The Odd Case of the Group MMSI
[The fourth number is the International Maritime Organization (IMO) number. it is assigned to big ships]
An individual MMSI identifies a ship's radio installation. In a sense it is the "digital" embodiment of the ship station call sign (e.g. Reboot's radio call WDB8435 identifies the same equipment as the MMSI 336 958 630.) Since automatic information systems (AIS) transmitters are radios it is also used to identify an AIS transmitter.
In a digital selective calling enabled radio one can "ring" another ship or shore station by using the DSC function and the called station's MMSI rather than making a voice call (Reboot, Reboot, Reboot this is OtherBoat, OtherBoat, OtherBoat over.) An alarm (ring ring) will sound on the called station's radio. This can be more efficient. It is also very useful when a ship is bearing down on you and not answering your radio calls! With the advent of AIS this has become much easier because one of the data fields of an AIS message is the other ship's MMSI.
MMSI numbers are a nine digit number constructed with a three digit country code (e.g.366 for the United States, a 5 digit unique number, and the number "0." The trailing zero identifies this as an "international" MMSI. In some countries the authorities have permitted other organizations to issue MMSIs for in country use. They will not end with the number zero.
What if one is participating in an event like a rally and would like to call multiple boats simultaneously? Or if one wants to place a DSC call to the Coast Guard but any Coast Guard station, not a particular station. The MMSI system takes this desire into account with "Group MMSI" functionality. A group MMSI will ring all of the radios that have been programmed to respond. Unlike the individual MMSI that is "burned" into your radio and AIS group MMSIs can be added and deleted at will.
A group MMSI is constructed by starting with a leading zero. Next one might enter the country code followed by a 5 digit unique identifier (e.g. 036 6xx xxx.) Since I am about the participate in the Salty Dawg Rally I thought it might be kind of cool if the rally had a group MMSI.
Here is where the story gets strange. I contacted the FCC to ask how one gets a group MMSI assigned. They had no knowledge of group MMSIs. I contacted a couple of different people at the FCC just to be sure that I had not stumbled upon someone who was not informed. None of them had a clue. Here we have this great functionality that does not seem to have a way of being implemented. Strange. I did a little more research and discovered that I am not alone in not being able to get the answer to my question. The common practice appears to be to make up a properly formatted group MMSI and use it.
Fair winds and following seas :)