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.
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