AIS Question

[redhead]

Last weekend we were meeting friends and they told us all they could see was our MMSI on AIS.

Yesterday we saw friends from another boat and they asked us if we had been to Sucia Island last week because they saw our boat name there on their AIS.

Why would we appear only as our MMSI number to the first set of friends?

Ohh...and please speak in words of one syllable, I struggle with this stuff.

Thanks all.

[Dsanduril]

A Class B AIS (the type used by most small yachts/boats) transmits your position information about every 30 seconds (simplified). That transmission identifies you by MMSI only. It has your position, speed, course, and MMSI. That's all.

About every 6 minutes or so there is a single transmission from your AIS that contains ship information; MMSI, vessel name, length, beam, draft, a few other details.

Many AIS systems "cache" that information - once they receive your ship data they will store it so that the next time you show up with a position transmission they can look you up in their own database using the MMSI. Until the other side receives your ship information all they will know is your MMSI. The ship information is low priority in the AIS system, and is not broadcast frequently, so it can be easy to miss or just not even sent in a "crowded" AIS area.

The ship data will also only be received if you put it into your AIS. In your case it seems at least the name is there because at least one friend received it.

Here is the message info

And the info on message priorities

[redhead]

Perfect! Now I can appear to know what's going on...

Thanks a lot, this is why I stay on CF.

[Brewgyver]

IIRC, the boat name is in the extended data string that is only transmitted every 6 minutes. The more important collision avoidance data is transmitted every 30 seconds for a Class B, if you're going over 2 knots.

I had first installed AIS before going on a rally, and was playing with finding the other boats. I noticed that when I first select a target I immediately get the MMSI, SOG, etc, but not the boat's name, but if I leave it selected, or go back to it later, then I can see the name.

Of course, if the boat is far enough away to be on the fringe of reception, a receiver might not have it acquired long enough to get the extended data.

[Brewgyver]

I guess it's also worth noting that the extended data string, including the ship's name, is also only transmitted at 6 minute intervals on Class A systems. Their collision avoidance data will transmit as often as every 2 seconds, and of course they transmit at 12.5 watts, vs Class B at 2 watts.

[Dsanduril]

If you want a little more...

AIS transmitters all have to share the airwaves (somewhat like mobile/cell phones). There are only two AIS channels and everyone has to use them. The more boats there are around with AIS the more crowded they get.

There are two classes of AIS; A and B. A is the big ships, B is the little guys. (There is also now a "B+" that is more like A, but that starts to complicate things).

A transmitters make a reservation for time on the air and then they use this reservation to make sure they broadcast. B type units try to find an empty hole to broadcast. If they can't find one (because everything is in use) they don't broadcast. So it can be quite easy, in a crowded environment for a B-level "static data" (ship name) message to get skipped because it is low priority.

Think of it like a restaurant. A transmitters have their people call and get a good table. Even if they don't show up for some reason the maitre d' will still hold their table for a while because, well, they're A-list people. B people stand in line and hope they can get a table. With the new B+ system the B people can get a reservation, but it may be a table by the bathroom or the kitchen door if there are too many A-list clients around.