Voyager’s traffic control display

Collision avoidance is an area of big interest to voyaging mariners. Since most yachts are not large enough or constructed of the right materials to show well on radar, most voyagers have a keen interest in radar reflectors, both passive and active (see Shout it loud Issue 144, March/April 2005). And now there is another tool for collision avoidance called the automatic identification system (AIS). A voyager equipped with an AIS unit can turn his or her electronic chart software into a traffic control display that presents detailed collision-avoidance information about commercial vessels in the area.

A number of AIS “listening” units have become available recently for yachts, such as the SafePassage AIS from SeaCAS in Kent, Wash., the AIS engine from Nasa Marine Instruments in England (sold in the United States under the Si-Tex brand name), the RadarPlus unit from SeaLinks in Port Ludlow, Wash., and the SLR100 and SLR200 units from Comar in England. Using an AIS listener, voyagers can obtain automatically, via VHF radio, the name, dimensions, position, course, speed and cargo of commercial vessels in their immediate area. According to Fred Pot, president of SeaCAS, the recent interest in AIS by marine electronics companies is good for voyagers. “It’s great to have this competition because it will raise awareness of AIS,” Pot said.

The impetus for AIS comes from International Maritime Organization rules requiring AIS capability on merchant vessels. The U.S. Coast Guard has ruled that all vessels of 300 gross tons or greater must carry AIS transponders in U.S. waters. These required units are pricey Class A AIS transponders (Class B units have yet to be introduced by any manufacturers). They can broadcast their own AIS data and receive it from other vessels.

The yacht-type units, like the SafePassage, are listen-only and don’t have the capability to broadcast their position on the two AIS-authorized frequencies 87b and 88b (161.975 and 162.025 MHz). But for voyagers, it’s the ability to see what the big guys are doing that makes AIS such a useful tool.

A Class A AIS-equipped merchant vessel picks up its position, speed, rate-of-turn and heading information from its instruments, then adds information such as its MMSI number, vessel name, cargo and even destination. This information is gathered into a 168-bit package. The shipboard AIS works out a time slot with nearby AIS-equipped vessels using a sophisticated self-regulating time-sharing scheme. Then in its allotted 26 microseconds, the AIS transponder automatically broadcasts its info every two to 10 seconds on the two AIS VHF frequencies. So, just as air traffic controllers use aircraft transponders to see both the radar blips of aircraft in their area and also the aircraft’s name and altitude, mariners equipped with an AIS listener can see similar information about ships in the vicinity. Since VHF is line-of-sight only, voyagers will only see nearby ships – the ones most important to a voyager in a yacht.

The result of listening in to this stream of data can be seen on a voyager’s electronic chart. Ship icons are displayed as they move across the chart, along with their navigational data, including closest point of approach and time to closest point of approach. This information makes collision avoidance a great deal easier for voyagers. This information also can be displayed on the screens of certain radar units that can accept the data, like the Furuno VX2.

One particularly elegant version of an AIS listener intended for the recreational market is the SafePassage AIS. This unit looks like a chubby VHF antenna with a single wire emerging from its base. Inside the white fiberglass package is a VHF antenna and, in the base, circuit boards that allow the unit to receive both AIS VHF channels and GPS signals as well. The SafePassage unit is a 16-channel GPS receiver that can access WAAS and EGNOS differential GPS signals via a patch antenna built into the top of the fiberglass enclosure. The single wire is a USB cable that can be plugged into a computer’s USB port. The SafePassage also draws its power via the USB cable. Should you wish to connect the SafePassage to a dedicated chart plotter, a serial connector is available.

The SafePassage unit works with most electronic chart/navigation software, such as the Cap’n, Fugawi Marine ENC, MaxSea Yacht, Nobeltec Admiral, Transas, GPSNavX and more. It has a suggested retail of $1,250.

Unlike the all-in-one antenna approach of the SafePassage, the AIS engine from Si-Tex/Nasa, the RadarPlus from SeaLinks and the SLR100 and 200 from Comar are all black-box-type units that require external GPS and VHF signal inputs. The RadarPlus is a dual-frequency device that can listen to both AIS channels and lists for $999. The Si-Tex/Nasa black box AIS listener is a single-frequency unit that sells for $499. (Si-Tex/Nasa also makes a an AIS listener called the Si-Tex Radar, $699, that displays AIS info on an LCD screen.) The Comar SLR100 is a single-frequency unit that sells for $525, while the SLR200 is a dual-frequency receiver that sells for $1,000.

By Ocean Navigator