Automated Identification Systems (AIS): Explanation of Transmitted Data
An automated identification system is a transmitter aboard oceangoing vessel that relays vessel data over a VHF radio band. The information can then be received by other vessels, shore-based stations, or satellites. This article provides a summary of what data is transmitted.
Introduction
Automated identification systems (AIS) came about in the 1990s as a method of tracking vessels within short ranges. Due to high efficacy in supporting traffic management — both at sea and through vessel traffic services ashore — AIS units proliferated and are now mandated for most commercial vessels.
In 2023, over 100,000 commercial vessels over one-hundred gross tons (UNCTAD, 2023) ply the world’s waterways. Most of these vessels send packets of AIS data in as little as two second intervals (IMO, 2015). These micro-transmissions result in a total worldwide output of 38 gigabytes of AIS data per day (Spire, 2023).
Today, the push is on to collect and analyze AIS data, both real-time and historical, in a value-driven manner. For example, due in part to the global effort aimed at sustainability, there is a strong relationship between vessel tracking and emissions reporting. This article’s purpose is to provide a summary of what kind of AIS data is being transmitted for the benefit of maritime data analysts.
Automated Information System Data
Generally AIS data is received through an application program interface (API). Depending on the provider, the API call will source data from terrestrial AIS stations, satellite AIS stations, or a combination thereof.
The API call usually involves geofencing the data through an upper and lower geographic boundary, and indicating the age of the returned positions (AISHub, n.d.; Spire, n.d.).
# Example of an API call
https://data.aishub.net/ws.php?username=A&format=B&output=C&compress=D&latmin=E&latmax=F&lonmin=G&lonmax=H&mmsi=I&imo=J&interval=KThe data is then usually returned as extensible markup language (XML), javascript object notation (JSON), or a comma-separated value (CSV) file.
# Example of an API return
id,created_date,modified_date,mmsi,tstamp,latitude,longitude,cog,sog,heading,navstat,imo,name,callsign,vessel_type,bow,stern,port,starboard,draught,dest,eta,point
570288791,2023-09-27 00:48:09,2023-09-27 00:48:09,316004328,2023-09-27 00:48:01,51.72445,-127.91442,164.0,7.1,167.0,0.0,8720292,CENTRAL COASTER,,80,38.0,6.0,5.0,4.0,2.8,COASTAL PORTS,00-00 00:00,SRID=4326;POINT (-127.91442 51.72445)An explanation of the data packet received follows.
Maritime Mobile Service Identity (MMSI)
316004328
The nine-digit maritime mobile service identity is analogous to a personal telephone number. One number is unique to one vessel, and therefore can serve as an identifier. The number itself has further coding embedded in it, for example the geographic area of the issuing agency (ITU, 2022).
The MMSI is considered, in the author’s opinion, a more reliable identifier than an International Maritime Organization (IMO) number. For example, the IMO number may carry over from scrapped vessels and cause datasets to become corrupted.
The MMSI value is entered into the AIS unit upon installation.
Time Stamp
2023–09–27 00:48:01
The date and time the signal was generated. Usually the time signal originates from an onboard sensor such as a global navigation satellite system (GNSS) and is considered a reliable data point.
Latitude
51.72445
Latitude is the measure of distance north or south of the equator. Latitude ranges from 0 degrees at the equator to 90 degrees at the poles. A negative number indicates the latitude is in the southern hemisphere.
Longitude
-127.91442
Longitude is the measure of distance on either side of the international reference meridian (IRM). Longitude ranges from 0 to 180 degrees east or west. A negative value indicates the longitude is to the west.
Both the latitude and longitude originate from the vessel’s onboard global navigation satellite system (GNSS).
Course Over Ground (COG)
164.0
Course over ground is the course inclusive of environmental factors. For example if the vessel is heading 090°, and there is a seasonal current heading south, the course over ground may show 095° due to the vessel being pushed south by the current. COG is provided by an onboard sensor, usually the GNSS.
Speed Over Ground (SOG)
7.1
As with course over ground, speed over ground is inclusive of environmental factors. The unit of measurement is knots. Therefore if the vessel is on a course of 090° at 15 knots, and the wind is coming from the east at 20 knots, the speed over ground may show 14 knots because the wind is pushing against the vessel. SOG is provided by an onboard sensor, usually the GNSS.
Heading
167.0
The heading is the direction the vessel is pointing at, however it may not be the direction the vessel is moving if there are environmental factors, or if the vessel is drifting.
For example, if the COG is 164°, and the heading is 167°, then we might infer that there is some environmental force coming from the south / southwest that is causing the vessel to adjust the heading to make good 164°. Heading is updated using an onboard sensor.
Navigational Status
0
Navigational status adheres to collision regulations (COLREGS). Unlike the prior values, navigational status is entered by the vessel officer. Examples:
0: Under way using engine
1: At anchor
2: Not under command
3: Restricted maneuverability
4: Constrained by her draught
5: Moored
6: Aground
7: Engaged in fishing
8: Under way sailing
Based on above, the navigational status of 0 indicates the vessel COASTAL VOYAGER is underway using their engine.
