PLB vs AIS MOB

[GuyC]

A couple of scenario's:

Crossing the channel at night in not very good visibility and one of your crew goes over the side. They have a PLB on them which they activate. Because of the sea state you can't see the strobe and try as you might you can't find them. Because they have a PLB, the signal is picked up by the CG and within 45 minutes a bedraggled crew member is picked up by the CG helicopter.

In this instance it would possibly be better to have a AIS MOB device which assuming you had an AIS receiver connected to your chart plotter would allow you to pinpoint the casualty as soon as they activated the device. Because it is a man overboard you could still send out a mayday but you would be able to recover the casualty rapidly.

If you were on your own and you went over the side, the kill cord would stop the engine but the windage on the RIB would put it tantalisingly out of reach. In this instance the PLB would be a lifesaver because your now empty RIB can't effect a rescue.

Is there anything on the market that combines the benefits of the two systems - PLB and AIS MOB?

Would a DSC handheld VHF fulfil those two functions as pressing the distress button would highlight it on you chart plotter and send a distress signal to the CG. It would obviously rely on everyone carrying a VHF which potentially isn't realistic.

[Poly]

Originally Posted by GuyC

A couple of scenario's:

Crossing the channel at night in not very good visibility and one of your crew goes over the side. They have a PLB on them which they activate. Because of the sea state you can't see the strobe and try as you might you can't find them. Because they have a PLB, the signal is picked up by the CG and within 45 minutes a bedraggled crew member is picked up by the CG helicopter.

I think you might be a little optimistic about the response time for a Helo to get airborne and mid channel... ...that might reinforce your argument though...

In this instance it would possibly be better to have a AIS MOB device which assuming you had an AIS receiver connected to your chart plotter would allow you to pinpoint the casualty as soon as they activated the device.

Is there a reason the CG couldn't relay you the coordinates the PLB was transmitting? It may be that they will have a time lag and won't get you close enough. Does the AIS require the casualty to be conscious? To my mind AIS is more suited to situations where there might be a delay in both getting help AND when you may not immediately realise the person had gone AWOL - e.g. on a yacht. it will be rare that someone can fall overboard on a rib unnoticed and if people are still on board will usually be able to begin a fairly localised search. Perhaps a laser flare, or the lifejacket with the "glow inside" is a better bet for finding them.

Would a DSC handheld VHF fulfil those two functions as pressing the distress button would highlight it on you chart plotter and send a distress signal to the CG.

Mid channel is a big ask for a handheld being used at water level (to get back to CG), especially if the chop is bad enough to hinder visual search. A VHF does have the advantage that the casualty can tell you if they can see you (a bigger, possibly better lit target).

Bear in mind though that I can't recall any incidents where someone had fallen overboard from a proper seat in a rib unexpectedly (i.e. when not pushing the limits or doing something silly) unless the whole crew got ejected at the same time.

[GuyC]

Thanks for the comments.

Interesting on the CG relaying the PLB co-ordinates. I wonder if they could convert it into an AIS type signal that would be visible on a chart plotter.

In terms of the modes of operation, I was looking at the Ocean Signal rescueme range.
The PLB is manual activation and the AIS MOB is automatic triggered by inflation of the lifejacket.

I'd never heard of the AIS MOB device until I started doing some research but I can see the point about them possibly being more applicable to a yacht miles from anywhere. I do like the idea of having a casualty's position visible on a chart plotter if you were to lose sight of them.

It would be good if there was a device that performed both functions simultaneously.

[BogMonster]

I think the PLBs only transmit coordinates periodically (to give reasonable battery life) but not sure how often - it may be every few minutes. Presumably the satellite system provides the CG with a time that the position was transmitted.

[gpsguru]

The response time of the CG to a PLB depends on a number of factors.....

The PLB waits about 1 min before sending a distress. This is part of the spec. If it was operated inadvertently, then 1 min gives the operator a chance to turn it off.

In the UK we are in the footprint of 2 GEO birds, so plenty of good quality coverage for us ...........

Once activated, the 406 beacon is digitally encoded with the serial number of the device and the LAT/LONG .............. this info is then retransmitted on the downlink to the earth stations. The station operator firstly see's the distress code followed by the serial number ....... the first 3 digits of the serial number give the country of registration, the next few digits give the device type and manufacturer, so they can quickly see if it is a PLB, or EPIRB,. In our case the Earth station will contact the UKMCC at RAF Kinloss, who will analyse the data.

