How Does AIS Work?

AIS (Automatic Identification System) is a maritime communication system that enables vessels to share and receive real-time navigational data to improve safety and collision avoidance. It operates using VHF radio frequencies and works based on TDMA (Time Division Multiple Access) technology to ensure efficient data transmission among ships and shore stations.

1. Key Components of AIS

AIS consists of the following main components:

• AIS Transceiver (Transmitter & Receiver) → Installed on ships, shore stations, or satellites.

• GNSS (GPS) Receiver → Provides accurate location data.

• VHF Antenna → Transmits and receives AIS messages over radio frequencies.

• Onboard Display/Software (ECDIS, Radar, or AIS Display) → Shows AIS data to navigators.

2. How AIS Data is Transmitted and Received

AIS transceivers continuously broadcast and receive real-time vessel information over two dedicated VHF frequencies:

• AIS Channel 1 → 161.975 MHz (VHF Channel 87B)

• AIS Channel 2 → 162.025 MHz (VHF Channel 88B)

AIS transmission occurs in short bursts of data, known as AIS messages, using TDMA (Time Division Multiple Access) to prevent data collisions.

AIS Communication Process:

1. A vessel’s AIS transceiver collects its position data from a GPS receiver.

2. The AIS unit transmits a signal via VHF radio, containing ship details (e.g., position, speed, course, vessel name, etc.).

3. Other ships and shore stations receive the AIS signal and display the information on electronic charts, radar, or AIS software.

4. AIS messages are updated periodically based on vessel speed and movement.

5. Shore-based AIS stations can rebroadcast the data for VTS (Vessel Traffic Services) and maritime authorities.

6. Satellite AIS (S-AIS) captures data from ships beyond coastal coverage, allowing global tracking.

3. Types of AIS Data Transmitted

AIS broadcasts different types of messages depending on the ship’s status and purpose. The key data transmitted includes:

Static Data (Fixed Information)

• MMSI (Maritime Mobile Service Identity)

• Ship Name & Call Sign

• IMO Number (if applicable)

• Ship Type (Cargo, Tanker, Fishing, etc.)

• Ship Dimensions

Dynamic Data (Real-time Navigation Information)

• GPS Position (Latitude & Longitude)

• Speed Over Ground (SOG)

• Course Over Ground (COG)

• Heading

• Navigational Status (Underway, Anchored, etc.)

• ROT (Rate of Turn)

Voyage Data (Updated by Crew)

• Destination Port

• ETA (Estimated Time of Arrival)

• Draught (Water Depth Clearance)

• Special Cargo or Navigation Warnings

4. AIS Update Frequency

AIS transmits updates based on vessel speed and movement:

Vessel Condition: AIS Update Frequency

Moving (>23 knots): Every 2 sec

Moving (14-23 knots): Every 6 sec

Moving (0-14 knots): Every 10 sec

At Anchor: Every 3 min

5. AIS Transmission Methods

AIS messages are transmitted using different TDMA methods:

• SOTDMA (Self-Organizing TDMA) → Used by Class A AIS for reliable and frequent transmissions.

• CSTDMA (Carrier-Sense TDMA) → Used by Class B AIS with lower priority and less frequent updates.

• RATDMA (Random Access TDMA) → Used for Base Stations and special messages.

• FATDMA (Fixed Access TDMA) → Reserved slots for shore stations and controlled environments.

6. AIS Coverage (Terrestrial vs. Satellite AIS)

AIS Type Coverage Range

Terrestrial AIS (Shore-based): Up to 40-70 km (~22-38 nautical miles)

Satellite AIS (S-AIS): Global coverage, but with data delays

• Terrestrial AIS is effective in coastal areas but limited by the VHF radio range.

• Satellite AIS (S-AIS) enables long-range tracking of vessels in open ocean areas.

7. Uses of AIS in Maritime Operations

AIS is essential for:

✔ Collision Avoidance → Ships can track nearby vessels and take action to avoid collisions.
✔ Vessel Traffic Monitoring (VTS) → Authorities track and manage ship movements.
✔ Search and Rescue (SAR) → AIS helps locate vessels in distress.
✔ Fleet Management → Shipping companies track their vessels in real-time.
✔ Illegal Fishing & Maritime Security → AIS aids in monitoring fishing zones and detecting suspicious activities.
✔ Environmental Monitoring → Authorities can track oil spills or vessel emissions.

8. AIS Limitations & Challenges

🚨 AIS data can be spoofed (false information input by users).
🚨 Not all vessels are required to use AIS (small boats may lack AIS transceivers).
🚨 Signal interference and VHF range limitations affect coverage.
🚨 S-AIS may experience data delays due to satellite orbits.

Conclusion

AIS is a vital maritime system that enhances safety, navigation, and global vessel tracking. It operates through VHF radio frequencies, using TDMA technology to share ship information. While terrestrial AIS covers coastal areas, satellite AIS (S-AIS) enables global tracking.