One-channel-at-a-time receivers, another Class B problem?
Well, whereas I stirred up a bit of a Class B AIS hornet’s nest yesterday, I may as well keep at it! It seems to me that another repercussion of Class B’s 30 second (at best) dynamic data rate is that the inexpensive “single frequency” receivers are only going to see Class B targets once a minute (at best). I put “single frequency” in quotes because I’m realizing that the nomenclature for these receivers has gotten pretty confused.
For instance, when I characterized the EasyAIS receiver as “dual frequency” back in April, I meant that it could listen to both AIS frequencies simultaneously. Now I realize that I was probably wrong about that, though EasyAIS is not exactly forthright about its receiver's specs. The company site calls it a “real 2 channel receiver”, which, when you think about it, does not mean that it listens to both channels simultaneously, and an English install manual I found (PDF here) doesn’t mention reception modes at all.
Meanwhile one retailer, YachtBits, also calls it a “double superheterodyne receiver” which sort of sounds like parallel reception on both channels, but another notes that “every few minutes it switches automatically between both channels” (Busse Yachtshop). And Y-tronic, a reliable source in my experience, says that the EasyAIS “monitors both AIS frequencies by alternating between both channels”, further noting that that it is quite well made. Conclusion: I don’t think that the EasyAIS listens to both channels at the same time, and therefore will miss 50% of the AIS messages sent out in its area. I also think that the industry should settle on some terms for these receivers and use them to properly inform customers.
PS, 12/8: It’s good to see that NASA is clear about its AIS Engine: “The unit can receive ships on either the A or B AIS channels. In default setting it alternates between the two channels.” But then in the very next paragraph NASA claims that the Engine outputs to a “NMEA 2000 input.” It absolutely does NOT, and, moreover, I’ve heard that its serial NMEA 0183 output is so flawed that it won’t drive the opto couplers that are a feature of many good multiplexers. Sigh.