Saturday, February 20, 2016

Introduction to AIS

The Automatic Identification System (AIS) is a worldwide electronic navigation system designed to prevent collisions and to facilitate maritime traffic control and security. It was developed for commercial vessels, but any vessel can take advantage of this important service on some level. It is now a key component of the electronic instrumentation of all vessels, recreational and commercial.

AIS shows where all ships and other commercial vessels within VHF radio range are located—they are required to have AIS—as well as all of the other vessels who have chosen to add optional AIS transmitters. AIS signals include the vessel’s dynamic data: Lat-Lon, COG-SOG, and true heading (requires a separate heading sensor), as well as certain static data such as name, MMSI (Maritime Mobile Service Identity), call sign, vessel type, length and breadth, and GPS antenna location. Signals from Class A broadcasts (discussed below) also include voyage data including destination, ETA, cargo, rate of turn, present draft, navigation status (moored, underway, restricted maneuverability, constrained by draft, etc).

You have two ways to take part: You can install a receiver alone to learn about traffic around you, or you can install a transceiver, to both see AIS traffic and broadcast your own AIS information as well. Both are optional for recreational mariners, but transceivers are mandatory for commercial traffic. Receivers alone cost $60-$300, whereas transceivers are in the $800 to $3,000 range. At minimum, an AIS receiver should be considered standard electronic navigation equipment on all recreational vessels. Reception alone is very easy to set up; some new VHF radios include AIS output as an option, sharing the same antenna. Transceiver set-up is required to be assisted by a certified technician.

There are a few simple, but important, basics: First, we need a way to display the targets. The higher end receivers include a display of text data—which is not so convenient if that is all you set up—but all receivers have an output in NMEA format. The simplest solution is input the NMEA signals to your electronic charting system (ECS). This might be some integrated package of instruments and displays, or without that, you can run echarts on a laptop. AIS receivers have a USB output that goes directly into your computer. The free OpenCPN program, for example, has many convenient AIS display options, as do all commercial echart programs.

Next, there are two classes of AIS signal transmission: Class-A transmission is the higher-end type required on ships and other commercial vessels, and Class-B transmission, available to vessels not required to carry Class A. All AIS receivers display both A and B signals, so the equipment decision is mostly receive only or transmit and receive. Recreational mariners can choose Class-A or Class-B transmission. Class A transceivers offer higher performance, but they cost more, so the Class-B transceivers are more popular with vessels not required to carry Class-A.

Vessels required to carry AIS Class A include: all commercial vessels over 65 ft, towing vessels over 26 ft with horsepower over 600, vessels certificated to carry more than 150 passengers, most dredging vessels, and all vessels with dangerous cargo.

Vessels required to carry AIS Class B if they do not carry Class A include: fishing industry vessels, and some passenger vessels that do not use traffic lanes nor have speeds over 14 kts. Vessels that would otherwise be required to carry AIS, but whose operations are restricted to a 1-nmi radius, can apply for an exemption to the AIS requirement. Other previous exemptions to the carriage rules all end on Mar 1, 2016. Vessels required to carry AIS must turn them on not later than 15 minutes before getting underway.

In short, ships, tow boats, and most larger commercial vessels use Class A, as do some fishing vessels, but most fishing vessels and most recreational vessels will be using Class B. Table 12.19-1 compares the specifications of Class A and Class B.


AIS can also be of great assistance in basic chart navigation in that prominent aids to navigation (buoys, lights, bridge pillar marks, etc) often send out AIS signals that show up on your echart display in the proper locations (labeled ATON). [I will add a note shortly on AIS ATON Symbols.]

Collision avoidance is also greatly enhanced by AIS because most echart programs read the AIS dynamic data of the targets and they know your own COG and SOG, so they can compute and display for you the closest point of approach (CPA) and the time to the CPA (TCPA) based on present conditions. Then as either of you alter course, you watch how this changes.

Search and rescue operations have special AIS modes that locate vessels and aircraft involved. There are even small, personal AIS transmitters that you can carry and activate if overboard so you show up on all local AIS receivers.

AIS reception also enhances your radar watch in the ways listed below.

