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Do power voyagers need an automatic ID system?

Jan 1, 2003

Few recreational power vessels have as much in common with commercial vessels as a power voyaging yacht. Some power voyagers are powerful enough to defray their costs of operation by doing a little towing as a sideline.

Given this, power voyagers might want to know whether they are required to carry automatic identification system (AIS) equipment like their commercial brethren. Before we discuss AIS requirements, however, let's look at how AIS works and why it's useful to mariners.

AIS is a shipboard navigation data broadcast system for collision avoidance. You can obtain the majority of the value of AIS - knowledge of the position and immediate future path of AIS-equipped vessels - by installing either an AIS receiver and associated data display device (chartplotter or radar display) or by installing a complete Class A or Class B AIS system on your boat.

Displayed on a chartplotter or radar the AIS data received from vessels transmitting AIS information with either Class A or B AIS equipment will provide the helmsman of a vessel equipped with an AIS receiver or a full AIS system with an easy-to-interpret view of nearby traffic (usually out to at least 6 nm) showing the movement vector of other vessels and, depending on the equipment used to process the AIS data, the point, time and distance of closest approach. This information can be invaluable in your effort to avoid and evade the other guy, a particularly useful tactic since you can't be sure he knows that you are nearby.

An AIS system transmits a comprehensive set of vessel position and navigation information on the VHF marine radio frequency band to other vessels and shore stations within radio range. The system, initially designed to reduce the likelihood of collisions between reporting vessels, is in addition being used to provide harbor traffic control systems with real-time vessel position and movement information and for maritime domain awareness - the identification and monitoring of vessel traffic in and around U.S. territorial waters. (Efforts are also underway to develop a long-range tracking capability for commercial vessels on the high seas.)

AIS information is sent "in the clear�VbCrLf (unencrypted) and can be received by any vessel equipped with an AIS receiver. The received data stream can be decoded and displayed on a compatible chartplotter or radar, providing the user with navigation safety information that will be of overwhelming value to vessels navigating in crowded waters and especially for recreational mariners who, for various reasons, will be the "give way�VbCrLf vessel in virtually every close approach situation.

The decoded AIS messages present the mariner with a "picture�VbCrLf of the relative position of all reporting vessels in range. The nature of the picture can vary from basic target identification, range, bearing and display of the target's maritime mobile service identity number (MMSI), to an on-chartplotter or radar screen display of icons of all of the AIS-equipped vessels in range. Even in its most basic form the range and bearing information will greatly simplify a complex navigation challenge. The availability of the MMSI of each target vessel makes voice communication quick and easy; just enter the MMSI into the VHF/DSC radio, select a working channel, press the radio's "enter�VbCrLf or "send�VbCrLf key and you will have the immediate attention of the watch stander on the selected vessel.

Two sets of information are transmitted by the AIS: dynamic movement data and static information related to the ship's registration, dimensions and destination. The dynamic or maneuvering information is sent at intervals between two and 10 seconds (depending on the speed of the vessel) when it is underway and once every three minutes when at anchor. The static information is sent every six minutes. Transmissions alternate between VHF channels 87B and 88B.

The dynamic data includes the sending vessel's MMSI, its navigation status (underway, at anchor, not under command), direction and rate of turn, speed over the ground (�0.1 knots), position accuracy statement (indicating if differential GPS or other methods are being used to refine the GPS position), longitude and latitude (to 1/10000 of a minute), course over ground (relative to true north �0.1�), true heading in degrees and a time stamp indicating UTC to the nearest second. The typical chartplotter display of the reporting vessel's icon will show its position and velocity vector. The vessel's identity and the remainder of the information are usually displayed in a callable data block since continuous presentation of the information could clutter the display.

There are two types of AIS equipment: Class A, which meets the requirements of the IMO carriage requirement, and a less complex Class B, which provides similar functions but reports less frequently, once every 30 seconds at speeds less than 14 knots, compared with every 10 seconds for Class A equipment. Class B AIS omits the vessel's IMO number and call sign, does not transmit ETA, destination, navigational status, rate of turn or draught. In addition, a Class B AIS need not be able to transmit text safety messages or binary messages.

