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Basics of AIS Transmission
A little history of AIS
Automatic Identification System (AIS) uses a transponder or transceiver a GPS and VHF antenna. AIS improves maritime safety, aids in collision avoidance, and helps to identify ships that are not in sight. AIS transmits the ship's data to other vessels capable of receiving AIS data. The transponder sends the most important ship data, position, speed, and course; this system is used worldwide through standardized message types or telegrams. The data is transmitted twice in case of failure. The data sends over designated VHF Operating frequencies 161.975 MHz (channel A = 2087) and 162.025 MHz (channel B = 2088). There are 4500-time slots available per minute, 2250 per channel—the AIS transceivers alternate one message on channel A next on channel B.
Garmin has transceivers (transmits and receives, also known as transponder) and AIS receivers. A Garmin AIS 800 is a Class B transponder. It sends vessel data and receives incoming data. Garmin also has VHF AIS radios that receive AIS data from ships with transponders. An AIS receiver helps the recreational user avoid larger vessels in heavily trafficked areas or dangerous areas.
Transceivers
AIS 800 Blackbox
AIS 600 Blackbox (discontinued)
Receivers
VHF 215 AIS Marine Radio
VHF 210 AIS Marine Radio
AIS 300 Blackbox (discontinued)
Types of AIS Service
There are two types of AIS service Class A and Class B.
Class A Service for commercial vessels is required (transponders)
Examples of standard ships required to have Class A service
Commercial Vessels 65 feet and longer
Towing vessels over 600 horsepower
Passenger vessels with more than 150 passengers
Dredges operating near a commercial channel
Cargo vessels carry specific types of dangerous materials
For the complete list of requirements for Class A, visit the US Coast Guard Navigation Center.
Class B Service
Pleasure boats are not required to carry AIS service. Class B is designed for smaller ships with fewer power options, less space, and fewer antennas than a commercial vessel.
Operation
Class B Transmit Data
Dynamic Data
Message number 18
MMSI number of vessel
Speed over ground
Position accuracy
longitude and latitude
COG (course over ground)
True heading
Timestamp
Class B Extended Position Report
Static Data
Message number 19
MMSI number of vessel
Speed over ground
Position accuracy
Longitude and latitude
COG (course over ground)
True heading
Timestamp
Ship's name (max characters 20)
Type of ship
Dimensions of the boat (to the nearest meter)
Type of positioning system (typically GPS or DGPS)
Class B Static Data Report
Message number 24
Message 24A:
MMSI number of ship
Name of the ship (max 20 characters)
Message 24B:
MMSI number of ship
Type of ship or pleasure boat
Vendor ID
Radio call sign
Dimensions of ship
Separated in two messages 24A and 24B, sent when possible, one after the other.
Static data takes longer to transmit. Transmit rate every 6 minutes, 24B typically sends a minute later than 24A.
Report Rate of AIS 800 Dynamic Data
AIS transponders send position updates based on speed over ground of the boat and transmit updates at different intervals based on the Class A or Class B used. Below is the information based on the AIS 800
Ship Speed | Nominal Reporting Interval | Increased Reporting Interval |
≤ 2 knots, at anchor, or moored | every three minutes | every three minutes |
2 - 14 knots | every 30 seconds | every 30 seconds |
14 - 23 knots | every 15 seconds | every 30 seconds |
> 23 knots | every 5 seconds | every 15 seconds |