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Author Topic:   Marine Mobile Service Identity Assignment
BellBoyBob posted 09-18-2008 09:38 PM ET (US)   Profile for BellBoyBob   Send Email to BellBoyBob  
I am considering the purchase of the new SH 850 hand held radio with internal GPS to use as a back up radio on my cruiser. My main radio is the SH GS3000X Matrix. For the GPS location to be transmitted to another radio, a MMSI number must be assigned to the hand held radio. Since I already have a MMSI assigned to the main radio, do I need another MMSI or should I use the same MMSI for the hand held? I would also like to be able to use this second radio to test the performance of the main radio and to see if they really can sent position via GPS polling from one radio to the other. Would identical MMSI numbers preclude that?


jimh posted 09-18-2008 09:55 PM ET (US)     Profile for jimh  Send Email to jimh     
Bob--That is a good question. Don't consider this answer authoritative.

I can't see any reason why you could not have more than one radio with the same Marine Mobile Service Identity (MMSI). The MMSI is related to the vessel, and it seems reasonable that a vessel might have more than one radio.

As for the question of how a radio will behave if it receives a digital selective call from another radio with the same MMSI, the best way to know would be to try it. I realize that with the limit of one MMSI programming in most of these new radios, if there were a problem, you would have to return one of the radios to be re-programmed.

I wonder if manufacturers know for sure exactly what would happen in the case of a DSC radio getting a call from another DSC radio with the same MMSI. I suppose a really careful reading of the full DSC specification might turn up some recommendations about what was supposed to happen, but, even if you found that information, there is no guarantee that a radio would actually follow it.

I have been thinking about getting a second DSC radio, too, and for the same reason--so I can test my first radio. I was planning on programming it with a different MMSI.

Now this brings up a related question: Can one person legitimately have more than one MMSI if they only have one vessel? There is no doubt you probably can register more than one, at least via one of the free MMSI registrars.

And another question? Is it worth the cost to get a FCC Station License for your vessel and thus get an MMSI that is registered in the international database?

Bella con23 posted 09-18-2008 10:07 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
This is a excerpt from Navgear -

“The MMSI number is assigned to the vessel, not the radio. All radios, whether fixed installations or handheld and no matter how many, permanently associated with the same vessel should all use the same MMSI number.”

The only problem I would see is if you were to take the portable on another boat. Rescue would be looking for the registered boat.

Fishmore posted 09-19-2008 12:55 AM ET (US)     Profile for Fishmore    
I used the same MMSI on both of my fixed radios when I had my larger boat. But I never tried to call one from the other as they were only about 6 feet apart. However, when I had one of the ships I work on call me both radios sounded off that there was an incming call. I think that what would happen is that when a radio transmits it is not recieving so there would be no problem when you called from one to the other using the same MMSI. By the if you read the manual on the handheld I believe there is a section that recomends that you turn off the DSC function if you take the handheld on a boat other than the one the MMSI is registered to. You do this for the same reason you do not take a full sized EPIRB from one boat to another in both cases the number is registered to the boat. The new personal EPIRBS are registered to the owner so you can take those wherever you go and I do.
BellBoyBob posted 09-19-2008 12:56 PM ET (US)     Profile for BellBoyBob  Send Email to BellBoyBob     
Well, the thing to do then is to purchase the SH x850s and see what happens with identical MMSI numbers assigned to both radios on board. If there is an issue, I could send the hand held back for code erasure and try a new MMSI.
Chuck Tribolet posted 09-19-2008 01:11 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The problem I see is that when someone calls you, both radios
would reply and might step on each other. Probably with
a 25W fixed mount and a 5W handheld, the fixed mount would


jimh posted 09-19-2008 02:27 PM ET (US)     Profile for jimh  Send Email to jimh     
When a DSC radio receives a DSC Message with its MMSI, it does not automatically go into transmit and reply. That functionality has to be programmed into the radio, if it is desired. For example, if Vessel A sends a Remote Position Poll request to Vessel B, and Vessel B happens to have several DSC radios with the same MMSI, they will all receive the message. None of the radios will automatically reply or acknowledge the message, unless the operator has selected that behavior in the radio DSC set up.

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