Normally, vessels with an AIS receiver connected to an external antenna placed on 15 meters above sea level, will receive AIS information within a range of 15-20 nautical miles. Shore side base stations on higher ground may extend the range up to 40-60 nm, even behind remote mountains, depending on elevation, antenna type, obstacles around antenna and weather conditions. The most important factor for better reception is the elevation of the antenna. The higher, the better.

The AIS unit receives data, which is processed by simple software and displayed on a chart display. Data received by the AIS unit are encoded in NMEA sentences. A sample is shown below:
 

!AIVDM,1,1,,B,1INS<8@P001cnWFEdSmh00bT0000,0*38

Messages include the following three basic types:
1. Dynamic Information, such as vessel’s position, speed, current status, course and rate of turn.
2. Static Information, such as vessel’ name, IMO number, MMSI number, dimensions.
3. Voyage-specific Information, such as destination, ETA and draught.

AIS is a useful source of additional information available to the OOW. Information received via AIS supplements and improves radar information and that derived from other navigational systems. AIS is therefore a valuable aid to assist in collision avoidance.

There always will be ships without AIS onboard. However, radar detects targets independent of the target’s onboard equipment. Therefore, AIS cannot replace radar, which is, in many ways, a ‘complete’ system.

In addition to radar, AIS has the following benefits:

• Automatic vessel identification;
• Automatic provision of heading, course over ground (COG) and speed over ground (SOG), as derived from external sensors;
• Automatic provision of CPA (Closest Point of Approach) and Time to Closest Point of Approach (TCPA);
• Improved vessel tracking (no target swap);
• Generally greater range, although in some circumstances e.g. mountainous areas, this may only be achieved with the provision of shore-based repeater stations;
• AIS can also be used as an aid to navigation and possibly replace racons;
• greater positional accuracy, dependent on the position input sensor;
• Provides information in radar shadow areas (‘sees’ around bends and behind islands);
• Near real time manoeuvring data;
• No loss of targets in sea, rain and snow clutter;
• In addition to radar, AIS has the following benefits:
• Automatic vessel identification;
• Automatic provision of heading, course over ground (COG) and speed over ground (SOG), as derived from external sensors;
• Automatic provision of CPA (Closest Point of Approach) and Time to Closest Point of Approach (TCPA);
• Improved vessel tracking (no target swap);
• Generally greater range, although in some circumstances e.g. mountainous areas, this may only be achieved with the provision of shore-based repeater stations;
• AIS can also be used as an aid to navigation and possibly replace racons; greater positional accuracy, dependent on the position input sensor;
• Provides information in radar shadow areas (‘sees’ around bends and behind islands);
• Near real time manoeuvring data;
• No loss of targets in sea, rain and snow clutter;

• Data received is only as good as the data entered into the AIS. To ensure that correct AIS information is broadcast to other vessels and shore authorities, mariners are reminded to enter current voyage related data such as draught, type of hazardous cargo, destination and ETA properly at the beginning of each voyage and whenever changes occur.
• Not all ships carry AIS. The OOW should always be aware that other ships, in particular pleasure craft and fishing vessels, may not be fitted with AIS. Also not all vessels accurately report all AIS information.
• The AIS unit may not, in all cases, be installed in accordance with the IMO Guidelines. This can result in poor performance and erroneous transmissions.
• The officer on watch should always be aware that AIS fitted on other ships as a mandatory carriage requirement, may, under certain circumstances, be switched off, particularly where international agreements, rules or standards provide for the protection of navigational information.
• AIS is subject to the vagaries and limitations of VHF-FM propagation.
• Mariners should be aware the accuracy of AIS positional information is the accuracy of the EPFD connected. For example, LORAN C can be used, but will typically have a far lower accuracy than GNSS.
• Mariners are reminded to periodically check that correct information is being broadcast by their own vessel, particularly position, heading (provided by the ships master gyro) and speed.
• The mariner must always remember that AIS is just one of the several tools available to watch-keepers, to fulfill their obligations under the Collision Regulations.