AIS Synergy with Radar

1. AIS can find and communicate with targets hidden behind land or around corners that radar cannot see, and then plots them on an echart as if seen by conventional radar.

2. A radar echo can be identified or contacted by vessel name and vessel class rather than using a generic hail.

3. Target interactions (CPA, TCPA) can be improved because the target vessel is broadcasting performance data such COG and SOG and even a rate of turn when turning.

4. Extended tracking range is obtained since VHF communications reach out farther than typical radar ranges and thus passing arrangements can be made long before the vessels meet.

5. Ship target intentions will be more clear, and will help with maneuvering decisions, because the target’s destination, ETA, cargo, draft limitations, etc. are broadcast along with other AIS data.

6. Some ECS radar displays and all commercial vessel radars have an input for AIS signals so they can display the AIS targets overlaid on top of the radar targets.

There are numerous AIS sources online and in mobile apps that present near-live AIS signals for most waters of the world. In principle we could use our smartphone with a 4G connection to actually see the AIS targets around us when we are underway. This however, is not at all a dependable or safe approach. There is often a large delay between actual AIS reports and the ones we see in these apps, and this could lead to serious navigational errors.

A sample of AIS targets displayed in an echart program is in Figure 12.19-1, with a detailed view in Figure 12.19-2.
Figure 12.19-1. AIS targets viewed in OpenCPN. Active or selected target is the one whose information is being displayed (marked with a square box), else the targets are called sleeping targets, which is misleading—they are sleeping only in that you have not chosen to read the data. Echart programs often have ways to annotate or color the symbols beyond the standard format. Projected vector lines show COG and SOG. The above are set to show 3-minute projected positions. Sometimes we can detect current when the true heading line is different from the COG. Destination information when present is more often wrong than right. This has to be manually updated and often gets overlooked.

Figure 12.19-2. ECS display can be set to show actual target size and precise location when zoomed in. On the right is our vessel, also scaled to right dimensions and precise location.  We can see that the target heading and COG are different. This is viewed with OpenCPN.

These notes were adapted from our text Inland and Coastal Navigation, 2nd Ed. We will follow up with a report on several specific AIS receivers, including this neat unit that is made here in Ballard, a few blocks of Starpath HQ, called the dAISy. It is powered via the USB that connects it to your computer.

A dAISy with a ducky (not included). It does include an adapter to a standard antenna connector.

References (We are lucky here, there are excellent references online.)

USCG Nav Center AIS section.

Wiki on AIS.

Added 3/23/17. Good overview looking ahead: AIS Meets IoT: How Technology is Set to Transform Global Ocean Trade and Supply Chain Efficiency


Unknown said...

Just my thought- I considered the reporting schedule of Class A vs Class B AIS. I chose to install a Class A system on our somewhat fast powerboat. Our normal cruising speed is 20kts+ with a top speed of over 45kt. The rapid position broadcasting is critical. 30 seconds and we could be a 1/4 a mile from we are actually reporting. I don't believe the Class A/B differences get evaluated often enough. Sailboats & displacement boats do fine with a Class B. The cost differential is large, the installation is slightly more difficult, but I believe the safety due to the increased accuracy is very much worth the trouble.

David Burch said...

I agree completely. If the decision is made to be seen, I think all vessels might consider the advantages of A over B, especially in the ocean where the range will be much farther as well. In fact, a legal option of any viewer is to shut off showing the B-class signals, which could be attractive to a ship in a very busy harbor, but I do not know how often this might be done. The IHO standards require that any filter set on the display must be shown prominently so it is not forgotten.

Unknown said...

Hi David,

I am an active duty Naval Officer and graduate student working on my master's thesis. My topic deals with trying to create an automated system for predicting future ship movements using historical AIS data. I really like this picture you use to demonstrate what AIS is and how it works, and I was wondering if it is ok with you if I use this same picture for my thesis? Please contact me at if you have any questions.

Unknown said...

Hi David,

I am an active duty Naval Officer working on my master's thesis at the Naval Postgraduate School in Monterey, CA. My topic deals with using historical AIS data as input to an automated model that can help predict future ship movements. I really like your picture on this blog because it helps explain what the user sees when they're interacting with AIS. Can I have your permission to use this photo for my thesis?
Please contact me at with any questions.