Marine Electronics Forum - Backup VHF on boat: Same MMSI number as primary?

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Flot
03-24-2012, 05:55 AM
The MMSI FAQ on Boat US's page implies that you can, but are not required to use, the same MMSI number on your primary and backup VHF.

I am just curious if anyone has a strong opinion one way or another.

Does an MMSI signal in any way identify the radio when sent? In the case of an actual emergency is there an advantage to having two radios with either the same or different numbers?


SaltyG
03-24-2012, 05:57 AM
All radios for a given boat should have the SAME MMSI number, this includes multiple fixed radios and also any handheld radios if primarily used on that boat. The radio's dont identify themselves, they just send the MMSI number, which is used to identify the boat, so the USCG can look up the information. So use the same MMSI number.

Flot
03-24-2012, 06:09 AM
That seems to be the recommendation but as far as I understand it, NOT a requirement.

If the radios do not identify themselves, then it seems having DIFFERENT MMSI numbers would be preferable, as if both were set off and received, it would be a practical guarantee that it was not a malfunction or operator error? Similarly if the two radios had different GPS data but the same MMSI, I am not sure how that would look to the receiving station?

The downside of course would be maintaining and updating two MMSI accounts. My last MMSI account I never updated in 3 years so not sure this is a downside.


capecuddy
03-24-2012, 07:46 AM
This issue is a conundrum....

Personally, I followed the conventional wisdom that the handheld should have the same MMSI and the vessel's VHF/DSC MMSI. I find that having the same identity helps me in checking out the functionality of both radios since each radio will acknowledge the actions of the other radio. It is hard to change your MMSI, since most radios give you one shot at setting it and you've got to return to the vendor to get it reset.

Operationally, I'm less sure it's a good idea to have them be the same. If the USCG followed up with a Position Request to a Man Overboard situation where the handheld went with the MOB, the CG would get two replies at two different positions with the same MMSI. I don't know what that would mean or how it would be handled.

The more I think about it, the more I wonder if they shouldn't be separate MMSI's.

If you have a personal handheld and are frequently on different vessels, I would definitely have a unique and independent MMSI for the handheld.

GTBecker
03-24-2012, 09:55 AM
I associated my DSC handheld to the dinghy (and use a different MMSI) because my Standard Horizon DSC radios cannot exchange position data if the MMSIs are the same. Consequently, at least with these radios, the dinghy cannot report DSC position to the boat unless the MMSIs differ.

Gary999
03-24-2012, 10:00 AM
I have had the same questions.

Here is another. Should you activate 2 VHF distress signals at one time? Would they interfere with each other being on the same frequency?

Flot
03-24-2012, 10:33 AM
Here is another. Should you activate 2 VHF distress signals at one time? Would they interfere with each other being on the same frequency?

The normal way of dealing with this with electronics is to send the signal immediately, then wait a slightly random amount of time, then send the signal again, etc. Unless you hit both buttons at exactly the same time on the same brand of radio I doubt this is a problem.

Given a real emergency I would likely hit one button and hit the other within the next 30 seconds, and hope that one of the two radios gets a signal off.

One radio has a built in GPS, the other has a NMEA connection. My assumption is that the CG would get two requests with ever so slightly different GPS data, however when they looked up the MMSI registration issue it would be obvious that it was the same boat/owner so it would not cause any special confusion on their end. I also assume that they would get the message that TWO requests from TWO radios went off and therefore it is not a false alarm.

I think I'll be submitting a 2nd MMSI number request.

caltexflanc
03-25-2012, 07:29 AM
It makes zero sense to me to have two different MMSI's for the same boat. I'm curious if they will even allow it. Almost like having two different license plates and registrations for your car. By having the same MMSI attached to your radios and EPIRB, with the same address and contact information, you avoid unnecessary confusion. When the CG sees multiple signals coming in with the same MMSI they know there is a big issue with that boat. The also know if it is coming from the EPIRB vs the DSC system on a VHF. Having a second MMSI does absolutely nothing to help the situation. I got my MMSI from the feds as part of the voluntary ships station license, so I could use it in other countries. I then added it to my EPIRB registration. Whether the BoatUS MMSIs allow for multiples assigned to the same boat, I don't know.

Now, having a separate MMSI for your tender, that makes sense, it is a separate boat with a much different description and aids the search effort. The same applies to a PLB, that is assigned to you, not the boat.

capecuddy
03-25-2012, 08:02 AM
If you always use the handheld with the same boat, I can agree with you most of the way.

What will USCG do with double waypoints whenever they do a Position Request of your MMSI and the MOB has the handheld?? Propably no big deal unless the MOB has drifted far away.

Both radios will respond to the Position Request near simultaneously on Ch70. One could interfere with the other.

Candidly, I'm not sure this has been thought out at the system level.

If two MMSI's in the same general vicinity are reporting the nearly same position, I would think that's less confusing to the system AND less likely to interfere between the boat and the handheld as they would be polled separately.

saxe point
03-25-2012, 08:08 AM
If you have a personal handheld and are frequently on different vessels, I would definitely have a unique and independent MMSI for the handheld.