The master should authenticate the information received by his AIS unit bycross-checking with his radar and/or by other means.

Rudder angle information is not a required AIS sensor input and may not always be representative of the actual movement of the vessel. Transmitting rudder angle information may mislead the recipient and can therefore be dangerous. On the other hand, Rate of Turn (ROT) information is included in AIS, as this is a SOLAS carriage requirement on board vessels of over 50,000 gross tonnage.

The display panel with the unit is often the only means of showing AIS received data. Together with a keypad, this basic configuration is known as a Minimum Keyboard and Display (MKD). The display part of a MKD, as a minimum, consists of three lines of data, each showing bearing, range and name of the target. In practice, most MKDs display more lines of data and may also have a simple graphical display, showing the relative location of targets, rather like the Plan Position Indicator of a radar. To achieve the full benefits of AIS, information ought to be display graphically on a radar, ECDIS or on its own dedicated display. Recognizing this, IMO has mandated that from 1 July 2008 onwards, all new radar installations must be able to display AIS targets. The ability to display AIS information on radar or ECDIS depends entirely if the radar/ECDIS has been designed or modified for this purpose. If so, the connection can easily be made by a qualified installer.Equipment manufacturers are the best source of information and available options in this regard.

The accuracy of navigational information such as position, course, speed etc. output by AIS, depends on the accuracy and proper operation of the sensors used. There is an indication of the positional accuracy transmitted by AIS (‘low’ or ‘high’), depending on whether GNSS or DGNSS is used. Further, the accuracy of data, such as voyage related data, depends on the accuracy with which this is entered and the frequency of its update. Masters must always bear in mind that a third party manually enters some the information they receive.

AIS should not be used as an anti- collision device in isolation. It should be used in conjunction with all means available to assist the mariner in assessing the risk of collision. It is important to note that there will always be other vessels that do not have AIS. IMO recognises the potential of AIS as an anti-collision device and may recommend AIS as such a device in due course. In summary, AIS is a valuable navigational aid, one of several on the bridge of a ship. It can assist in the early appraisal and subsequent resolution of a close quarters situation, or of a risk of collision. Initially, detection by AIS alone should be considered in thesame way as detection by radar alone, with particular caution being exercised until the AIS information has been verified by other means.

Yes. All AIS messages have a predefined structure and length, as specified in ITU-R Recommendation M.1371-1. Each AIS message occupies at least one slot of the VHF data link. This can extend to a maximum of 5 slots (as in the of short safety related text messages; which translates to approximately 158 characters of text).

Ideally, yes. All sensor information should be integrated on a single display. This will enhance situational awareness. The operator should have the ability to customize the display to suit the task at hand.

Yes. ITU-R Recommendation M.1371-1 limits transmission on AIS designated channels to maritime safety related messages. In addition, IMO has permitted the exchange of seven other ancillary messages, for a test period of four years. Examples of such messages are number of persons on board, meteorological and hydrological information, indication of dangerous cargo, status of fairway etc.

AIS should always be in operation when ships are underway or at anchor. If the master believes that the continual operation of AIS might compromise the safety or security of the ship or where security incidents are imminent, the AIS may be switched off. In port, AIS should be operated in accordance with port requirements. Oil terminals in particular, may have special requirements. Actions of this nature should always be recorded in the ship’s logbook, together with the reason for doing so. The master should restart the AIS as soon as the source of danger has disappeared. Ship’s own data will be transmitted after a two minute initialisation period.

No. Commercial messages should not be transmitted on the designated AIS frequencies – channels 87B and 88B (AIS 1 and AIS 2). Further, AIS is not the ideal tool for routine commercial correspondence. It is best to use other available means for this purpose.

Although it is possible to generate and transmit false or misleading messages (‘spoofing’), mariners are cautioned that there may be severe penalties imposed by national administrations for these actions, in accordance with national legislation.

