 minute and Latitude - to 1/10000 minute
* Course over ground - relative to true north to 1/10th degree
* True Heading - 0 to 359 degrees derived from gyro input
* Time stamp - The universal time to nearest second that this information was generated
In addition, the Class A AIS unit broadcasts the following information every 6 minutes:
* MMSI number - same unique identification used above, links the data above to described vessel
* IMO number - unique referenceable identification (related to ship's construction)
* Radio call sign - international call sign assigned to vessel, often used on voice radio
* Name - Name of ship, 20 characters are provided
* Type of ship/cargo - there is a table of possibilities that are available
* Dimensions of ship - to nearest meter
* Location on ship where reference point for position reports is located
* Type of position fixing device - various options from differential GPS to undefined
* Draught of ship - 1/10 meter to 25.5 meters [note 'air-draught' is not provided]
* Destination - 20 characters are provided
* Estimated time of Arrival at destination - month, day, hour, and minute in UTC