AIS (Automatic Identification System)

AIS, which stands for Automatic Identification System, has been mandated by IMO to enhance the safety of life at sea, the safety of navigation and the protection of the marine environment. Its correct title is Universal Automatic Identification System (UAIS) but the ‘U’ is rarely used. The purpose of AIS is to help identify vessels, assist in target tracking, assist in situational awareness and to simplify safety related information exchange between vessels and between vessels and the shore.

AIS has an important role in improving the capabilities of Vessel Traffic Services (VTS) and can also be used to enhance environmental protection monitoring and coastal state security. In fact, a major oil spillage arising from the grounding of the tanker Braer on the Shetland Islands in the UK was an important instigator in driving the IMO legislation.

A. AIS Communications 

At the basic level, AIS is a Very High Frequency (VHF) radio system that continuously exchanges navigational information between ‘stations’. Stations include ships (and, in the future, small craft), shore stations and (again, in the future) aids-to-navigation (AToNs), such as buoys and other navigational marks.

The navigational information exchanged includes ‘static’ parameters such as ship’s name, type and length, as well as dynamic data including position, heading, and speed over ground (SOG). It also includes voyage-related data such as destination, estimated time of arrival (ETA) and draught (draft).
In order to increase the number of ships that can use AIS at any one time, the system operates on two separate channels in the VHF band. To enable this to happen, ships’ AIS equipment embodies two independent VHF receivers and a single transmitter, which alternates its transmissions back and forth between the two frequencies. With these two frequencies the ship station can cope with up to 4,500 reports per minute from other stations.
The AIS operating frequencies, referred to as AIS1 and AIS2, are assigned by shore stations. Many countries have adopted Channels 87B and 88B for AIS (operating at 161.975 and 162.025 MHz) but this is not universal. In the overlap between regions that use different frequencies, shore station commands will instruct the ships’ equipment to change frequency. This is an automatic function that needs no operator intervention.

Frequency change can also be instructed through the use of Digital Selective Calling (DSC) on Channel 70. This means that a further receive channel is included in ships’ AIS equipment in order to receive DSC messages, making a total of three receivers, (AIS1, AIS2 and DSC).

In any one area all stations are using the same AIS1 and AIS2 dedicated channels, therefore there has to be a sophisticated system for ensuring that only one station is transmitting on a particular frequency at any one time. This is arranged by dividing time into precisely defined slots. There are 2,250 of these time slots in each minute, with each minute being aligned exactly to Coordinated Universal Time (UTC). All AIS units have a built-in position-fixing receiver, such as GPS (Global Positioning System), which is primarily used to determine UTC accurately. This ensures that the transmissions from all units are time-aligned correctly.

When making its first transmission, the AIS receiver ‘listens’ to establish what time slots are free. It then uses predefined algorithms to decide when it subsequently transmits. Transmissions are equally spaced and alternate between the two AIS frequency channels, AIS1 and AIS2.

Using a radio transmission system that is based on using defined time slots is known as Time Division Multiple Access (TDMA). The self-organising (SO) AIS use of TDMA is therefore referred to as SOTDMA. 

B. AIS Modes 

The ‘normal’ mode of operation is known as Autonomous and Continuous. This is the mode that has been described in the section above. There is also a mode of operation known as Assigned in which the ‘competent authority’ (in the form of coastal stations) can individually assign ships their data reporting transmission intervals and time slots.
This mode automatically times-out after a period (4-8 minutes), when the ship reverts to Autonomous and Continuous mode. There is no user action required.

A further mode is known as Polled. This allows coastal stations to request specific data from ship stations or obtain data updates from the vessel in a shorter time interval than that occurring in Autonomous mode transmissions. Again, no user action is required for the AIS to respond to this mode.
Because AIS equipment can automatically transmit information in response to requests from other stations they are often called Transponders.

C. AIS Data
 

The information reported by the ship station is divided into four groups: static, dynamic, voyage related and short safety-related messages.

Static information

o IMO number (where available)
o Name
o Call sign
o Maritime Mobile Services Identity number (MMSI) 
o Type of vessel
o Length and beam
o Height over keel
o Location of the position-fixing antenna (referred to bow and centreline)
The location of the position-fixing antennas (eg GPS) is needed to ensure accuracy in close-quarters situations, when the system is perhaps being used to aid collision avoidance. The position of the internal ‘position-fixing’ antenna (which is normally just used for the generation of UTC time) also allows the internal system to be used to obtain position, in the event of a malfunction in the ship’s primary position-fix system.

Dynamic information

o Ship’s position, (with accuracy and integrity, if available) 
o Time in UTC
o Course over ground (COG)
o Speed over ground (SOG)
o Heading
o Rate of turn (where available)
o Navigational status, eg at anchor, not under command, etc
It is important that the AIS has been configured by the installer to transmit the navigational data that is identical to that being used to navigate the vessel.

Voyage related information

o Ship’s draught
o (Hazardous) cargo type 
o Destination and ETA
o Number of passengers
o Route plan (optional)

Short Safety related messages

Limited to about 160 characters 


D. DATA Checks

Static data checks

The static data consists of the following:

o IMO number (where available)
o Ship’s name
o Call sign
o Maritime Mobile Services Identity (MMSI) number
o Type of vessel
o Length and beam
o Location of the position-fixing antennas (external and internal)
This data is available for view on the MKD by menu selection. The data should be compared with a master printed list kept close to the MKD. If a discrepancy is found the vessel’s Master must be immediately informed, who may authorise correction to this data. This data must only be changed on specific authorisation by the Master and for this reason is password protected. 

Voyage related data checks

The voyage related data consists of: 
o Ship’s draught
o Hazardous cargo type, if required by competent authority
o Destination and ETA (at Master’s discretion)
o Total number of persons on-board (obligatory for some port administrations)

This data is available from the menu structure of the AIS.
It is essential that the Master gives clear rules concerning entering both the hazardous cargo type and the requirement for destination and ETA and that these are followed. Disobeying these rules could compromise the security of the vessel and contravene port and flag state regulations. 

Dynamic data checks

From time-to-time during the watch the OOW should briefly select the screen that shows the data that is actually being transmitted. By inspection, the OOW should ensure that the dynamic data is replicating the readings given on the ship’s navigation sensor displays. The frequency of checks should be increased in coastal waters.
Heading offsets are a particularly common problem and are often caused by the offset being incorrectly set at the heading sensor. This can happen on older equipment when a digital converter has had to be retro-fitted in order to provide data to the AIS. On these systems, since the user normally uses only the ‘analogue’ display, the digital offset is not always immediately obvious.
Unfortunately, on some AIS equipment, viewing of own ship’s transmitted data is under password protection in an ‘Engineering Mode’. The password for entering this area also allows static data to be altered. On ships with such equipment it is often recommended that the Master, or the Master’s appointee, checks this data on a once-daily basis. 

The only dynamic data that needs to be manually input is the Navigation Status of the vessel. This is normally accessed by a dropdown list in the AIS menu structure and will have terms such as:
o Underway by engines
o At anchor
o Not under command (NUC)
o Restricted in ability to manoeuvre (RIATM) 
o Moored
o Constrained by draught
o Aground
o Engaged in fishing
o Underway by sail, etc
Changes in navigational status should take place concurrently with the necessary changes in lights and shapes.