Yes, if the administration decides to do so. The message structure and technical approach have been defined in the IALA AIS Guidelines. It is important to note that such corrections will only correct the internal GNSS. Any external GNSS equipment interfaced to the AIS will not be corrected by such messages.

A message structure has been adopted by IMO that allows the transmission of meteorological and hydrological information. At this time, broadcast and display of this information is a matter for manufacturers and authorities.

Shipboard AIS units autonomously broadcast two different AIS messages: a 'position report' which includes the vessels dynamic data (e.g. latitude, longitude, position accuracy, time, course, speed, navigation status); and, a 'static and voyage related report' which includes data particular to the vessel (e.g. name, dimensions, type) and regarding its voyage (e.g. static draft, destination, and ETA). Position reports are broadcasted very frequently (between 2-10 seconds-depending on the vessels speed-or every 3 minutes if at anchor), while static and voyage related reports are sent every six minutes; thus it is common and likely that an AIS user will receive numerous position reports from a vessel prior to receipt of the vessels' name and type, etc.

AIS users are required to operate their unit with a valid MMSI, unfortunately, some users neglect to do so (for example, use: 111111111, 123456789, 00000001, their U.S. documentation number, etc). A valid MMSI will start with a digit from 2 to 7. AIS users whom encounter a vessel using MMSI: 1193046 or named: NAUT should notify the user that their AIS unit is broadcasting improper data; see Nauticast AIS-MMSI Technical Bulletin for further information. All AIS users should check the accuracy of their AIS data prior to each voyage, and, particularly units that have been shutdown for any period of time.

A Class A AIS transponder broadcasts the following information every 2 to 10 seconds while underway, and every 3 minutes while at anchor:

• MMSI number - unique referenceable identification
• Navigation status - “at anchor”, “under way using engine” or “not under command”
• 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

AIS transponders and receivers use two VHF radio frequencies: 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B). 

Yes, but, be aware that AIS safety related text messages are not -currently- received, processed, recognized or acted upon as Global Maritime Distress Safety Systems (GMDSS) messages would be by some coast guards or other maritime first responders. Therefore, AIS should not be relied upon as the primary means for broadcasting distress or urgent communications, nor used in lieu of GMDSS such as Digital Selective Calling radios which are designed to process distress messaging. Nonetheless, AIS remains an effective means to augment GMDSS and provides the added benefit of being 'seen' (on radar or chart displays), in addition to being 'heard' (via text messaging) by other AIS users within VHF radio range.

Practically all AIS units have an in-built GNSS receiver (such as GPS or GLONASS). This is required primarily to provide accurate UTC. The GNSS antenna and receiver are part of the ship’s AIS equipment, separate from any other GNSS equipment onboard. AIS can use positional and other data from the ship’s GNSS or other radio-navigation system that is being used to navigate the ship. However, if positional information is lost from the ship’s GNSS or other equipment, the AIS will transmit position and other data from the internal GNSS. Regardless, any GNSS connected to the AIS should comply with IMO Resolution MSC.112 (73) (Revised performance standards for  ship-borne Global Positioning System (GPS) receiver equipment) and IEC Test Standard 61108-1.

Vitally, GPS units that comply with the older IMO performance standard should not be interfaced to the AIS. This is because differences between the two standards are considerable. Under the new standard, there is a requirement for integrity monitoring (RAIM), satisfactory operation in typical interference conditions, higher accuracies for position, Course Over Ground (COG) and Speed Over Ground (SOG) output, higher update display rate (every second, as opposed to every two seconds). The most significant of these are the RAIM requirement and interference rejection standards.

AIS requires the input of the ships gyro or transmitting heading device (THD), which meets the relevant IMO and IEC standards. Without heading information, the presentation of the ship shape will not be generated. However, in this case, the isosceles triangle representing the ships position will be aligned with the COG (without the heading marker) on the display of receiving stations.

All AIS receivers essentially have the same four connections. One connection is for a standard marine VHF antenna. This is usually via a BNC connector. The second connection is NMEA position data input from either GPS/Chartplotter or your PC. The Third is AIS Data output which is connected to your chartpotter/PC. The forth connection is for 12 volt DC power.

