Maritec AIS_Overview

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Automatic Identification System (AIS) – A basic overview

What is AIS and how does it work?

AIS is an initial requirement set by IMO (International Maritime Organisation), developed by the relevant authorities and industry. IALA (International Association of Lighthouse and Marine Aids to Navigation Authorities) played an instrumental roll in the development of AIS. The primary requirement of Automatic Identification System (AIS) is to provide accurate identification and navigational data, assisting safe navigation.

Recommendation ITU-R M.1371 defines the AIS on-air (VHF Data-link) behaviour. There are various types of AIS which is addressed in more detail below. Each AIS unit consists of at least one VHF transmitter, two VHF TDMA receivers (or a combination thereof), a processor, GNSS and a communication interface. Timing information is obtained from an integral GNSS receiver. Positional information is obtained from an integral or external GNSS receiver (Global Navigation Satellite System e.g. GPS). DGNSS can be used (Differential GNSS) in order to enhance the accuracy of positional information.

In terms of the IMO SOLAS Chapter V (Safety of Lives at Seas), all vessels greater than 300gt are mandated to carry AIS as from 31 December 2004. Some of the respective marine authorities have implemented local AIS requirements. AIS is used in inland waters and waterways.

AIS works autonomous, automatic, continuous and operates primarily in broadcast mode, but also in an assigned and in an interrogation mode using time division multiple access (TDMA) schemes. The access schemes include SOTDMA (self organized), RATDMA (random access) and FATDMA (fixed access). Transmissions use 9.6 kb GMSK FM modulation over 25 or 12.5 kHz channels, using a HDLC type packet protocol. Two VHF radio channels are used concurrently. Two frequencies have been allocated internationally for the use of AIS namely AIS1 (161.975MHz) and AIS2 (162.205MHz). Other frequencies in the maritime band can be used, subject to local authority requirements.

Each AIS channel (frequency) is divided in to 2250 time periods (slots) per 60 seconds (frame), thereby providing 4500 slots per minute. Each slot is equal to 26.6’ milliseconds. Twenty six (26) AIS message types have been defined to date, ranging from single slot to five slot messages. Messages types include navigational data (1 or 2 slot/s), ship/AtoN static and/or voyage related data (1 or two slot/s) and safety messages (1 to 5 slots).

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 (DSC) 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.

AIS Types

All AIS types provide identification, positional and time stamp data. Performance test standards have been developed and published or are in the process of being developed for the respective AIS types.

Class A

Class A AIS is a mandatory carriage requirement for all for vessels greater than 300gt in terms of IMO SOLAS convention. Navigational message are broadcasted every 2 to 10 seconds, depending on the vessels speed and rate of turn. At anchor requires an update rate of every 3 minutes.

Navigational data includes the following:

MMSI number - unique identification
Navigation status (as defined by the COLREGS - 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 (input from rate-of-turn indicator)
Speed over ground - 1/10 knot resolution from 0 to 102 knots
Position accuracy - differential GPS or other and an indication if (Receiver Autonomous Integrity Monitoring) 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 ship static and voyage data every 6 minutes:

MMSI number - same unique identification used above, links the data above to described vessel
IMO number - unique 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

The Class A AIS transmitter output power levels can be set to 12.5 watt (default) or 2 watt when commanded by an AIS base station in a specific operational area. Default mode of operation is autonomous and continuous using the SOTDMA access scheme. The Class A interfaces with various onboard sensors such as Gyro’s (True Heading), etc. The standard onboard marine NMEA interface/s is used. A Class A may interrogate or be interrogated by other shipborne AIS Class A and base station units.

Class B/CS (Carrier Sense)

Class B/CS shipborne mobile equipment is generally used by vessels not subject to the IMO mandatory carriage requirement. This includes leisure craft, etc. The Class B/CS functionality is similar to Class A except the Class B:

Has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to every 10 sec. for Class A)
Does not transmit the vessel’s IMO number or call sign
Does not transmit ETA or destination
Does not transmit navigational status
Is only required to receive, not transmit, text safety messages
Is only required to receive, not transmit, application identifiers (binary messages)
Does not transmit rate of turn information
Does not transmit maximum present static draught

The Class B/CS AIS transmitter output power level is set to 2 watt (default). Default mode of operation is autonomous and continuous using the Carrier Sense (CS) access scheme. The Class B/CS is not required to have any interfaces. The Class B/CS may not impair the Class A operation, a function provided by the CS access scheme. A Class B/CS may not interrogate other shipborne AIS units.

Aids to Navigation (AtoN)

There are three AIS AtoN categories:

Physical - actual AtoN AIS installed on an AtoN (buoy, lighthouse, etc.)
Virtual - displayed as a electronic formatted symbol on an Electronic Chart System (not a physical device)
Synthetic - information received from other physical non-AIS AtoN devices, ported to and broadcasted by and AIS station

There are three types of Physical AtoN AIS stations:

Type 1 - no receiver (transmit only)
Type 2 - receiver for control functions only
Type 3 - receiving processes for autonomous mode

Physical AIS AtoN units can be installed on fixed or floating aids and off-shore structures. The general mode of operation is automatic, continuos using FATDMA access scheme. A pre-defined transmit schedule is configured for this purpose. RATDMA is also used when appropriate. Reporting rates can vary from installation to installation however, every three to ten minutes should be sufficient. Identification and positional data is broadcasted. ON- / OFF-POSITION indicator and type of navigation aid is included. The AIS AtoN may eventually replace the RACON.

Provision for telemetry monitoring and control may optionally be provided. Meteorological and Hydrological data can be broadcasted, if interfaced to the appropriate sensors.

AIS Base Station

AIS shore based stations provide identification, positional and time stamp data, normally reporting every ten seconds. Additionally base stations provide VDL Management (VHF Data-link), Channel Management and Assign Mode Command functions. An AIS base station may interrogate any shipborne AIS, when required. The AIS base station may transmit and process all received shipborne AIS safety messages.

AIS base station transmitter output power levels can be set to 12.5 watt (default) or 2 watt in a specific operational area, as implemented by the competent authority. Default mode of operation is autonomous and continuous using the FATDMA access scheme. RATDMA may be used when appropriate.

Repeater Station

AIS repeater stations are used in order to extend the required coverage AIS area.

Search and Rescue Aircraft

Aircraft mobile equipment, normally reporting every ten seconds.

Stakeholders and Beneficiaries

AIS data can be provided to and applied by all relevant stakeholders and beneficiaries, which include however are not limited to:

Onboard navigators, masters and captains
Vessel Traffic Services
Port operations, including terminal operators, tugboats, pilots, service crews, etc.
Logistic operators, including warehousing and transport
Customs and Duties
Coastal surveillance / Homeland security
Ship owners, ship management and agents
Lighthouse and marine aids to navigation authorities
Marine and environmental management
Search and rescue
Coast guards
Police services
Defence and Navy
Marine recreational clubs
Many others