That's why I got a separate MMSI for my handheld - to take on my friends' boats that don't have any VHF. And I honestly don't think that separate DSC distress calls from my fixed VHF and my handheld are going to cause confusion because they will be rare and happen in very defined circumstances. First, both fixed and handheld MMSI's are registered in my name, so the CG will probably figure out it's me. Second, the handheld is always a backup to my fixed VHF when I am in my boat, so if there's a problem, and I have activated both, the GPS coordinates will be the same (if the fixed VHF has died after the first distress call) or close (if I am out of the boat and sending a distress DSC call with the handheld). Where I am, DSC distress or urgency calls are rare, and I would hope and expect that the CG would throw all its resources to getting to both the coordinates because they will be so close.

The Canadian authorities allow you to register your handheld MMSI as "any Canadian vessel 'Smith' handheld", which is how I registered mine.

Rode Warrior
03-25-2012, 08:13 AM
been through this thought process also. big boat has 2 DSC and dingy/ditchbag has the handheld DSC (Standard Horizon) and figure that when the Coasties look up the multiple messages, they will note the same owner name and figure the rest out.

capecuddy
03-25-2012, 08:27 AM
So Rode, how are your MMSI's configured?

My biggest worry is interference between the HH and Fixed.

Abbor or Fairbanks, if you see this, what do you guys think about the interference potential between a fixed and HH with the same MMSI answering a DSC poll of any kind? 6W vs 25W but it could persistently and consistently garble, like being stepped on verbally.

fairbank56
03-25-2012, 08:35 AM
This scenario is currently in discussion by the GMDSS task force. The UK has already addressed this issue and requires that you obtain a portable license for the handheld and you are issued a separate MMSI for it.

DSC radio's do not auto-reply to position request's unless you have selected the auto-acknowledgement setting in the setup menu. The default for this is OFF.

You do not have to repeatedly press the distress button to send an alert. The radio will continue to automatically do so every couple minutes or so until it has received an acknowledgment from the CG or RCC.

These radio's are FM and FM receivers exhibit what's known as "capture effect". The weaker signal will be completely suppressed in the receiving radio. If your handheld and fixed mount at 25W send at the same time, the fixed mount will get through.

Eric

capecuddy
03-25-2012, 08:45 AM
I'm not going to have my position request reply disabled as I'm floating away from my vessel or it has sunk. That's a bad time to figure out menus on a HH to re-activate it.

Looks like I need to figure out how to flash my HH radio and get a separate MMSI.

Thanks!

fairbank56
03-25-2012, 08:53 AM
The CG is not going to do a position request via DSC to you. When you send a distress call, your position is sent with it. They already have your position. After they have acknowledged your call via DSC they will be contacting you via voice on channel 16 which your radio has already switched to. If they did decide to poll you for some reason and you have auto-reply OFF, your radio will sound an alert and prompt you with a message asking if you want to reply manually. You just press a button to do so.

Eric

capecuddy
03-25-2012, 09:20 AM
The CG is not going to do a position request via DSC to you. When you send a distress call, your position is sent with it. They already have your position. After they have acknowledged your call via DSC they will be contacting you via voice on channel 16 which your radio has already switched to. If they did decide to poll you for some reason and you have auto-reply OFF, your radio will sound an alert and prompt you with a message asking if you want to reply manually. You just press a button to do so.

Eric


Although I get the logic of your answer and the fact that it would prevent interference, not sure I want to have to be screwing around with "protocol" while I am trying to stay afloat. (MOB situation.)

If my wife goes overboard, she ain't gonna get that. I don't want the HH position to be constantly stepped on by the fixed unit. If I go overboard, she's not going to be able to set up a position request from our vessel (which would work btw even if the MMSI's are the same. Fixed can't receive while transmitting the Pos Req.)

fairbank56
03-25-2012, 10:45 AM
If I go overboard, she's not going to be able to set up a position request from our vessel (which would work btw even if the MMSI's are the same.

Have you tried this? Will a radio allow you to send a DSC request to itself? If it does, does the other radio respond to a request from a radio that has the same MMSI as it's own? Good to know info and may be radio specific.

Eric

saxe point
03-25-2012, 11:41 AM
Eric, how do you know so much about this stuff? Not just the DSC stuff, but the technical radio stuff?

LMychajluk
03-25-2012, 12:12 PM
... I don't want the HH position to be constantly stepped on by the fixed unit. If I go overboard, she's not going to be able to set up a position request from our vessel (which would work btw even if the MMSI's are the same. Fixed can't receive while transmitting the Pos Req.)

If you go overboard (presumably w/ the HH?), why would she need to set up a position request? By activating the DSC Emergency call from the HH, won't the HH put the HH's position on the boat's radio (and connected chartplotter), asuming integrated GPS on on the HH? Won't the boat's radio also relay the DSC emergency call (w/ the HH's position) until the CG acknowledges it?

