Reader Ron Heinze recently posted a comment on a Navagear item entitled MMSI Confusion: Group MMSI Numbers?. Here are Ron’s questions, along with my responses:
Can you program both your individual MMSI number AND the group MMSI number into your VHF?
Yes, absolutely. Group MMSI numbers are treated like an optional feature, whereas each vessel’s unique, individual MMSI number is critical to DSC functionality. None of the DSC functions work until you’ve programmed this number in, and many radios prevent you from changing it, or limit the number of times you can change it. Group MMSI numbers are much more flexible.
My Raymarine radios, for example, allow me to be enter up to three Group MMSI numbers. If you’re a member of more than three groups, you’ll have to switch those MMSI numbers in and out of the available slots. Luckily, unlike your vessel’s unique MMSI number, there are no constraints on the number of times you can replace Group MMSI numbers.
Can the group number be used to locate other boats in your group on your GPS chartplotter? How can that work? Does it locate every other boat with the group MMSI number?
Not exactly. You can use the Group MMSI number to hail all the other boats with that Group MMSI configured, and the individual vessel operators could then acknowledge the hail and switch to whatever working channel you specified in the hail. But they will NOT transmit position data automatically.
As far as I know (but I may be wrong), you can use DSC to request a position report from one vessel at a time. Think of it this way: If you sent a position request to the whole fleet, and they all responded, how would they know who should transmit first? Without some sort of traffic handling, their position report transmissions would walk all over one-another. DSC, as I understand it, isn’t quite this sophisticated.
Even if it did work that way, however, you would need to ensure that your DSC-equipped VHF radio was capable of sending the position information to your chartplotter over NMEA. Not all DSC-equipped radios can do this. Even many that can are not installed correctly, so there may be no way for the data to get to the chartplotter. For more on this sort of issue, look back at the following:
- Raymarine Ray49: First Impressions
- Crazy NMEA Wiring: AIS + 2-Way DSC
- DSC Positions Display On Chartplotter
I hope this helps, Ron!