Remember it is not just seafarers that use PLB's ...........

RAF Kinloss will then pass the info and mission command to the MCA at the Aeronautical Rescue Coordination Centre (ARCC) which is based at the National Maritime Operations Centre (NMOC), Fareham, Hampshire.

The MCA will then use the serial number info for the owner details ...... if the PLB is linked to a vessel with wih MMSI, then part of the mission will be to call the vessel ................ also remember that the PLB also has the aircraft distress homing beacon the the SAR craft use to locate it when close in.

IIRC, typical response times are in the 45 - 90 mins bracket .............


Looking fo a MOB in the English Channel in the dark is not something you want to waste time doing ............ the time would be better spent in activating SAR who can pile in the necessary resources quickly ........ looking for the MOB yourself is like looking for a needle in a haystack.

Try this ........... go out in a decent sea (1+ m @ 5s swell) and get a crew member tolob an A3/A4 orange buoy over the side and shout MOB........... the helm should hit the MOB on the plotter, then into a williamson turn and recover the buoy ........... on a good day that will be 15 - 30 min ............... now do the same at night, I doubt that you would recover the buoy.

AIS makes life a bit easier, but not a lot .............. at night you need searchlights and as many eyes as you can muster.

A PLB is by far the most sure fire way of being rescued in a timeframe that will save your life or that of your crew.

My PLB is in my lifejacket pocket ......... (SeaGo 3Dynamic 300N) .... also attached is a Knife and high power strobe ............

[Poly]

Gpsguru - you forgot to mention that the PLB which has been switched off needs to get a fix which will probably require downloading the almanac from the satellite- a time consuming process.

However I have no idea why it would take 15-30 mins to do a MOB drill on a RiB. Those are wafi times.

[SteveHall]

Just a question on the PLBs - I personally have one, and it's all registered with CG and all that with the CG66 as most of my time with the PLB is afloat.

Now in a few weeks I am off walking, so decided to pack the PLB as a just in case thing - but how do I tell them my walking details?

Will they still use the same emergency contacts?

[gpsguru]

Originally Posted by Poly

Gpsguru - you forgot to mention that the PLB which has been switched off needs to get a fix which will probably require downloading the almanac from the satellite- a time consuming process.

However I have no idea why it would take 15-30 mins to do a MOB drill on a RiB. Those are wafi times.

Most of the PLB's are 66ch receivers and most use 'Quickfix' software, so the aquisiton time will be quite a small window.

15 - 20 mins is what I have seen from inexperienced crews that dont practise like they should. In daylight, somebody who knows what they are doing will be alongside in 5 mins ........... nighttime is a whole different ball game

[lakelandterrier]

So.....broadcast the mayday and don't bother to look for the MOB???

Agree first thing is to hit the MOB on the plotter AND broadcast Mayday...THEN retrace course to search -as Mr Poly says that;s not 15-30 mins in a RIB - unless the MOB went unnoticed for a long time.

The PLB does require the wearer to physically activate it which means they have to be conscious.....the recommendation for high-speed boats like ribs is to hae auto-inflate LJs, as hitting the water at speed may render the MOB unconscious,in which case they won't be able to activate the PLB.

We practice MOB with a small cylindical fender weighted with chain - maybe 6" diameter so smaller than a human head and inflated LJ, and done it in in seas of 1m+ - with good course retrace and lookouts, - not lost "Fred" yet, but not as easy as flat calm

[gpsguru]

Originally Posted by lakelandterrier

So.....broadcast the mayday and don't bother to look for the MOB???

Agree first thing is to hit the MOB on the plotter AND broadcast Mayday...THEN retrace course to search -as Mr Poly says that;s not 15-30 mins in a RIB - unless the MOB went unnoticed for a long time.

The PLB does require the wearer to physically activate it which means they have to be conscious.....the recommendation for high-speed boats like ribs is to hae auto-inflate LJs, as hitting the water at speed may render the MOB unconscious,in which case they won't be able to activate the PLB.

We practice MOB with a small cylindical fender weighted with chain - maybe 6" diameter so smaller than a human head and inflated LJ, and done it in in seas of 1m+ - with good course retrace and lookouts, - not lost "Fred" yet, but not as easy as flat calm

We use an A3 buoy with weight attached ........... my experience is that many inexperienced cews get flustered and often disorientated ..........

