TWO DSC radios on boat - unique or common MMSI numbers?

jfm

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I have 2 DSC VHF sets, one on the mothership and one on the tender. Both are built-in, not handheld. I'm about to finalise the licence on the Ofcom website,, then punch the MMSI number into the hardware (which is once-only task; no going back once you've done it).

I am trying to work out whether I should (a) register/license them separately and have a unique MMSI number for each one, or (b) register them both on one licence and so have the same MMSI on both radios.

It seems I can do either, because the ofcom registration/licensing thing on the web is quite flexible.

Can anyone see a problem if I get just the one licence, and so input the same MMSI number into 2 separate VHF sets? I don't think the world will stop spinning if I do this, but am I missing something?

By the way, like probably 99% of people, I never use the MMSI feature anyway. If calling a mate, I would use ch16 or a mobile phone, but not his mmsi number. BUT, with MMSI radio on the tender, maybe I will use that to call the tender. What happens if I call the shared MMSI number from the mother ship? Will the tender radio get the call or will the mother ship VHF refuse to call its own number?

Any advice would be gratefully received, so I can finalise my license application
 

yesod

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i would image you'd want different mmsi numbers - otherwise things get confusing. eg if you want to request the position of the tender, but it has the same mmsi as the main station - what occurs? kiss principle rather than rely on the software boys to have thought about it...
 

aquapower

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I would have seperate numbers, what if you press the distress button while out on the tender and they go looking for mothership instead, I know they will have your position but posibly wont know what they are looking for
 

SteveE

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When you register the MMSi, do you not let them know the size of boat etc so they are prepared in case of emergency, so if the tender called up they would be looking for a 78ft Fairline and not a 8ft Tender?
 
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MapisM

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Assuming that a distress call from the tender is a VERY unlikely scenario, there are two other features to consider.
One is the selective call, and for that alone, I wouldn't even bother reading the manual to learn how to program ANY mmsi on the tender radio.
But since I suppose you'll have also a GPS on the tender, there's another DSC functionality that might interest you, and it's the possibility to get the rib position from the mother ship.
And if you're interested, I think - though I'm just speaking by common sense here - that a shared mmsi could create some conflicts/problems in this case.
 

jfm

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But since I suppose you'll have also a GPS on the tender, there's another DSC functionality that might interest you, and it's the possibility to get the rib position from the mother ship.

Thanks all. OK, you're all convincing me it makes sense to have 2 separate MMSI numbers.

MapisM, yes I have a garmin plotter on the tender, N2K-linked to the VHF. Can you tell me how to get teh tender's position from the mothership? I'm very interested in doing that. I was going to put an AIS Tx on the tender but if I can avoid that £500 expense/hassle I would like to hear how to do it. Thanks
 

MYAG

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I dont think the advice/thoughts above are correct. I am pretty sure the yacht's tender (or even life raft) should carry the same MMSI number as the parent vessel unless the craft is not normally fitted aboard or in davits etc, which in this case it is.
 

MapisM

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Can you tell me how to get teh tender's position from the mothership?
For the specific buttons to press, it depends on each equipment, but below is a nice Icom video showing how the system works from a logical viewpoint.
http://www.youtube.com/watch?v=b3UhzETSTsQ
Btw, on second thought, I actually can't think of a valid technical reason why it shouldn't work also with a shared mmsi.
Also, aside from regulatory requirements I'm not aware of (though MYAG might well be correct that there are some), there's another side advantage of having a shared mmsi: a totally redundant radio available on the mother ship, which could be used for distress calls with the correct mmsi even in the worst case scenario of a totally disabled boat due to a catastrophic electrical problem (heaven forbid!).
 
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MYAG

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For the specific buttons to press, it depends on each equipment, but below is a nice Icom video showing how the system works from a logical viewpoint.
http://www.youtube.com/watch?v=b3UhzETSTsQ
Btw, on second thought, I actually can't think of a valid technical reason why it shouldn't work also with a shared mmsi.
Also, aside from regulatory requirements I'm not aware of (though MYAG might well be correct that there are some), there's another side advantage of having a shared mmsi: a totally redundant radio available on the mother ship, which could be used for distress calls with the correct mmsi even in the worst case scenario of a totally disabled boat due to a catastrophic electrical problem (heaven forbid!).