Access to the flow of AIS broadcast information can be of inestimable value to any mariner. AIS information overlaid on the radar will change the undifferentiated maggots on the screen into named targets, greatly simplifying what is often a confusing picture and supplementing whatever ARPA or MARPA capability may be available. The velocity vector projected from another vessel will provide an at-a-glance picture of the navigation situation. Many of the AIS data processing programs incorporated in chartplotters compute the point, distance and time of closest approach, assisting the assurance of a safe passage. The frequently updated navigation picture will clearly display the effect, if any, of your maneuvers on other vessels.

There are three ways to obtain the benefits of access to AIS data. Install a Class A or Class B AIS setup or install an AIS receiver, which only receives AIS information, but does not transmit your own vessel information. The Class A or Class B AIS decision is fairly straightforward: Class A equipment prices range from about $4,000 upward, Class B units are available for about $1,500. AIS data transmissions alternate between 161.975 MHz (channel 87B in the U.S.) and 162.025 MHz (channel 88B in the U.S.). The receivers in Class A and Class B AIS equipment monitor both channels, ensuring that they receive all the transmissions from each vessel in range. Receive-only systems are available in both single-channel and dual-channel configuration with single-channel receivers available for less than $200 and dual-channel units available for $440 and up.

The functional difference between the single-channel receiver solution and the more costly dual-channel unit is the frequency at which the AIS data displayed on the chartplotter or radar is updated. The lower-cost, single-channel receiver will provide half the updates from each reporting vessel (since it can't hear the transmission on the second frequency). Dual-channel units will provide all the updates.

Depending on the speed of your vessel, the effect of missing half the updates, which are transmitted at least every 10 seconds (vessels moving faster than 14 knots transmit every two seconds), can be relatively minor. For example, a boat moving at 6 knots will have traveled 202 feet during the 20-second interval between receipt of successive data transmissions from a vessel moving at 14 knots or less. High-speed vessels will obviously benefit from using a two-channel receiver in the same way they benefit from the use of high-rotation-speed radar scanners. (When using a single-channel receiver a delay of as much as 12 minutes can elapse between updates of the static data, however, that information is not likely to be critical in assuring safe navigation, especially since the vessel's MMSI, the key to establishing radio contact, is part of every navigation data transmission.)

The vessel receiving the AIS data must have a GPS and a decoding and display device. Some of the AIS receiver packages include the GPS; others rely upon the GPS associated with the vessel's chartplotter. Some of the receiver packages are marketed with chartplotter software designed to run on a general-purpose computer (versions are available for both Windows O/S and Macintosh computers). The data output from the receiver is usually in NMEA 0183 format, however the data rate is 38.4K baud, not the more familiar 4,800 baud. If a multiplexer is needed for connection to the display system it must be able to handle the higher data rate. (The MiniPlex-Lite NMEA multiplexer can handle the high-rate data and provides a trouble-free interconnect means for all types of NMEA 0183 data streams - Alternatively, a Keyspan USB to serial adapter.)

So, which types of vessels are required to have AIS capability? Carriage of Class A AIS equipment on SOLAS vessels is mandatory for all ships of 300 gross tons and upwards engaged in international voyages, cargo ships of 500 gross tonnage not engaged in international voyages and virtually all passenger ships. In addition to the SOLAS requirements AIS is mandatory on other vessels operating in U.S. waters, including towing vessels longer than 26 feet and powered with 600 hp or more. While recreational vessels are not required to carry AIS equipment they may install either Class A or Class B equipment designed primarily for use on small craft.

Excellent examples of what the user will see on the chartplotter are available on various chartplotter software manufacturers' Web sites. An interesting overview of the use of AIS by the world's commercial fleets can be viewed at the AISLive site ( - you must register to use the site). Shine Micro ( - registration also required) also provides a real-time view of AIS reporting vessels in portions of the Pacific Northwest, Pacific Southwest and Northeast U.S. The display for Port Townsend to Victoria, B.C., can be particularly interesting, although the site does not provide access to the tabular data for each reporting vessel that are available on board an equipped vessel.