In a real live situation the our 2 man crew procedure is .......... the helm hits the MOB on the plotter and the distress button on the radio whilst slowing the boat and getting a visual on the MOB .......... then a williamson turn and come along side the MOB ................with a 2 man crew once visual is lost the disorientation seems to set in ............ remember stop and LOOK ...........

However, if you are more than 30 miles out then the radio will not be effective other than alerting passing ship traffic of your situation.

Yes, part of the PLB spec is no auto activation which is allowed on EPIRBS.

[Poly]

Originally Posted by gpsguru

Most of the PLB's are 66ch receivers and most use 'Quickfix' software, so the aquisiton time will be quite a small window.

I'm not sure how that would work when it needs the almanac which only lasts for 180 days - I believe a fix will require 12.5 minutes to download it, albeit an initial distress message might have been sent first.

15 - 20 mins is what I have seen from inexperienced crews that dont practise like they should. In daylight, somebody who knows what they are doing will be alongside in 5 mins ........... nighttime is a whole different ball game

I have no idea what you are doing wrong. But in a "drill" situation when there is little delay between the MOB and the "shout" I can't see how it takes even 5 mins in a RIB.

Even if we assume 20s from the shout to taking any action (thats 10-20x longer than it should be), and we assume that you make a wide slowish turn - a radius of 100m (probably 3x what you could comfortably do on most ribs) - that is ~600m which at 18 kph* (<10knots) = 2 minutes; back on the reciprocal course and say 10s for the final approach. My kids can do that. So my kids would be back at you within 2.5-3 minutes... In a decent rib with a confident helm I'd expect to be back at an announced MOB in 1 minute - as I was in my PB2 course**, which I assume is normal.

I'm not disputing that at night it is a totally different ball game, and certainly without a LJ light it will be extremely difficult to find them, and operating at speed may no longer be the best plan.

*I picked this because it made the mental arithmetic easier - actually a higher speed turn is probably better. But also at this speed I can maintain steerage easily whilst still operating the VHF.

** The only time it took longer was when the casualty was quietly slipped over the side unannounced, and it took a good few minutes for it to be noticed.

[gpsguru]

Originally Posted by Poly

I'm not sure how that would work when it needs the almanac which only lasts for 180 days - I believe a fix will require 12.5 minutes to download it, albeit an initial distress message might have been sent first.


I have no idea what you are doing wrong. But in a "drill" situation when there is little delay between the MOB and the "shout" I can't see how it takes even 5 mins in a RIB.

Even if we assume 20s from the shout to taking any action (thats 10-20x longer than it should be), and we assume that you make a wide slowish turn - a radius of 100m (probably 3x what you could comfortably do on most ribs) - that is ~600m which at 18 kph* (<10knots) = 2 minutes; back on the reciprocal course and say 10s for the final approach. My kids can do that. So my kids would be back at you within 2.5-3 minutes... In a decent rib with a confident helm I'd expect to be back at an announced MOB in 1 minute - as I was in my PB2 course**, which I assume is normal.

Yes you are right about the almanac .......... I had to make a phone call to refresh my brain on the subject as a lot has changed since I retired from the industry ............

The global Cospas Sarsat system can get a rough fix on the origining beacon .......... I always understood that it was the LEO's (Low Orbit birds) that did this and it is time consuming (up to 90mins) but now I'm not so sure as the Copas Sarsat info says the following,

"Some Cospas-Sarsat satellites only relay the distress message transmitted by the beacon. Other satellites relay the message and also gather details about the signal that will enable computers on the ground to estimate the beacon’s location"

However, 12.5mins aquisition time is not great when you take into account other natural delays in getting the info through to chain of command to the CG ............ I am often out alone and fishing , for me the PLB could be a lifesaver. Properly dressed the survival time in UK waters is OK(ish) but not great.

Thinking about it, I have never really timed myself on an MOB shout ....... I was guessing 5 mins but I know its quick, so you could be right.

What I do know is that it is not practised enough and an inexperienced crews reaction is often horrendous when tasked to do it ! ...... I often wonder if their brain disappears out their bum crack when MOB is shouted

[gpsguru]

Originally Posted by Poly

I'm not sure how that would work when it needs the almanac which only lasts for 180 days - I believe a fix will require 12.5 minutes to download it, albeit an initial distress message might have been sent first.