International Maritime Organization Number
8720292
International Maritime Organization (IMO) Number is a seven digit number used for identification in addition to the MMSI number. The MMSI number, in the author’s opinion, is a more reliable identifier.
However IMO numbers are used on commercial documents, such as charter parties, and can be used to establish a relationship between technical (AIS) and commercial (charter party) data.
Name
COASTAL VOYAGER
The vessel’s name. Use of capitalizations is common with vessel names and assist with highlighting an important element (the vessel’s name) in legal and technical documents.
Call Sign
WHF74
The call sign is of little use in almost all cases and should be ignored.
Vessel Type
80
A vessel type indicates whether a vessel is a fishing boat, tanker, or any other such vessel type. A partial list of vessel types are as follows:
30: Fishing
31: Towing
33: Dredging
36: Sailing
52: Tug
60–69: Passenger ship
70–79: Cargo ship
80–89: Tanker
In the example above a type 80 vessel is a tanker. The type value is generally applied upon AIS installation.
Flag
Canada
The vessel’s country of registry. A common misconception is that the vessel’s flag reflects the nationality of the crew. Except in the case of cabotage vessels, this is almost never the case.
Draft
2.8
The depth of water that the vessel is drawing. The draft may not come with a unit of measurement. In the United States, the draft is sometimes — but not always — relayed in feet. In most other parts of the world, the draft is relayed in meters.
In the case above it could be inferred that the draft is in meters. The draft is usually entered by the vessel operator. It is easy for vessel operators to forget or not update draft, therefore it could be assumed the draft is unreliable unless it is checked against other information.
Estimated Time of Arrival
00–00 00:00
Estimated time of arrival is the time that a vessel will reach its destination in universal coordinated time (UTC). The value is entered by the vessel operator.
ETA is unreliable for two reasons. The first is it is common for the vessel operator to simply not enter the value, as is the case with the COASTAL VOYAGER. The second is the conflation of local time (LT) and universal coordinated time (UTC). ETA does not usually come with a designator, so analysts are often left wondering if the ETA is indeed correct, and if a time conversion is necessary.
Destination
COASTAL PORTS
The geographic destination of the vessel. In this example a generic name is used. The destination is entered by the vessel operator.
Conclusion
Some AIS data elements were omitted above, such as the length and breath of the vessel, which is usually shown in meters. The position of the transmitting antenna is omitted within this article as well, and is generally not of much use for data analysis.
AIS data can be siloed in three ways: static, dynamic, and voyage-related. Static AIS data is data that is rarely changed, such as the MMSI number. This data can generally be trusted. Dynamic AIS data is sensor data, and as long as the sensors are operational — the data can be deemed for the most part reliable. There are cases when sensors, particularly a GNSS sensor, may have bad reception which may result in an error. Voyage-related data, such as draft and cargoes, can be considered unreliable unless compared against another source of data. The reason is that crews may not update the data or omit data entirely. Unit of measurement or time conversions are often conflated. Crews are by default overworked and tired, and AIS management is low on a vessel officer’s list.
Managing AIS data by shoreside personnel is the easiest it has ever been. Computer speed, and general proficiency in handling large data by newer generations of analyists and shore operators are the primary reasons. This bodes well in unpacking value-driven outcomes using AIS data, especially for small developers, and may serve to innovate and push the maritime domain further ahead in terms of sustainability.
References
AIS data API (XML / JSON / CSV Webservice). (n.d.). AISHub. Retrieved October 1, 2023, from https://www.aishub.net/api
Conventions — Convention on the International Regulations for Preventing Collisions at Sea, 1972 (COLREGs). (n.d.). International Maritime Organization. Retrieved October 4, 2023, from https://www.imo.org/en/About/Conventions/Pages/COLREG.aspx
Expected AIS data volumes | Spire Maritime Documentation. (n.d.). Spire Maritime Documentation. Retrieved October 4, 2023, from https://documentation.spire.com/static-data-files/expected-ais-data-volumes/
GUIDELINES FOR THE ONBOARD OPERATIONAL USE OF AIS. (2015, December 14). ClassNK. Retrieved October 1, 2023, from https://www.classnk.or.jp/hp/pdf/activities/statutory/ism/imo/imo_a1106-29.pdf
M.585 : Assignment and use of identities in the maritime mobile service. (2023, March 6). ITU. Retrieved October 4, 2023, from https://www.itu.int/rec/R-REC-M.585/en
WGS 84 IMPLEMENTATION MANUAL. (1998, February 12). ICAO. Retrieved October 4, 2023, from https://www.icao.int/safety/pbn/documentation/eurocontrol/eurocontrol%20wgs%2084%20implementation%20manual.pdf