Once these connections have been made, simply configure your chartplotter or PC software to utilize the output data stream. Note that AIS receivers use 38400 baud by default so make sure you configure your PC serial port and program appropriately.

Yes, you can use an existing VHF antenna on its own or you can use an antenna splitter to share one VHF antenna between your VHF/DSC radio and your AIS receiver. While transmitting on the VHF radio, you may see some interruption of incoming AIS signals. Since AIS broadcasts from each ship are repeated every few seconds, this is not normally noticeable. Alternatively, use a dedicated AIS antenna.

AIS provides:

• A built-in integrity test (BIIT) running continuously or at appropriate intervals;
• Monitoring of the availability of the data;
• An error detection mechanism of the transmitted data; and,
• Error checking of the received data.

If no sensor is installed or if the sensor (e.g. the gyro) fails to provide data, the AIS automatically transmits the "not available" data value. However, the integrity check cannot validate the accuracy of the data received by the AIS. The AIS requires that an alarm output (relay) be connected to an audible alarm device or the ship’s alarm system, if available. Alternatively, the BIIT alarm system may use the alarm messages’ output on the Presentation Interface (PI), provided its alarm system is AIS compatible.

No – a correctly installed AIS unit should not need any further calibration to continue operating. The Built-In Integrity Test component of each AIS will activate an alarm should the unit fail to operate in accordance with specified parameters.

Yes. Static data (vessel name, call sign, MMSI, IMO number etc) must be entered upon installation in accordance with the ship’s registration documents. Voyage-related data (draught, destination, ETA, navigation status etc) must be entered at the commencement of each voyage or if there are any changes. It is recommended that both static and voyage-related data be checked and updated at appropriate intervals, as required. The failure to correct the vessel’s navigational status when it changes (a vessel comes alongside from being underway, for example) is commonly overlooked.

All AIS manufacturers provide operating manuals may well offer initial training on their equipment. It is recommended that ship owners and managers take advantage of these sources and incorporate AIS training in their ISM procedures. All About AIS offer web based training on AIS, visit our events section for more information.

No. Chapter V of the SOLAS Convention does not apply to them. However, national administrations may require the installation of AIS on board naval vessels. National administrations may dictate the use of AIS on board naval vessels. Many navies have outfitted their vessels with AIS. However, they may not always be transmitting AIS information (e.g. operating in a ‘receive only’ mode).

Maybe. Most AIS do not need additional equipment (sensors) in order to operate; a few however, do require interfacing with an external global navigation positioning device (e.g. dGPS, GPS, GLONASS) in order to accurately calculate and broadcast position, course, and speed--thus requiring this equipment to properly operate. Although not required for the operation of AIS, Chapter V, Regulation 19 of the Safety of Life at Sea Convention (SOLAS), as stated in 33 CFR § 164.46(a)(2), does require certain vessels--those on international voyage--to also interface other onboard equipment (i.e. transmitting heading device, gyro, rate of turn indicator) to the AIS; domestic vessels, not on international voyage, are not currently required to do so, however are highly recommended to.

A unique and official Maritime Mobile Service Identity (MMSI) number is required for every AIS unit.

It normally takes a few seconds for ships to appear since the receiver needs to pick up a transmission from the remote ships’ transponders. The system allows for ships to rebroadcast their information every few seconds so within a minute you will typically see nearby ships appear on your navigation package.

Just wait. Ships broadcast voyage information every few seconds but also broadcast full ship information every 6 minutes. So after a few minutes, you should see complete information for every ship that the AIS receiver has picked up.

Most should, but, soon all will. Although all Class A devices will receive Class B information; unfortunately, some older Class A models are unable to display this information on their Minimum Keyboard and Display (MKD) or may only have available the Class B vessel’s dynamic data (i.e. position, course and speed) but not its static data (i.e. vessel name, call-sign).

