Understanding the Basics of AIS

Written by Capt. Juan Watson

One cannot but marvel at the strides and innovations in onboard marine electronics.

Today, equipment traditionally found on the sophisticated bridges of commercial vessels and mega yachts are now available in simpler forms to the recreational yachtsman. One such item that has become available and affordable to the recreational boater is Automatic Identification System, or better known as AIS

How it works:

AIS broadcasts a vessel’s onboard GPS readings and vessel data via VHF band channels to other vessels within range. Provided they are also fitted with AIS equipment these vessels will be able to receive information such as your vessel’s name, speed, position and heading. It basically sends and receives two types of data. The first is your static data which is your MMSI number, vessel name, vessel dimensions, vessel type and location and position of your GPS antenna. It is basic information regarding the details and characteristics of your vessel. The second is your dynamic data which consists of your position, universal time coordinated t (UTC), course over ground and speed over ground. This is information that is constantly changing when you are making way. The timing of theses updates is dependent on what type of unit you have and vessel speed.


There are two types or classes of units available, A & B. The Class A units are primarily used by larger commercial vessels and Class B units are for small commercial and recreational vessels. They essentially provide and receive the same information with a few differences. The Class A units have a higher transmitting power providing increased range.  They provide more specific vessel and voyage information and vessel updates are every 2-10sec (depending on vessel speed) when underway and not every 30sec as found on the Class B units.

Hardware and equipment:

The AIS unit itself can be mounted at or near your navigation station. It will require a VHF antenna and GPS feed to function. The information can be displayed on a designated AIS display but can also be fed into your radar and chart plotter systems. This will make your radar significantly more effective as you will be able to identify targets and get more specific information about each target (i.e. speed and closest point of approach/ CPA) and be able to make more educated decisions.

Some important things to remember:

  • Learn the limitations of the unit. As with all electronic aids to navigation there can be errors, so factor that into your evaluation of each collision situation.
  • Ensure that you are familiar with all the AIS screen symbols and functions associated with your specific unit. Also, understanding what certain terms mean i.e.TCPA (time to closest point of approach)  and so on are crucial because without a grasp of these terms you will not be able to effectively use the information or equipment and put yourself at risk.

If used correctly, AIS is a wonderful aid to navigation and most certainly gives us all an added level of safety on the water. If you have been thinking about purchasing an AIS unit, get one! You would certainly not be wasting your time or money, especially if you are on the water in high commercial traffic areas and or with dense seasonal fog.

Stay warm!

Juan Watson is a 14-year mega yacht veteran. Currently, he owns and operates Pelorus Yacht Consulting, LLC in Annapolis, MD. PYC focuses on educating yachtsmen and owners.