Collision Avoidance 2.0
By Evans Starzinger
How Does AIS Work?
An AIS transceiver has an integral GPS and a VHF transmitter. It broadcasts the vessel's name, GPS course, speed, and, if enabled, data on its route and cargo on a VHF frequency (most units are dual frequency using VHF 87B and 88B). AIS units on other vessels receive that data and, assuming both vessels continue on at their current course and speed, calculate when and how close they'll be at their closest point of approach (CPA). The units sound an alarm if they will be closer than some minimum preset distance.
In a potential collision situation, the AIS units can do two things that radar can't. First, AIS can "see" around corners, over islands, and through sea clutter that would block a radar return from a vessel on a collision course. Second, it gives you a specific vessel name to hail on the VHF radio. You'll be answered much more quickly and be sure the vessel answering is the correct vessel if you hail it by name than if you call, "Vessel near the Bay Bridge." Some AIS units will interface with your VHF and allow you to make a DSC (digital selective calling) call to a specific target, but our experience has been that a VHF 16 or 13 (bridge-to-bridge channel for ships) voice call is answered more frequently and faster than a DSC call.
If we had had an AIS when we were entering the Chesapeake, the alarm would have gone off long before we realized the danger, probably before we entered the first traffic separation zone. We might even have been able to plot a course behind the freighter by angling across the outbound channel. If we'd gotten into a collision situation, we could have done one of two things. As the give-way or burdened vessel, the vessel that had to change course to avoid the collision, we could have stopped at the edge of the outbound channel and allowed the freighter to pass before proceeding across. Or we could have hailed the freighter using its name, visible on our AIS screen, and asked what its captain preferred that we do to stay out of the way.
In situations where there's more room to maneuver, the give-way vessel should take clear and decisive action to avoid the collision. Where it's unclear which is the give-way vessel, you should hail the other vessel on VHF 16 or 13 and discuss each other's intentions and on which side you'll pass one another.
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