AIS (automatic identification system) transponders put out a signal that describes your boat and what it is doing to other ships. It also receives the same information from any other ship within range. Commercial ships are now required by most flag countries to carry a transceiver—for big ships it is a class A—so most large vessels will show up as well as a good proportion of smaller ones.
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I will second the ravings about the Vesper Marine Watchmate 850. I purchased mine in 2014 and have nothing but praise for it. I also purchase the Vesper Marine SP160 VHF Splitter at the same time and use my masthead antenna…never had a problem with either of them. It performed perfectly during my roundtrip voyage to Hawaii in 2016 from Washington State and was especially helpful when hove-to about 150nm off the coast of Washington during a gale monitoring commercial traffic in/out of the Strait of Juan de Fuca. I have it connected to my laptop and it provides both GPS and traffic information. It’s a keeper.
I also had a Vesper Watchmate 850 on my Bristol 41.1 and really liked everything about the unit. Unfortunately it failed to powerup at the beginning of last season and I learned unfortunately that Garmin had purchased Vesper and discontinued this great standalone AIS unit (I presume to remove a product competing with Garmin products). The only similar (but nowhere near equal) replacement standalone AIS unit I found was the ICOM MA510. I have found the ICOM unit to be far less user friendly than the Watchmate 850 which I miss dearly.