Class B AIS

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It’s important to note that signals sent over SOTDMA are prioritized and slotted ahead of those sent over CSTDMA. “Stronger signals, which are usually closer, bump weaker signals,” Arroyo says. “This tends to impact Class B more [than Class A] because they [Class B signals] report at lower power, thus providing a weaker signal. And, because they report at a less [frequent] rate, it’s all in the law of averages as to who bumps whom.” As for areas where this can be a problem, Arroyo points to portions of the lower Mississippi River. As with all other areas, this situation is exacerbated when Class A users fail to update their navigational status when docked or moored, which reduces their broadcast rate to once every 180 seconds and frees up 18 to 90 additional slots.

Screen clutter is another obvious concern. As a ship enters a busy harbor, the number of AIS targets in a given range can quickly mushroom, potentially overpopulating the screen. To compensate, Arroyo says, users can zoom in for tighter views. Strong says Class A users can also apply several different filters to their AIS overlay views (but not their MKD), including ones that minimize targets based on their range, their potential collision danger or whether they are a Class A or Class B user.

While these filters may sound dangerous, it’s crucial to understand that radar operators rely heavily on both their AIS and radar, typically cross-referencing potential threats between their automatic radar plotting aid (ARPA) tool and their AIS. When judgment calls are required, captains, including Strong, trust their radar, which — with an update rate of once every two to 2½ seconds — is far more accurate than their AIS.

Boat speed is also a serious factor. Hughes and Strong say fast-moving vessels can outrun their Class B AIS target, creating confusion. “The one real problem that I have with Class B is its refresh rate,” Strong says. “It’s fine if you’ve got a sailboat going 6 knots, but … if you have a [Class B AIS target] doing 30 knots, in 30 seconds it’s gone a quarter-mile. That’s a substantial chunk of real estate that it’s just covered, without [updating] its information … and if it has made a course change in that time, you may not know it.”

Hughes says that when a fast-moving Class B vessel outruns its target, it creates a serious situation for a ship’s radar operator, because a single boat can appear to be two threats for a number of moments: its outpaced Class B target and its ARPA return. Sprinkle in other nearby Class B users, Hughes says, and the situation quickly compounds itself, especially in fog or rain — dead-common scenarios on both Puget Sound and Penobscot Bay. Because of this, Hughes recommends that Class A AIS units be used on all vessels that travel faster than 5 knots.

Eric Kunz, Furuno’s senior product manager, agrees with Hughes’ assessment. “There are times when a signal gets missed by a transponder,” he says. “If it misses [the Class B signal] the one time, you [might not] see it again for another minute.” Manufacturers can’t legally change the update rates of their Class B units, which must conform to IMO standards.

For recreational boaters, current Class B technology provides the security that heavy metal objects probably know that you’re out there, provided that they haven’t applied any filters. Arroyo says a new type of Class B AIS is expected out this summer with a reporting rate of once every 15 seconds. While it will be interesting to see how this affects channel congestion and target separation, a Class A system nearly guarantees that your yacht will be visible to nearby AIS users, without fear of a skipped transmission, target separation or class-type filtration. Irrespective of AIS, however, it’s crucial to remember Hughes’ point about maneuverability, and the unflinching laws of gross tonnage.