Frequently Asked Questions

The Automatic Identification System (AIS) is an automatic tracking system used on ships and by vessel traffic services (VTS) for identifying and locating vessels. AIS transponders broadcast vessel data — including the vessel's MMSI number, name, type, position, course, speed, and navigational status — at regular intervals over dedicated VHF radio frequencies (161.975 MHz and 162.025 MHz).

When satellites are used to receive AIS transmissions, the term Satellite AIS (S-AIS) is used. AIS was designed to supplement marine radar by providing additional vessel identity and track information to watchkeeping officers and maritime authorities. AIS information can be displayed on screen or overlaid on an electronic navigational chart (ENC).

AIS operates using Self-Organising Time Division Multiple Access (SOTDMA) technology, which allows multiple vessels to share the same VHF frequencies without collisions. Class A transponders (fitted to SOLAS vessels) use SOTDMA and transmit position reports every 2–10 seconds when underway. Class B transponders use Carrier Sense TDMA (CSTDMA) and transmit less frequently.

Each AIS transponder automatically manages its own transmission slot, listening to other transmissions to avoid overlap. Shore-based base stations receive these broadcasts and feed the data into maritime information systems, such as MariWeb , where vessel tracks can be viewed, analysed and managed in near real time.

Yes. Class A AIS transponders, as fitted to SOLAS vessels, have priority over Class B transponders on the VHF data link. In situations where channel capacity is under pressure, Class A SOTDMA transmissions take precedence. Class B CSTDMA transmissions will defer or be delayed when they detect an occupied slot, ensuring that larger commercial and passenger vessels maintain reliable transmission rates even in high-traffic areas.

AIS can be used by a wide range of users:

• SOLAS vessels over 300 GT on international voyages, and all passenger vessels, are required by IMO regulations to carry Class A AIS.
• Smaller vessels and pleasure craft can voluntarily use Class B AIS transponders to improve their visibility to other vessels and maritime authorities.
• Maritime authorities and VTS operators use shore-based AIS receiving systems to monitor vessel movements in their area of jurisdiction.
• Ports and harbours use AIS data integrated into port management and vessel traffic systems.
• Coast guards and naval authorities use AIS for maritime domain awareness and security operations.

• Improved collision avoidance — AIS provides vessel identity, position, course, speed and closest point of approach (CPA) calculations to watchkeeping officers.
• Enhanced situational awareness in restricted visibility and congested waterways.
• Improved coordination with VTS and port control, particularly during vessel traffic management in busy channels.
• Faster, more accurate search and rescue (SAR) operations — AIS last known position data significantly aids SAR coordination.
• Monitoring of Aids to Navigation (AtoN) using AIS AtoN units to confirm buoy positions and report status remotely.
• Transmission of safety-related messages and meteorological data to vessels at sea.

• Identification and tracking of vessels entering or transiting territorial waters and exclusive economic zones (EEZ).
• Detection of anomalous behaviour such as AIS spoofing, sudden course deviations, or unexplained AIS outages (dark vessels).
• Support for port security assessments and pre-arrival vessel notifications.
• Integration with national maritime domain awareness systems to support law enforcement and border protection agencies.
• Enhanced coordination between coast guard, customs, and naval authorities during joint maritime security operations.

• More efficient port operations through improved vessel scheduling, berth allocation and turnaround planning.
• Reduced fuel consumption through optimised routing and reduced waiting times.
• Lower incident costs through improved situational awareness and collision avoidance.
• Potential reductions in marine insurance premiums for vessels operating within well-monitored VTS areas.
• Commercial fleet monitoring enables owners and operators to track vessel performance and cargo delivery schedules in real time.

• Optimised routing and reduced idle waiting time lowers fuel consumption and greenhouse gas emissions.
• Improved traffic management in environmentally sensitive coastal and marine protected areas.
• Monitoring of vessel activity in restricted zones to support environmental compliance and enforcement.
• Early warning of vessels at risk of grounding or collision in ecologically sensitive areas.
• Integration with environmental monitoring sensors (weather, sea state, water quality) to build a comprehensive maritime environmental picture.