I am not sure about being a regulatory requirement, more of a guidance/standard from GMDSS. BTW, it would also include handheld units and based on some theories for registration, searchers could be looking for a swimmer!

Actually, the point on knowing what to look for on receipt of a distress call is moot. All that is transmitted is the MMSI mnumber and the Lon/Lat, any good samaritan receiveing the signal would not know what they were looking for anyway, that info is only held at the homebase coastguard (Falmouth for UK) they should pass on assuming they are aware.
 

jfm

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Thanks MYAG. See your point. And thanks Mapis for the video. That woman was quite, er, earnest, wasn't she?

That "position request" would work for sure if I had 2 different MMSI numbers (if Garmin supports it, which i will check), but i guess it would work if i had the same MMSI becuae I'd think the mother ship VHF set will not receive the position request due to the fact it cannot receive at the same time as transmit. So the tender would receive the request, and radio its position back. And vice versa if you are in the tender and lose (how carelss...!) the mother ship

So I'm thinking I should have one MMSI, but I'll think about it some more. I get the idea above that if the tender transmits a mayday the authorities will think a 78 footer is in distress with 15peeps aboard and 7000litres of fuel, which could lead to quite a big response! Also, if the tender transmits a mayday I want it to be heard on the mother ship (and vv), and there could be a software glitch whereby the non-maydaying radio will not hear a message sent with its own MMSI number, if you see what i mean. It seems a bit of a 50-50 decision. I could email Garmin about the technicalities/software i suppose
 

MapisM

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i guess it would work if i had the same MMSI becuae I'd think the mother ship VHF set will not receive the position request due to the fact it cannot receive at the same time as transmit. So the tender would receive the request, and radio its position back. And vice versa if you are in the tender and lose (how carelss...!) the mother ship
Yep, precisely my second thought!
Btw, with a nice rib as you have, which allows some longish tours, in some places with many islands/bays (Croatia springs to mind) it's not so impossible to "lose" the mother ship, if you don't think to fix her wp in advance.
there could be a software glitch whereby the non-maydaying radio will not hear a message sent with its own MMSI number, if you see what i mean.
Naaah, I don't think so.
I do see what you mean, but it would be a nonsense to build in the VHF such control. The receipt of distress calls surely must be kept as "open" as possible.
Not that I know for sure, though. It would be neither the first nor the last case of "overprogramming" useless functionalities in electronic equipments...
 
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dt4134

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I don't believe a DSC call is anything like a circuit or packet-switched call from a mobile phone.

I believe it just sends out a message on Ch70 with the MMSI of the intended recipient which is received by all VHF radios in range but the MMSI is matched by only one (or more) radios that workout "Hey, that's me".

I'm sure someone with ooudles of time of their hands can check the standard to make sure (although it is probably an ITU doc so they'd need to work for someone who's paid the subscription).

So anyway, you should (according to my theory) be able to initiate a call from one DSC VHF to another with the same MMSI.

The main problem with two DSC radios is you'll have twice as many irritating alarms, but as most are caused by all stations broadcasts it doesn't really matter whether they have the same MMSI or not.

There is another approach, you could ask Ofcom, or even the Coastguard.
 
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timbartlett

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Actually, the point on knowing what to look for on receipt of a distress call is moot. All that is transmitted is the MMSI mnumber and the Lon/Lat, any good samaritan receiveing the signal would not know what they were looking for anyway, that info is only held at the homebase coastguard (Falmouth for UK) they should pass on assuming they are aware.
If you go to the ITU website http://www.itu.int/online/mms/mars/ship_search.sh you can type in the MMSI and get all sorts of information about the vessel, usually including its name, nationality, tonnage, sometimes the owner's name etc. etc.

The search process works the other way round, too: you can type in the name and get the MMSI.
 

Bilgediver

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If you go to the ITU website http://www.itu.int/online/mms/mars/ship_search.sh you can type in the MMSI and get all sorts of information about the vessel, usually including its name, nationality, tonnage, sometimes the owner's name etc. etc.

The search process works the other way round, too: you can type in the name and get the MMSI.
Thats cos you gave it to them when you registered...Dont forget you also gave them Grannies or mother in laws details as notifiable contact for emergencies.

