Anyone that has been reading marine electronics WEBlogs lately knows that AIS (Automatic Identification System) is a hot topic. AIS consists of a transponder fitted on most ships within navigational waters of the US and high seas. AIS that is used in shipping is known as AIS Class A. The transponder provides identification and GPS positional data for the ship it is installed on. This information is then used to ensure the safe passage of ships, as well as enhancing port and harbor security.
AIS is not required for pleasure boaters, however the private boatowner can purchase an AIS receive-only system that can pick up the AIS Class-A information transmitted by the ship. This provides important safety information if you boat in an area of shipping, since you will know each ship's destination, course, and speed. That information can be used to help the boater stay clear of any shipping.
Recreational boaters can optionally install an AIS transmit/receive system (AIS Class-B) intended for recreational boats. The signalling in Class A and Class B systems differ, as potentially, many more AIS Class B transmitters can exist, as well - there are more recreational boats than ships. The Class B signalling is designed to limit congestion on the VHF channels dedicated for its use. Further discussion is limited to Class A receive-only AIS systems as this is the system most recreational boaters will likely want to install in their boat.
AIS Class A is mandated by the US Coast Guard for most commercial shipping and passenger vessels on coastal waters in the US, international voyages, and even on the Great Lakes. AIS position information is transmitted every few seconds by each ship. Other less critical information, such as the ship's name is generally transmitted once every 6 minutes. AIS information may include:
As a pleasure boater, should you be operating in the vicinity of shipping, it goes without saying that shipping can be a real hazard. For example, I use AIS because the access to my marina is via a shipping channel . It can be very difficult to enter or exit my marina if a ship is in the channel, so AIS allows me to plan my departure and arrival when there are no ships nearby.
To receive AIS information, you need an AIS receiver, antenna, and a display unit. AIS Class A is transmitted on one of two VHF channels. Ships may transmit on either or both VHF channels, and may alternate between channels. Normally this would require a 2 channel AIS receiver, however, single channel receivers exist on the market that automatically scan each AIS channel in succession. An inexpensive and popular single channel AIS receiver is Smart Radio's SR-161 receiver, imported by Milltech Marine. There are many other AIS receivers on the market - single and dual channel, but the Milltech receiver is one of the least expensive, and sufficient for recreational boating.
AIS uses the maritime VHF band, so a standard VHF antenna can be used. Finally, the output of the AIS receiver is NMEA0183 data; therefore, either a PC with AIS software or AIS capable chartplotter may be used to display the data.
Since AIS information is relatively new for pleasure boaters, only a handful of chartplotters can display AIS data. However, as time goes on, more and more chartplotters are including this feature, including some models by RayMarine, Garmin, and others. My RayMarine C-Series display with the latest firmware update can display AIS data, and this is the system I used.
Some thought must be given to connecting an AIS receiver to a chartplotter. If the receiver is the only device connected to the chartplotter, it's a straightforward connection as shown here:
It may be likely that your chartplotter is already using its NMEA0183 port for other purposes. In this event, you may wish to install a NMEA Multiplexer designed for AIS receivers, which are tailored for the various speed requirements of the AIS receiver. Manufacturers of thes multipluxers include Actisense, Brookhouse, and ShipModul. Interconnection of these devices are beyond the scope of this article, however, the multiplexer providers typically have an installation guide that will provide all of the information you will need.
I am using is a standard 3ft VHF stainless steel whip antenna mounted atop my radar arch for the AIS receiver. I have found a 10 to 15 mile range is possible with this setup.
When I initially powered up the system, I was astonished that three AIS targets appeared. The screen above shows my home port on Lake Michigan with one ship in the channel, and two ships out on the lake. Each ship's current position is shown, along with a vector showing the current heading. If a ship is headed in the direction of your vessel, two additional pieces of information become available, the CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach).
While a other brand displays may show information a bit differently, the basic information should be similar.
This ship is currently 6.049 miles from my vessel, and if we both maintain our current speed and heading, the closest point of approach will occur in 18 minutes and 33 seconds. However, if you notice the angled point on the target icon, the ship is currently turning to Port.
Remember that AIS positional data is transmitted by each ship, so the accuracy on the display is determined by the accuracy of the GPS units on each vessel.
The full range of AIS information can be obtained by simply selecting the AIS target.
Real time AIS World Tracking Map
www.navcen.uscg.gov/enav/ais/default.htm((US Coast Guard AIS overview)
www.millitechmarine.com (makers of inexpensive AIS receivers)
www.brookhouse.com (NMEA0183 multiplexers)
www.shipmodul.com (NMEA0183 multiplexers)
www.actisense.com (NMEA0183 multiplexers)