You typically have two options.

The first option is to use a serial to USB adaptor. Connect the serial end to the AIS receiver and the USB end to a spare USB port on your computer. Make sure you know which COM port has been assigned to the USB serial port and configure your software appropriately.

The international requirement for the carriage AIS as ship-borne navigational equipment on vessels is detailed within Chapter V (Safety of Navigation) Regulation 19, of the SOLAS Convention. This requires that:

“All ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size shall be fitted with Automatic Identification System (AIS), as follows:

• Ships constructed on or after 1 July 2002;
• Ships engaged on international voyages constructed before 1 July 2002;
• In the case of passenger ships not later than 1 July 2003;
• In the case of tankers, not later than the first survey for safety equipment* after 1 July 2003;
• In the case of ships, other than passenger ships and tankers, of 50,000 gross tonnage and upward, not later than 1 July 2004;
• In the case of ships, other than passenger ships and tankers, of 300 gross tonnage and upwards but less than 50,000 gross tonnage, not later than the first survey for safety equipment after 1 July 2004 or by 31 December 2004, whichever occurs earlier; * and
• Ships not engaged on international voyages constructed before 1 July 2002, not later than 1 July 2008.

As determined at the IMO Conference of Contracting Governments to the International Convention for the Safety of Life at Sea, 1974: 9-13 December 2002.

Ships to which Regulation 19 of Chapter V of SOLAS do not apply are broadly fishing vessels, pleasure craft, support vessels and inland waterway vessels. It is expected that national administrations and the operators of these vessels will quickly realise the potential of AIS and its capability to enhance the safety of navigation.

There are numerous AIS networks and commercial purveyors that do provide AIS data and track information on the internet.

All networks that can handle NMEA VDM serial data at the speed 38400 baud will be able to handle AIS data.

All the units can receive AIS information from either AIS channel. The single channel switching receiver can only receive information on one channel at a time but automatically switches between both channels.

The dual channel receiver can receive all AIS broadcast information from both AIS channels simultaneously and consolidate the information from both channels into a single data stream. This generally means you will acquire new vessels sooner with the dual channel units and you will also get the full information about a vessel in a shorter period of time.

A type-approved AIS can range in price between $500 (AIS Class B) and $4,000 (AIS Class A), not including installation cost which will vary considerably depending on the level of integration of the AIS with other shipboard systems (e.g. radar, speed log, rate of turn indicator, navigation positioning system, ECDIS, etc.).

Class A devices are designed to meet the current IMO Performance Standards. SOLAS Chapter V (Safety of Navigation) dictates their carriage requirement. Carriage of Class A units may be required for other vessels as domestic or regional carriage requirements dictate.

Class B devices may not necessarily meet all the performance requirements specified by IMO MSC Resolution 74 (69) Annex 3. They are designed to operate harmoniously with Class A units on the VHF data link. The Class B units may be used on craft not subject to SOLAS.

The VHF antenna should fulfill at least the following requirements:

Antenna type: Vertical radiator

Antenna gain: 0 – 3 dBd

Impedance: 50 ohm

This is a matter for national administrations to regulate. However, the protection of AIS frequencies and information should be embodied in individual national legislation. As regards Class A shipborne mobile stations, a security mechanism is provided to detect disabling of the AIS and to prevent unauthorized alteration of input or transmitted data. Means are also provided to automatically record all periods when the AIS installation is non-functional. The most recent ten instances when the kit was non-functional for more than 15 minutes, is captured (UTC and duration) in a non-volatile memory.

International regulations do not prohibit a vessel to depart a port with an inoperative AIS. However, national administrations may require its operation prior to entering port. If the vessel does not fall under SOLAS, it depends on the regulations of the national maritime administration.

This is a matter for individual administrations.

All About AIS is an independent source of information on AIS. It is a one stop shop for all the facts on AIS that will help you decide which AIS product you buy, how it was tested and certified and how to install it.

No, All About AIS is independent and does not advocate one AIS brand over the next.