The AIS format uses TDMA radio access that allows for just 4,500 time slots per minute. One time slot equates to a single vessel transmission. Any more than that and the individual signals start to interfere with each other, corrupting the data held within. The terrestrial AIS.
- Using the UTC time signal transmitted by GPS as a reference a frame of one minute length is divided in 2225 single time slots. The transmission of one AIS data telegram should take normally one time slot. The maximum number of slots to be occupied for longer telegrams is five. For initialisation of its VHF transmissions the AIS transponder.
- 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.
- All AIS entities observe the VHF medium trying to find free time slots to transmit data. For that reason a started AIS device will wait and listen for a specific time to identify other AIS entities and to find time slots for transmitting its data. The amount of time slots per minute is restricted to 4500 slots.
It's not entirely clear who it was that came up with the first AIS vessel tracking and identification system, but like so much other technology that we take for granted these days, it came out of the introduction of GPS for civilian purposes, which achieved global coverage in the early 1990s. It was then the integration of GPS time and position data with long-standing VHF radio technology that enabled the development of AIS. We'll delve into a bit more into the history of AIS in this article.
The early years
In its early years its primary use was as a ship-to-ship anti-collision system for use in poor visibility and at night, in support of radar and conventional watch keeping. Over time the amount of information that could be transmitted in the VHF signal grew and its usefulness increased. In 2002 it finally went global when the IMO in its landmark SOLAS convention mandated that all passenger ships and other commercial vessels over 300 GT should carry Class-A AIS transceivers. At the time this affected around 100,000 ships, but since then use has expanded as the unit cost of transceivers has fallen and both compulsory and voluntary adoption has increased.
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Originally developed as a short range identification and vessel tracking system, at the start of the 21st century it was discovered that AIS transmissions could be received at ranges of up to 400km above the surface of the earth, whereas on the surface the maximum effective distance is around one tenth of that. This revolutionised AIS, taking it from a coastal and ship-to-ship tracking application to a vessel management system with global coverage. Cobra hearts slot machine. However, the challenge for satellite operators looking to develop this opportunity now is managing the enormous volumes of data that this creates for individual satellites each monitoring thousands of square kilometres of ocean.
Future-proofing
The AIS format uses TDMA radio access that allows for just 4,500 time slots per minute. One time slot equates to a single vessel transmission. Any more than that and the individual signals start to interfere with each other, corrupting the data held within. The terrestrial AIS infrastructure with its short range and higher density does not have the same capacity problems.
The satellite developers are, however, working on ways of receiving and processing incoming data at faster rates and rapid advances are being made.
An important tool in the box
There is no perfect vessel tracking system, but AIS is becoming increasingly effective as accuracy and refresh rates get ever better. Liberty casino midleton opening hours today. Its ability to interface with other detection sources makes it an important component of integrated navigation and warning systems, and the addition of supplementary environmental and situational data makes it yet more versatile. Without a doubt AIS is now one of the most valuable information sources available for anyone involved in the maritime sector.
About BigOceanData
BigOceanData is a leader in maritime tracking and telemetry, delivering global vessel tracking and monitoring services along with fleet management tools via its sophisticated browser-based interface. Key features of the BigOceanData product include its ability to fuse data from AIS signals and a range of onboard satellite reporting systems so as to both improve position accuracy and reduce data costs. The system also integrates a series of data feeds and management tools that show users not only vessel locations and movements, but situational data such as marine charting, terrestrial mapping, weather and sea-state (current and forecast), and piracy and other security alerts.
For more information contact sales@bigoceandata.com or call +44 (0) 207 998 3048.
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.
What is broadcast by AIS
For more information contact sales@bigoceandata.com or call +44 (0) 207 998 3048.
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.
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