If you don't want to initiate the emergency call, you should also be able to initiate a Position Report from your HH to the main unit (i.e., in case of a HH in a dinghy). But, I think any MOB situation must be reported to the CG anyway, so might as well use the emergency button in that scenario...

fairbank56
03-25-2012, 12:14 PM
Eric, how do you know so much about this stuff? Not just the DSC stuff, but the technical radio stuff?

Iv'e been a marine electronics service technician for 37 years. Everything from installation to component level PC board level repairs. I'm one of the last of a dying breed. Few shops do the kind of repairs I do any more.

Eric

fairbank56
03-25-2012, 12:27 PM
Won't the boat's radio also relay the DSC emergency call (w/ the HH's position) until the CG acknowledges it?...

Absolutely not! Class D DSC radio's are not capable of relaying calls.

Eric

capecuddy
03-25-2012, 01:55 PM
Have you tried this? Will a radio allow you to send a DSC request to itself? If it does, does the other radio respond to a request from a radio that has the same MMSI as it's own? Good to know info and may be radio specific.

Eric

I have actually done this. I set the ACK ON for Position Request on my Handheld and transmitted a Position Request to the shared MMSI from the Fixed unit. Of course, it can't receive a request from itself since it is actively transmitting. But the HH does receive it and responds.

The fixed is GX-2150, the HH is HX-851. The GX-2150 sends the "waypoint" to the HDS for display as well.

An unsolicited Position Report from the HH is received by the Fixed and the WP is shown on the display via the GX as well. I did this from the back of the boat and it was enough of a difference in position to show on screen.

For reference, when either the fixed or the HH transmits a DSC Test, both record the response for the test if the transmission is strong enough for both. I can create a test where only the fixed unit acknowledges (beeps) in response to the DSC Test reply. For both to beep, I use the USCG shore station MMSI which hits Chatham. For only the fixed to beep, I send the DSC Test to USCG SENE in Falmouth.

capecuddy
03-25-2012, 02:07 PM
If you go overboard (presumably w/ the HH?), why would she need to set up a position request? By activating the DSC Emergency call from the HH, won't the HH put the HH's position on the boat's radio (and connected chartplotter), asuming integrated GPS on on the HH? Won't the boat's radio also relay the DSC emergency call (w/ the HH's position) until the CG acknowledges it?

If you don't want to initiate the emergency call, you should also be able to initiate a Position Report from your HH to the main unit (i.e., in case of a HH in a dinghy). But, I think any MOB situation must be reported to the CG anyway, so might as well use the emergency button in that scenario...

Eric's earlier post cited the fact that the ACK for Position Request on the handheld is defaulted to OFF, a true statement. Thus when the wife's HH receives a PosReq, it would not respond unless she answered a prompt on her radio as "Yes". No way would that happen if she is in the water.

A DSC Emergency from the HH would, I suspect, put its position on the HDS (via the fixed unit) just as with a response to a Position Request OR an unsolicited Position Report. You can't test the DSC Emergency, so that's why I'm saying I suspect.

As Eric answered, these radios are not supposed to relay an emergency request since the originating unit will repeat the Emergency until it receives and ACK from the CG.

LMychajluk
03-25-2012, 04:14 PM
Eric's earlier post cited the fact that the ACK for Position Request on the handheld is defaulted to OFF, a true statement. Thus when the wife's HH receives a PosReq, it would not respond unless she answered a prompt on her radio as "Yes". No way would that happen if she is in the water.
.

True enough, but you can easily change the setting on the radio to automatically acknowledge position requests. Some consider this a 'privacy' issue, but unless you're broadcasting your MMSI (maybe as part of your boat data w/ a AIS transciever), you can only request positions from MMSI #'s in your radios Address Book, or one that you enter manually.

A DSC Emergency from the HH would, I suspect, put its position on the HDS (via the fixed unit) just as with a response to a Position Request OR an unsolicited Position Report. You can't test the DSC Emergency, so that's why I'm saying I suspect.

No, you can't. But if tests with the Position Request and Position Report functions are successful, I'd be fairly confident in the Emergency Call function.

As Eric answered, these radios are not supposed to relay an emergency request since the originating unit will repeat the Emergency until it receives and ACK from the CG.

I stand corrected. I thought all DSC radios included the ability to relay distress signals.

BTW, what plotter are you using? I have the same radios (SH 2150 and HX851), and am using them w/ a Simrad NSE.

saxe point
03-25-2012, 10:45 PM
Iv'e been a marine electronics service technician for 37 years. Everything from installation to component level PC board level repairs. I'm one of the last of a dying breed. Few shops do the kind of repairs I do any more.

Eric

That explains it!

capecuddy
03-26-2012, 07:34 AM
BTW, what plotter are you using? I have the same radios (SH 2150 and HX851), and am using them w/ a Simrad NSE.

I have an HDS-10. I had to move just before the NSS gear came out last year and had waited a month for my 3G. Always a moving target...



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