How does my AIS know that my MOB is an emergency.

[Privilege]

I've just about finished setting up my Vespa 8000 AIS. It's working correctly and now I'm just fitting the separate alarm. I have an Ocean Signal rescue me MOB device for my lifejacket, (which I haven't yet set up). If the MOB is activated how does my AIS know that it's an emergency and why does it not recognize it as just another vessel in the water? I'm assuming that the activation sets off some emergency signal that isn't just a normal AIS signal but I can't find anything in the instructions or on the web. Will my alarm just continue to activate until I turn it off?
I know that it can also send out a DSC signal but I'll learn about that after I've understood the AIS stuff. Thanks in advance.

[MartinR]

Yes, the MOB device sends out an Emergency AIS signal that MODERN AIS receivers can recognise.

[Privilege]

Thank you. I wonder why the alarm needs four NMEA connections (two in and two out) as well as three relays? It's just an alarm box activated by the AIS? I just can't seem to find a proper set of instructions.

[GordMay]

When activated, an Emergency Position Indicating Radio Beacon (EPIRB) transmits its details on 406MHZ and, if GPS-enabled, the vessel’s position to within 100m.
This signal is relayed by satellite to a base station, then the Maritime Rescue Co-ordination Centre (MRCC).
Simultaneously, the EPIRB transmits a continuous signal on 121.5MHz, which allows search and rescue services to home in on its location using radio direction finders.
PLBs are basically miniature EPIRBs, for personal use. They transmit on both 406MHz and 121.4MHz.
Both EPIRBs & PLBs must be registered with the Maritime Rescue Co-ordination Centre (MRCC) in your country.

[a64pilot]

I believe though that he as an AIS MOB alarm, that just notifies any local AIS receiver and will not transmit to a satellite and therefore won’t trigger the rescue coordination center.
It’s a different set of frequencies, it uses two freqs, but I don’t know why.
We have one on each of our vests, it’s the best plan in my opinion for self rescue, (for the boat you fell off of to find you).
https://en.m.wikipedia.org/wiki/AIS-SART

The built in alarm on my Vesper would wake the dead

Other than the time we tested it, it has never alarmed in the five years or so that we have had it, so false alarms must be exceedingly rare?

[MartinR]

Originally Posted by GordMay

When activated, an Emergency Position Indicating Radio Beacon (EPIRB) transmits its details on 406MHZ and, if GPS-enabled, the vessel’s position to within 100m.
This signal is relayed by satellite to a base station, then the Maritime Rescue Co-ordination Centre (MRCC).
Simultaneously, the EPIRB transmits a continuous signal on 121.5MHz, which allows search and rescue services to home in on its location using radio direction finders.
PLBs are basically miniature EPIRBs, for personal use. They transmit on both 406MHz and 121.4MHz.
Both EPIRBs & PLBs must be registered with the Maritime Rescue Co-ordination Centre (MRCC) in your country.

NOT PLB, the OP has an AIS MOB 1

[MartinR]

The external alarm should normally only be a buzzer? To be connected via the power supply connector.

One AIS in-out for heading sensor
One AIS in-out for Chartplotter

If you have an NMEA 2000 network, you do not need to connect these.

[billknny]

Originally Posted by GordMay

When activated, an Emergency Position Indicating Radio Beacon (EPIRB) transmits its details on 406MHZ and, if GPS-enabled, the vessel’s position to within 100m.
....[SNIP]....
Both EPIRBs & PLBs must be registered with the Maritime Rescue Co-ordination Centre (MRCC) in your country.

All of that is completely accurate, and completely irrelevant to an AIS based MOB device like the OP was asking about.

The alarm box you are installing can respond to multiple alarm sources, hence the multiple nmea inputs. It has outputs because it re-transmits the NMEA0183 to other devices if needed, since NMEA0183 is really designed as a one-to-one protocol.

Once you get everything setup, you'll find the actual MOB device has a "test mode" where it will broadcast a test signal. Since this will show up on everybody's AIS display within range, it is a good idea to NOT test it in a crowded harbor since there are always a few dim bulbs who miss the fact that the signal is labeled "TEST"

Wherever you test it, it is good manners to broadcast a "Securite'" announcement ahead of time that you will be activating an AIS emergency beacon in test mode.

[boat driver]

The PLB AIS unit (MOB1) is operating on the 161.975 and 162.025 frequencies.
It transmits a AIS address starting with a "9", which alerts AIS units within the area.
It is a 1 watt transmission. transmitting seven times a minute.
Range is approximately one mile, although three may be expected.
To maximize the benefit, have the vessel AIS antenna at minimum ten feet above the water to permit line of sight minimum efficiencies.
Be aware,
the AIS receiver must be programmed to receive the digit "9" as well as the plotter if location is to be displayed. Units prior to 2015, particularly lower priced models, may not have the updates available.
Reliance on this equipment should be utilized as part of a bigger MOB system plan. A good number of criteria must be met by the electronic hand offs to make it work and MANY EXPECTATIONS/APPLICATIONS ARE OVERCONFIDENT of the reliability.

At minimum, test the unit via activation to assure it activates your vessel's AIS system.
Additionally, be aware that specifically the MOB1 DSC function is entirely dependant on the LOCATION it is programmed- I.E. in the US it will only activate the MMSI of the mother vessel, In Canada it is not a permitted function period, in many other countries it is an all ships DSC call.