Be Safe and Be Seen with AIS

AIS screen

By Tom Burden, Last updated 8/16/2019

What is AIS?

Picture a radar-chartplotter display, with overlaid electronic chart data, that includes an icon for every significant ship within VHF radio range, each with a velocity vector (showing that vessel’s speed and heading). Each ship icon reflects the actual size of the ship, with positions accurate to GPS precision. By “clicking” on a ship mark, you can learn the ship’s name, course and speed, classification, call sign, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation data is also available. Information previously visible only to the Vessel Traffic Service is now available to every AIS-equipped vessel.

competing sailboats shown on a display with AIS input from a NAIS400 class b transceiver

Looking at my competitors in a race near the Farallon Islands, with AIS input from the NAIS400 Class B transceiver.

The Automatic Identification System is a digital VHF radio-based transponder system that can prevent collisions, and can protect your boat from being run down by a huge, fast moving ship. It’s like digital radar with precise position information. AIS uses GPS, VHF radio and sophisticated digital processing to automatically communicate between vessels without any operator interaction.

The system’s range is similar to that of your VHF radio, essentially depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends in rivers or over islands if the landmasses are not too high. AIS information is not degraded by rain-clutter like radar, so it works the same in all weather. The consensus among safety experts is that AIS is extremely valuable, especially if you travel at night or in crowded shipping lanes with restricted visibility.

Do You Want Class A, Class B or Receive Only?

Three classes of AIS units

Class A: Full-function transceivers are required for ships over 300 gross tons that travel internationally, and for passenger ships. More classes of commercial vessels have been added to these requirements as of March 2016 (see below). Class A AIS units include both transmitters and receivers, and automatically transmit every two to 10 seconds when the ship is under way, and every three minutes when anchored. Transmitting at 20 watts, they consume substantial amounts of electrical power. Class A units are also often pricey.

Class B: This class offers a somewhat reduced set of functions at a much lower cost, and is intended for recreational vessels and others not required to carry Class A AIS. Class B was approved for use throughout the U.S. in September 2008. It is nearly identical to Class A, except that Class B:

  • Has a less frequent reporting rate than Class A (every 30 seconds when traveling at more than two knots, as opposed to 10 seconds for Class A).
  • Does not transmit vessel destination, ETA, draft, IMO Number or rate of turn.
  • Consumes less electrical energy than Class A (transmits at 2W instead of 20)

Receive-Only: These units let you identify what ships are in the neighborhood while you remain off the AIS grid. Think of a receive-only unit more like radar than a radar unit with radar reflector. These receivers, containing only the two-channel VHF receiver and decoding software, are not Class B, since they are not active transmitters on the AIS system.

Is the Unit Programmable After Purchase?

FCC regulations require that all Class B AIS units have their “static data,” your boat’s MMSI (Maritime Mobile Service Identity) Number and other dimensional and identifying information, programmed by a professional—in this case by trained technicians at our warehouse. If you do not have an MMSI number, you can get one for free at

If your boating takes you to international ports, you must obtain a Ship Station License and your MMSI number from the FCC. Go to FCC Ship Station Licensing for instructions. Currently there is a fee for this license. If you believe that your vessel may not be a recreational boat due to use or size, this site will also provide information defining whether or not you will need a Ship Station License.

When you order an AIS transceiver, collect and submit the following information online at: Your AIS unit will then be pre-programmed for your boat and shipped to your address.

  • Order Number
  • Shipping and Contact Information: First and last name, address, phone number and email address
  • Product Information: Manufacturer and model
  • MMSI registrant: First and last name
  • MMSI Information: MMSI number (nine digits), Serial number of AIS unit (only needed for em-trac brand AIS units)
  • Vessel name (up to 20 characters)
  • Vessel call sign (if available)
intended GPS antenna location diagram

Intended GPS antenna location. Must be in meters (1 meter = 3.28 ft).

  • A—Distance from bow
  • B—Distance from stern
  • C—Distance from port side
  • D—Distance from starboard side

AIS units like the em-trak B100 can be operated (in Receive Mode only) before the boat’s MMSI number and data are programmed on an SD Card.

Do You Need NMEA 2000 Networking?

Do you have an NMEA 2000 network on your boat? About half of the AIS units in our assortment are compatible with the newest networking protocol, and they’re also backward-compatible with the older NMEA 0183 standard.

Do You Want to Match the Brand of Your Other Electronics?

Does the AIS unit have special features that only work when networked with same-brand multifunction displays? For example, Garmin’s AIS 800 integrates with a Garmin chartplotter and VHF radio via NMEA 2000, giving you the ability to initiate a call to any MMSI target directly from the chartplotter by selecting the “call with radio” function. Raymarine’s AIS350, when used with Raymarine multifunction displays, supports Raymarine’s Buddy Tracking feature, which allows you to save AIS targets of interest with a special icon and “friendly” name.

Is an Antenna Splitter or GPS Antenna Included?

A few AIS units include an antenna splitter, allowing it to share an antenna with your VHF radio. Garmin’s AIS 800 includes a built-in splitter. Simrad and em-trac make add-on antenna splitters. Most AIS units include a GPS antenna.

Expansion of AIS Carriage Requirements

Here’s some information regarding expansion of the types of commercial vessels who must carry an AIS transceiver. In 2005, the US Coast Guard announced that it intended to significantly expand the carriage requirements for AIS transceivers to a wide range of commercial work boats, ferries and fishing boats. On December 23, 2014 this legislation was finally approved for publication by the USCG. The regulation gave operators of the effected types of commercial vessels 12 months, from March 2, 2015, to install a USCG Certified AIS transceiver. 

The new USCG AIS Rule requires most commercial boats operating in the US to install and operate a USCG certified AIS transceiver. Previously, under the international SOLAS regulation, only vessels over 300 Gross Tons on an international voyage were required to have an AIS Class A transceiver. The new USCG AIS Rule expands the existing carriage requirement to include virtually all commercial boats of any type and size operating in US waters.