The Automatic Identification System, or AIS, is a radio based tracking system used by many ships to share their identity, position, speed, and course. It helps crews see nearby vessel traffic even when visibility is poor or ships are beyond direct sight. AIS is important for collision avoidance, harbor management, search and rescue, and marine science.
On an infographic, AIS is often shown as radio signals spreading from a ship to other vessels, coast stations, and satellites.
A ship uses GPS or another navigation source to find its position, then sends that information through a VHF antenna in short digital messages. Nearby ships and shore receivers decode the messages and display the vessel on electronic charts. Satellites can also receive AIS from space, which allows wide area monitoring of open ocean traffic.
Submarines normally do not transmit AIS while submerged because VHF radio waves do not travel well through seawater, but a surfaced submarine may transmit or receive AIS like other vessels when appropriate.
Key Facts
- AIS stands for Automatic Identification System.
- AIS broadcasts commonly include MMSI, vessel name, position, speed over ground, course over ground, heading, and navigation status.
- Basic speed relation: distance = speed × time.
- AIS uses VHF radio, mainly near 161.975 MHz and 162.025 MHz.
- VHF signals travel mostly by line of sight, so antenna height affects communication range.
- Approximate radio horizon in kilometers: d ≈ 3.57√h, where h is antenna height in meters.
Vocabulary
- AIS
- AIS is an automatic VHF radio system that broadcasts a vessel's identity, position, and movement information.
- MMSI
- MMSI is a unique nine digit Maritime Mobile Service Identity used to identify a ship or station in radio communications.
- VHF
- VHF means very high frequency radio waves, a band commonly used for marine voice and data communication.
- Course over ground
- Course over ground is the direction a vessel is actually moving across Earth's surface.
- Radio horizon
- The radio horizon is the approximate maximum line of sight distance over which a radio signal can travel before Earth's curvature blocks it.
Common Mistakes to Avoid
- Confusing AIS with radar is wrong because radar detects objects by reflected radio waves, while AIS depends on vessels actively transmitting data.
- Assuming every vessel always appears on AIS is wrong because small boats may not carry AIS, equipment can be turned off, and submarines usually do not transmit while submerged.
- Treating AIS position as perfect is wrong because it depends on navigation inputs, message timing, receiver coverage, and possible data errors.
- Ignoring antenna height is wrong because VHF AIS range is limited by line of sight, so higher antennas can usually communicate over longer distances.
Practice Questions
- 1 A ship broadcasts an AIS report showing speed over ground of 18 knots. How far will it travel in 2.5 hours if its speed stays constant?
- 2 A shore AIS antenna is 25 m above sea level. Using d ≈ 3.57√h, estimate the radio horizon distance in kilometers for that antenna.
- 3 A submerged submarine does not appear on a nearby ship's AIS display, but a surface cargo ship does. Explain why this can happen and why AIS should not be treated as a complete picture of all objects in the water.