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Old 30-06-2019, 10:32   #1
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AIS versus PLB - a dumb question

I have an EPIRB, properly registered and needed to choose between a PLB (personal EPIRB) and a personal AIS. I went with the latter. Although only about a 3 mile range I can generally get that close even if someone fell overboard a while ago. My chartplotter gets the signal and I can go to the victim. With the PLB, as I understand it, the signal goes by satellite and the Coast Guard initiates the search. Unless I have a satphone (which I do) there does not appear to be a way for me to determine where the person is and the process appears cumbersome at best. I think it far more likely I will find the person much more quickly than a search and rescue team perhaps hundreds of miles away. Am I missing something here?
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Old 30-06-2019, 11:13   #2
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Re: AIS versus PLB - a dumb question

PLBs let them locate your corpse. Not good for much else IMHO.
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Old 30-06-2019, 11:15   #3
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Re: AIS versus PLB - a dumb question

You're not missing anything. Both EPIRBs and PLBs are 'requests for help' to the local Search & Rescue authority. That's their function and purpose. S&R (Coastguard in US I think) have people ready and waiting for the call. No different than Police, Fire and Ambulances.

PLBs are NOT personal EPIRBs.

I'd add when someone falls overboard that's a very serious event, and if it were me in the water, I'd want the maximum people looking, especially in cold water. And sadly jt11791 is on the money. Much of the time when someone falls in it is a body recovery operation. But body recovery is a far better result for the family than no body
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Old 30-06-2019, 11:33   #4
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Re: AIS versus PLB - a dumb question

The MMSI for a personal AIS starts with 972. If you see one of those, that is a person in distress.
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Old 30-06-2019, 11:39   #5
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Re: AIS versus PLB - a dumb question

You're not missing anything.


For MOB, the personal AIS beacon is the killer app, at least if you're not single handed. The better ones have DSC, like the Ocean Signal one.


I have had one of these since they first came out, starting with the old fully manual dildo-shaped McMurdo one with the safety pin like a hand grenade. This year I upgraded to the Ocean Signal one, which is automatic and adds DSC.


But I also have a PLB in my life jacket. Not at all for MOB -- where I sail, in cold water, I would be dead long before anyone would rescue me based on a distress signal from the PLB. But rather to have always on my person a means of sending a satellite distress message, in case in a chaotic abandonment we somehow get separated from the EPIRB. Or in case something happens on land somewhere. Or in the dinghy.


When we were in the Arctic last summer, I required everyone on board to keep a PLB on his or her person.


And contrary to what some have written, my own opinion, FWIW, is that a PLB is a reasonable substitute for an EPIRB in some conditions. It works the same way and should get the same response, but requires aiming the antenna and has half the battery life.
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Old 30-06-2019, 11:41   #6
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Re: AIS versus PLB - a dumb question

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Originally Posted by jt11791 View Post
PLBs let them locate your corpse. Not good for much else IMHO.

What about if you're in the life raft and you didn't manage to take the EPIRB with you in the chaos of abandoning? Or if you don't have an EPIRB, or if your EPIRB malfunctions?


Or if you're exploring on land in a remote place, and fall and break your leg?
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Old 30-06-2019, 15:47   #7
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Re: AIS versus PLB - a dumb question

Great point several made about it being a backup to an EPIRB in case you leave home without it. I originally went with the isatphone instead of iridium go for a similar reason. With the iridium go you need both the hockey puck and a phone in order to transmit. I ended up later going with the Iridium and the inreach. A well thought out poster in another forum said he wouldn't have a liferaft "probably won't need it, if I need it probably won't work, and in any case if I need it and don't have it I won't regret it for long". I might think that way if I didn't travel with so many people I love.
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Old 30-06-2019, 15:51   #8
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Re: AIS versus PLB - a dumb question

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Originally Posted by Dockhead View Post
What about if you're in the life raft and you didn't manage to take the EPIRB with you in the chaos of abandoning? Or if you don't have an EPIRB, or if your EPIRB malfunctions?


