Automatic identity system

Automatic identity System AIS: The best navigation aid to boaters since the GPS. One of the fastest and biggest growth areas in marine navigation is AIS. Having a AIS transponder onboard, will turn your boat into a radio beacon continuously transmitting your speed and course and your identity to anyone in range. Conversely having an AIS receiver allows you to track a ships COG, SOG. This can be very important information for navigating in shipping lanes and channels.

AIS description

ACR Nauticast™ AIS receive only
ACR Nauticast™ AIS receive only

AIS stands for Automated Identity System. AIS is data sent via two frequencies in the marine VHF band. The data includes the transmitting vessels MMSI number, speed and course which are just some of the information avaiable. AIS has been developed to help prevent collisions between commercial shipping and since 9/11 gives the authorities information about shipping movements. AIS has been a legal requirement on ships over 300GT for a while. In US waters it is required by commercial ships over 65ft and tugs over 26ft and over 600 HP.

Vessels with the proper equipment can receive/send the AIS signal over a special VHF radio frequency. The signal includes COG, SOG, CPA, call sign and MMSI which can then be plotted on a standalone AIS unit or the existing navigation or radar displays. With the call sign you can identify a ship and call them via VHF.

The display shows the AIS target as long triangle symbol depicting vessel and the same symbol is used for a large ship or small fishing vessel. The triangle points in the direction the target vessel is moving. This allows shipping to monitor traffic always knowing whats happening around them at all times.

For more information go to Navcen @ USCG


Marine Traffic

Marine traffic is a web site which tracks AIS signals worldwide. This map shows the entrance to New York harbor and the ships in view via their AIS signals. This shows the position course and speed of each vessel in the harbor. With an AIS device you can as a recreational vessel see each ship on your chartplotter and therefore be able to navigate safely in crowded channels.

By clicking on a target you will see the information that is provided by an AIS signal. This ship is Liberian based and is leaving Ney York en route to Seven island. This ship is underway, 245m long, speed is 14.1 knots and course 99 degrees.

Another option to track AIS traffic is Shipfinder

AIS Benefits and Features

The basic benefit of AIS is safety and navigation. AIS signals include course, speed and graphic displays showing AIS targets make navigating in crowded channels less stressful.

The Marine Accident Investigation Branch,(UK). MAIB has a report of a mid channel collision between a commercial Ferry and a small sailing vessel which resulted in the sailboat sinking and loss of life. The reports conclusion showed the sailing vessel did not appear on radar and because of the height of the bridge the sailboat was not seen by eye. The recommendation was for better radar reflectors so the sailboat could be seen on the ferries radar and or the use of AIS.

One of the many features of the AIS system is that it has the potential to notify you well in advance of an oncoming ship, along with potential collision avoidance alarms.

Closest point of Approach or CPA tells you how close a approaching vessel comes to your course. From this you can decide if you need to alter course. TCPA – The Time to Closest Point of Approach tells you how much time you have to work with.

Rate of turn ROT also in addition to CPA class A signals includes rate of turn. With this it is possible to predict the path of a ship which is turning. Rate of turn ROT information looks like this ROT L>5 degrees. The ship is turning left at a rate of turn under 5 degrees.

Call sign and ship details; AIS also includes information about the target including call sign and ship details. With this you can call a vessel knowing you have the right vessel. Instead of calling hey ship coming toward me, you can call the ship by name. If you have a VHF with DSC you can type in the ships MMSI number and call the bridge directly.

Filtering of AIS messages is possible with some devices. If the area is crowded with signals you can filter some of the AIS signals by choosing say CPA and speed, range and TCPA to prioritize targets.

Additional benefits include, Safety related Message (SRM) i.e. a mayday or Pan Pan call. The SRM will contain the vessels details and position. Another development is channel markers with AIS transponders, and now SARTS.

AIS Equipment Needed

You need an AIS receiver or transponder, then you need a display, an antenna, power, and GPS (some transponders have integrated GPS). These are the basic needs and you can use a PC as a display or a chartplotter, or have a standalone AIS display which just needs an antenna and power. Another recent development is combination VHF and AIS devices.

The most basic is an AIS receiver, which receives the AIS signals which can then be displayed on your boats existing chartplotter, repeater or PC.

Standalone displays look like Radar plots (see ICOM right). You are centered on the screen and AIS targets are shown relative to you. This is different to seeing a AIS signal on a chartplotter or PC, which shows the target at its Lat & long with its speed and direction.

If you decide on sending your AIS signals you will need a transponder either a class A or class B. Class B is for recreational boaters.

Antenna or splitter

You will need an antenna, either a dedicated antenna or using a splitter to your VHF antenna. VHF antennas work across a wide range of frequencies, while AIS works on two frequencies and 2 watts. AIS frequencies; 161.975 and 162.025 MHz in marine band.

if you are using your VHF at the same time as a transmission is scheduled the AIS signal may get lost. So if you use the VHF a lot a dedicated AIS antenna may be worth it.

Choosing a VHF and positioning the antenna should take these requirements into account. line of sight, gps and ais 5m apart interference, not on same plane as other antenna, 360 deg view of horizon

MMSI number

If you elect to get a Class A or B transponder you will need to program the unit with certain information including your MMSI number. If you have not already got one You will need to apply for a MMSI number just as you do for DSC radios. Someone receiving your AIS signal will have important information about you and be able to call you by name and call sign.

Register here for MMSI numbers

Once you have a AIS device you will need to get programmed into the transponder include; Ship’s name (20 characters maximum), Vessel’s radio call sign (7 characters maximum), MMSI number, Vessel type and certain vessel’s dimensions.

AIS types

The simplest AIS device is a receive only device, either single or dual channel. A “receive only device” allows you to receive AIS signals and display those signals on a chartplotter. You do not send your AIS data.

“Receive only devices” do not have the regulations, which cover transponders.

If you elect to broadcast your AIS data you will need a transponder and you have the choice of Class A or Class B transponders. Transponders need FCC approval.

AIS Class A

Class A is intended to meet the IMO mandated carriage requirements for commercial ships. Class A AIS transponders transmit, via 2 channels and typically are fully integrated into the ships navigation systems.

Class A transmits at 12 W and receive all types of AIS messages. Class A has a range of 20 miles, and transmits every 2 seconds,.

Shown is Comars Class A AIS for recreational boats. The Comar Class A costs around $2,500.

AIS Class B

Class B transponders are for smaller vessels. Class B signals are sent every 30 seconds, compared with as frequent as 2 seconds for Class A. Class B transponders are also dual channel. range is around 10 miles and 2 watt power.

Class B can be set to receive only. This maybe to avoid clutter in a crowded channel or to avoid being seen, Fishing boats unfortunately are in this category, so are Pirates.

The Class B is nearly identical to the Class A, except the Class B:

  • Has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to every 10 sec. for Class A)
  • Does not transmit the vessel’s IMO number
  • Does not transmit ETA or destination
  • Does not transmit navigational status
  • Is only required to receive, not transmit, text safety messages
  • Is only required to receive, not transmit, application identifiers (binary messages)
  • Does not transmit rate of turn information
  • Does not transmit maximum present static draught
  • another interesting feature of B is that transmission spots are not reserved as class A. instead class B listens for a empty slot, so there is no chance of crowding out Class A signals