WHen the coastie gets a DSC call he has to go to his puter to find out the boat details. Nice to know if it is JFM in his big Squadie or the QM2 :)

There is a big discussion going on at present regarding the numbering of other equipment such as PLB's and hand held Walkie Talkies so that there is reduced chance of confusion. There has already been confusion apparently where PLBS have been taken on camping trips inland and activated and not got the prompt response hoped for due to being considered rogue events. (not a marine environment).

I suspect that if two radios have the same MMSI this would not prevent one responding to an alarm call from another however I will ask a man who may know. Remember Joe from the Cserve sailing forum :) USA coastguard!
 

PaulGooch

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I'd go for separate MMSI's. If you set them up correctly, you can do position send.requests from/to both sets.

You program the mothership MMSI into the tender MMSI directory and vice-versa. If someone was out on the tender and you, on the mothership, wanted to know where it was, you bring the tender MMSI number up in the mothership VHF menu and send a position request. Having previously set the tender VHF to automatically send it's position, you know exactly where the tender is.

You've gone ashore for a meal in the tender and when you want to return to the mothership, fog has descended. Same deal, you bring the mothership MMSI up in the tender VHF menu, send a position request and the mothership position pops up on the plotter.

I'm sure there are other instances where it would be best if they had separate MMSI's too. Like for instance, someone who knows your MMSI sends a position request. A crew member is heading ashore in the tender, whos position does your friend get ?
 

PaulGooch

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I suspect that if two radios have the same MMSI this would not prevent one responding to an alarm call from another however I will ask a man who may know. Remember Joe from the Cserve sailing forum :) USA coastguard!

Both would still receive DSC alerts from 3rd parties as normal. Both would still be able to send DSC messages/alerts to 3rd parties.

3rd parties would not be able to distinguish between the two vessels.

Neither vessel could send or receive any form of DSC transmission to/from the other.

Be just like having two mobile phones with cloned SIM cards (in terms of DSC functionality).
 

MapisM

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I'd go for separate MMSI's. If you set them up correctly, you can do position send.requests from/to both sets...
I see what you mean, and that was in fact my first thinking behind post #5.
But I couldn't see why the position requests shouldn't work from/to both sets even if they share the same mmsi - hence my "second thought" in post #9.
If you can see any technical/logical problem, I'm curious to hear about it.

Mind, I agree with the conflicting scenario in your last paragraph, but in jfm boots I wouldn't be much worried about it.
 

MapisM

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Neither vessel could send or receive any form of DSC transmission to/from the other.
This is the point. Why not?
It shouldn't actually matter from a technical viewpoint.
I disagree that it's the same as two mobiles with cloned SIMs - that's a completely different kettle of fish imho.
 

PaulGooch

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This is the point. Why not?
It shouldn't actually matter from a technical viewpoint.
I disagree that it's the same as two mobiles with cloned SIMs - that's a completely different kettle of fish imho.

Let's say i have your MMSI number programmed into my VHF MMSI directory. I'd like to know exactly where you are, so i can come and empty your drinks cabinet. I bring up you MMSI number in my VHF directory and send a position request. This is sent out on CH70, but only your VHF set will respond, as it has the matching MMSI number. Your position will pop up on my plotter, assuming you have your system set to auto allow the position request, or you manually allow it.

If i also have JFM's MMSI number and do the same, what happens when his tender and mothership VHF sets both receive the position request and transmit their coordinates ? Pretty sure it's not going to work correctly/reliably.

How about if he brings the tender MMSI up in his VHF menu on the mothership and sends a position request ? The mothership transmits the call and it will be accepted by the tenders VHF because it matches the MMSI that the call is being sent to, the mothership cannot receive it, as it's transmitting. The tender then sends it's position, which the mothership alone receives, as the tender is transmitting.

Hmm, i was working through that last paragraph as i was typing. Looks like you're right, that side of things should work. I guess i was forgetting that each set that was transmitting wouldn't be able to receive at the same time.

All assuming you can also enter your own MMSI in the MMSI directory and the radio wouldn't "object" to transmitting a DSC call to that entry.

Still think i'd have separate MMSI's though. :)
 
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