Or if you're exploring on land in a remote place, and fall and break your leg?
I had a PLB packed in my raft the last time it was reinspected.
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Old 30-06-2019, 17:12   #9
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Re: AIS versus PLB - a dumb question

The big difference is that the PLB is asking someone else to help you. An AIS beacon is for your own crew to turn around and find you. At least, it is down here where the chances of someone else being nearby is slim to none.
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Old 30-06-2019, 17:28   #10
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Re: AIS versus PLB - a dumb question

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The big difference is that the PLB is asking someone else to help you. An AIS beacon is for your own crew to turn around and find you. At least, it is down here where the chances of someone else being nearby is slim to none.
Perhaps off topic but why isn't there a 406 receiver available that could decode the ID and GPS co-ordinates of the 406 data bust and place a marker on the plotter.
The technology already exists and the coding isn't secret. I can only speculate that the demand isn't sufficient.

The unit could even incorporate a 121.5 homer however antenna aspects would be more complicated and really the homing isn't the issue if the PLB / EPIRB is embedded with GPS.
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Old 30-06-2019, 17:59   #11
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Re: AIS versus PLB - a dumb question

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Perhaps off topic but why isn't there a 406 receiver available that could decode the ID and GPS co-ordinates of the 406 data bust and place a marker on the plotter.

The technology already exists and the coding isn't secret. I can only speculate that the demand isn't sufficient.



The unit could even incorporate a 121.5 homer however antenna aspects would be more complicated and really the homing isn't the issue if the PLB / EPIRB is embedded with GPS.
The main issue is that for a surface based receiver to be able to hear the 406MHz beacon signal emitted by a 6 inch antenna barely a foot off the water, the receive antenna would have to be within visual distance of the EPIRB to decode it. That’s why they use satellites to detect the EPIRB. One or more satellites always has a line of site path to everywhere on the planet. 406MHz doesn’t propagate far along the curvature of the earth.

A 121.5 homing receiver is likewise not much use unless it is in line of site with the beacon. Thus the useable radius at the surface is at best maybe 10-20 miles. Aircraft beacon DF receivers have a much greater range due to the height of the receive antenna. Which is why SAR aircraft carry them. But nowadays the 406MHz signal can give accurate DF making 121.5MHz mostly obsolete.
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Old 30-06-2019, 18:16   #12
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Re: AIS versus PLB - a dumb question

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Originally Posted by Paul L View Post
I had a PLB packed in my raft the last time it was reinspected.

I think that's very good practice
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Old 30-06-2019, 18:34   #13
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Re: AIS versus PLB - a dumb question

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The main issue is that for a surface based receiver to be able to hear the 406MHz beacon signal emitted by a 6 inch antenna barely a foot off the water, the receive antenna would have to be within visual distance of the EPIRB to decode it. That’s why they use satellites to detect the EPIRB. One or more satellites always has a line of site path to everywhere on the planet. 406MHz doesn’t propagate far along the curvature of the earth.

.........
Yes the range would be short but with a mast mounted 406 antenna, I would expect it to be in the order of 3 to 5 miles (maybe more, maybe not???). This would suffice for most MOB situations.

So very similar (I believe) to a personal AIS.

BTW What is the TX power of a personal AIS?
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Old 01-07-2019, 03:15   #14
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Re: AIS versus PLB - a dumb question

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Yes the range would be short but with a mast mounted 406 antenna, I would expect it to be in the order of 3 to 5 miles (maybe more, maybe not???). This would suffice for most MOB situations.

So very similar (I believe) to a personal AIS.

BTW What is the TX power of a personal AIS?
I think most of them are 1 watt when sending a MOB alert.

Yes, the range of AIS and EPIRB should be similar. However, the AIS PLB sends updates more frequently and most well found boats already have an AIS receiver integrated into the plotter making locating the MOB dead simple.

The EPIRB or PLB are not designed for rapid recovery of a MOB. That is what a personal AIS MOB device is designed to do. I have read several cases of PLBs in the open ocean being heard by SAR but never heard of a PLB alone resulting in recovery of a live MOB from open water. On land, many lost souls have been saved by a PLB.

If you are overboard in warm water where you hope to survive for hours best to have both PLB and AIS MOB alert. One is not a substitute for the other.
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Old 01-07-2019, 07:06   #15
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Re: AIS versus PLB - a dumb question

Thanks for all the great comments. Perhaps a dumb conclusion I draw: There is no real value in having a PLB. They are not very useful in live recovery (at sea). Several mentioned the good idea of having one in or for the liferaft, but if I'm having a second I'll spend a couple hundred bucks more getting a second EPIRB. Because I already have a satphone and inreach I think I'll stick with a single EPIRB. For years I've had the question but was afraid to ask because it seemed there must be a simple answer as to why they sell "Personal Locator Beacons". That implied to me crew recovery, not body recovery.
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