The equipment required depends on your specific use case:

• Vessels transmitting and receiving AIS: A Class A or Class B AIS transponder with a dedicated VHF antenna, connected to the vessel's GPS and optionally to chart display equipment.
• Shore-based receiving only: A VHF antenna, an AIS base station or receiver, and software to display and manage the received data.
• Full maritime information system: Multiple AIS base stations networked to a central server running a system such as MariWeb , providing coverage over a defined geographic area with a web-based operator display.

IMIS Global can advise on the most suitable configuration for your requirements. Please contact us for more information.

An individual vessel owner wishing to transmit, receive and view AIS data requires:

• A Class B AIS transponder (for non-SOLAS vessels) with a suitable VHF antenna.
• A GPS source — either internal to the transponder or connected from the vessel's navigation system.
• A chart plotter or MFD (Multi-Function Display) capable of displaying AIS targets, connected via NMEA 0183 or NMEA 2000.

Once installed, the transponder will automatically broadcast the vessel's position, course and speed, and will display nearby AIS-equipped vessels on the connected display. No internet connection or shore-based infrastructure is required for vessel-to-vessel AIS operation.

An individual wishing to receive and display AIS data from a fixed shore location requires:

• A VHF antenna mounted at an elevated position to maximise reception range.
• A standalone AIS receiver or AIS base station connected to the antenna.
• A PC or display device running AIS viewing software, connected to the receiver via serial or USB.

Coverage range depends primarily on the antenna height above sea level. A well-sited antenna at 10–20 metres elevation can typically receive AIS from vessels up to 20–30 nautical miles offshore. For greater range or networked operation, IMIS Global's MariWeb system provides a scalable solution.

For commercial organisations operating a fleet of vessels, each vessel requires a Class A or Class B AIS transponder (depending on vessel type and trading area) with a VHF antenna and GPS source. Fleet operators can monitor all vessels simultaneously using a shore-based maritime information system such as MariWeb , which aggregates AIS data from satellite and terrestrial sources to provide a comprehensive fleet picture regardless of geographic location.

MariWeb 's commercial fleet module provides voyage tracking, event alerting, and historical reporting for effective fleet management. Please contact IMIS to discuss fleet monitoring requirements.

Organisations requiring a shore-based AIS receiving capability — such as a port authority, harbour master, or commercial terminal — typically deploy one or more AIS base stations connected to a central maritime information system. IMIS Global's MariWeb system can aggregate data from multiple base stations and satellite AIS feeds to provide complete coverage of the relevant operational area.

MariWeb provides a web-based operator interface accessible from any modern browser, with no software to install at operator workstations. The system supports multiple simultaneous users with configurable access roles and permissions.

National maritime and coast guard authorities typically mandate AIS carriage for vessels operating within their jurisdiction, in line with IMO SOLAS requirements and national regulations. For government-operated patrol and survey vessels, Class A AIS transponders are standard equipment. National authorities also use AIS to monitor their own fleets centrally via a national maritime information system.

IMIS Global works with national maritime authorities worldwide to design, deploy and support national AIS networks integrated into MariWeb . This includes coordination with IALA and ITU standards, spectrum management, and integration with adjacent national systems.

A national AIS shore network consists of a series of AIS base stations distributed along the coastline and at key ports, connected to a National Node server running a maritime information system such as MariWeb . The National Node aggregates all base station data and provides a single, authoritative vessel traffic picture for the entire jurisdiction.

MariWeb supports IALA R0124 compliant national AIS networks and can be connected to the IALA AIS network of networks, enabling data sharing between adjacent national authorities. IMIS Global provides full turnkey project delivery including system design, hardware specification, installation support, integration, training, and ongoing SLA-backed support. Please contact IMIS Global to discuss your national AIS requirements.