As many of us use these PLB's, I think is important that we get to the bottom of this one ............

I had it in my head from my professional days that the above was not the case, but couldnt remember how or why ......... old git syndrome !

Last night I made a call to an old colleague who told me most of what we need to know and this was confirmed this morning by a call to Ocean Signal.


The cold start TTFF (Time To First Fix) for any marine beacon device is one of the quickest of any GPS device, mainly becuase it requires no time/date from the birds and also has a very wide mask angle . That means it is looking for birds through a 90 degree arc ............

Also to get the fix it requires NO almanac data and no Ephemeris data .......

The TTFF for most devices is typically 1 - 4 mins .......... the Ocean Signal PLB1 is sub 90 secs ............. I tested mine today and it got a fix in 65secs, and this is only the second test in 2 years !

Having looked at TTFF times the GME quotes 90secs and the ACR is also 90 secs ........ these will be worst case times.

Ocean Signal tell me that the first transmission at 50secs will not have encoded GPS data, but the 2nd and subsequent 50sec transmissions will have GPS data accurate to 10m (ish). It was pointed out that they will all be similar because a lot of them use the same GPS engine ! (Made in China)

I was sent a defintion of TTFF ...............

TTFF (Time To First Fix) is a measure of how quickly a receiver performs the signal search process. The search process, or ‘signal acquisition’, has two dimensions. The C/A code (The GPS SPS Coarse Acquisition ranging code) dimension associated with the replica PRN (Pseudo-Random Number) code, and the Doppler dimension associated with the carrier.
A Cold Start TTFF is defined as the time between application of power to a receiver and it obtaining the first valid navigational data point, when the following criteria are met:

• Time unknown
• Current almanac and ephemeris unknown
• Position unknown

When you do a GPS test on your PLB you can measure the TTFF .............

[Poly]

Reports today in maib report give some interesting idea of timescale to get accurate fix and then deploy resources. There were some added mixups in this case, but even then :

It took 18 mins from device being activated to Falmouth getting the coarse position information (5 miles off the actual). It took 6 minutes more for the fine position data to get from the epirb to the uk rescue services. It took 6 minutes from the local Mrcc being given coordination responsibility for them to request lifeboat and five more for helo support, then there is a further delay to page the crews and launch.

Best case the ALB would have launched 48 minutes after the epirb was activated.

[ShinyShoe]

Originally Posted by Poly

Reports today in maib report give some interesting idea of timescale to get accurate fix and then deploy resources. There were some added mixups in this case, but even then :

It took 18 mins from device being activated to Falmouth getting the coarse position information (5 miles off the actual). It took 6 minutes more for the fine position data to get from the epirb to the uk rescue services. It took 6 minutes from the local Mrcc being given coordination responsibility for them to request lifeboat and five more for helo support, then there is a further delay to page the crews and launch.

Best case the ALB would have launched 48 minutes after the epirb was activated.

MustRIB reported similar delays when he went in the drink on the way to the Round Ireland. I know there was a delay because there was some uncertainty if he had already arrived in Ireland (the rest of the fleet has but MustRIB was crossing later the same day). But I think we all imagine that pretty much the instant we press the magic SOS button a claxon sounds in Falmouth and a teleprinter prints off the details and a 54inch monitor instantly shows the location on a chart. The reality may not be quite so slick.

Its all dependent on the device being steady enough to get a fix and steady enough to send its data out too.

[Poly]

Originally Posted by ShinyShoe

Its all dependent on the device being steady enough to get a fix and steady enough to send its data out too.

The system is still based on faxes!
But even once it makes it to the UK there is quite a lot of time "wasted" before anyone starts a motor on the rescue resource. Even when the LB launching authority gets paged or Kinloss get asked if they can help several minutes pass before the pagers actually get activated.

[jonp]

Here in Australia it's a legal requirement to have a registered epirb on any boat venturing more than 2nm offshore, plbs don't meet the legal requirement and are considered more a bush walkers land based tool for if you get lost. Beacon types and models - Beacons

[jonp]

Here in Australia it's a legal requirement to have a registered epirb on any boat venturing more than 2nm offshore, plbs don't meet the legal requirement and are considered more a bush walkers land based tool for if you get lost. Beacon types and models - Beacons

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