Search and Rescue Direction Finders

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apward

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Given the use of PLB's which some crew are now carrying I was wondering how effective onboard direction finding equipment might be in locating an MOB transmitting on 121.5 MHz.

Having spoken to one manufacturer of such equipment who claims a 2 to 3 mile distance can be expected in calm waters (as a result of their testing), I was interested in how well these devices might perform in a 3 - 4 meter swell.

Their response was surprisingly that they were unsure as they've never tested their equipment in such conditions, and that I should ask the search and rescue operators who will have used such equipment in those conditions.

I'm trying to determine if DF an MOB transmitting on 121.5MHz is any easier / better than locating an MOB transmitting an AIS signal.

Anybody got any experience of locating a target by these methods in rough conditions
 
nigelmercier said:
However, if I was looking for someone, I'd much rather have a point on a chart I could navigate to than mess around with DF gear.
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Agreed. But I guess the idea is to be able to have a single carried device that both alerts the coastguard and allows the boat to find its own casualty. Unless and until someone makes a combined AIS / 406 device, having the boat DF the 121 signal seems like the only way of doing that.

Personally I think the answer is for singlehanders to carry a PLB and people on crewed boats an AIS SART (and something that does a good job of alerting). A definite position from AIS will be so much better than a vague DF bearing from portable or yacht-sized kit. Plus it's expensive and bulky whereas anyone contemplating this probably already has AIS.

That said, I carry neither, hold on in good weather and clip on in bad.

Pete
 
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apward said:
Given the use of PLB's which some crew are now carrying I was wondering how effective onboard direction finding equipment might be in locating an MOB transmitting on 121.5 MHz.

Having spoken to one manufacturer of such equipment who claims a 2 to 3 mile distance can be expected in calm waters (as a result of their testing), I was interested in how well these devices might perform in a 3 - 4 meter swell.

Their response was surprisingly that they were unsure as they've never tested their equipment in such conditions, and that I should ask the search and rescue operators who will have used such equipment in those conditions.

I'm trying to determine if DF an MOB transmitting on 121.5MHz is any easier / better than locating an MOB transmitting an AIS signal.

Anybody got any experience of locating a target by these methods in rough conditions
Click to expand...

I work on a seismic research ship as coxswain, we have tried numerous MOB systems across the company (23 ships), the early ones had some hand held DF equipment that was a pain in the ass to use and needed frequent training with new FRC crew etc , the system we have now works on VHF it transmits on 16 PLUS a DSC alert on 70 with GPS position report, made by company called mobi alarm , its water activated so we do get an occasional false alarm when its rough on the workboats/FRC and the life jackets get wet, but its been reasonably reliable , AVERAGE range is probably 6 kilometers in a 2/3 meter swell, I have tested them at our tail buoys which are 9k back and have set the alarms off on the bridge of the ship which i can barely see !

There are over 50 crew on board and all our life jackets are fitted with these alarms and I test them routinely , battery life seems pretty good and having pulled the units apart the batteries are 9v and easily changed if you needed too, the next generation units will be sending via AIS as well, so Id get a visible hit on a chart plotter / computer as well as a DSC alert on the DSC radio PLUS an automated voice call on 16 , they seem to be covering all the bases with these units...WAY simpler than using DF equipment and we are on the water every day with this kit, with very few failures.

Remember this MOB equipment is not transmitting to MRCC stations unless you are in range of a coastal station , so its no use if you are back of beyond and sailing solo , a dedicated PLB will raise the alarm at a far greater range as it works the same way as an EPIRB via satelites,



not definitive and Im sure someone else will have an opinion - hope that helps
 
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ribrage said:
I work on a seismic research ship as coxswain, [...] the system we have now works on VHF it transmits on 16 PLUS a DSC alert on 70
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That's interesting. Where is this research ship (or perhaps more relevantly, what is her flag?). I read something a while back about DSC distress and synthesised voice transmissions not being legal unless manually activated by a human being - but I'm not sure whether that applied to the UK, the EU or the US (or all three).

Pete
 
The system is company wide and in use all around the world, the ships have numerous flags of convenience around the world.

The site map says pretty much world wide

http://mrtsos.com/

And these are what we are updating too (AIS Upgrade) , I have a case full of them on board , just waiting for the correct pouches to mount them to our life jackets

http://mrtsos.com/products/sMRT-v100-vhf-dsc-ais-maritime-survivor-locating-device

What I like best is that the range is greatly increased and any vessel nearby also gets to know I'm in the water and come bloody find me ! No special kit required to become a SAR vessel , no additional base stations to fit to the rescue boats or workboats, Ill update you when Ive fitted and tested them if your interested ?

ian
 
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ribrage said:
The system is company wide and in use all around the world, the ships have numerous flags of convenience around the world.

The site map says pretty much world wide

http://mrtsos.com/

And these are what we are updating too (AIS Upgrade) , I have a case full of them on board , just waiting for the correct pouches to mount them to our life jackets

http://mrtsos.com/products/sMRT-v100-vhf-dsc-ais-maritime-survivor-locating-device

What I like best is that the range is greatly increased and any vessel nearby also gets to know I'm in the water and come bloody find me ! No special kit required to become a SAR vessel , no additional base stations to fit to the rescue boats or workboats, Ill update you when Ive fitted and tested them if your interested ?

ian
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Super information - meets some of the questions I'm having to answer, single-handing in the Med (EPIRB needs re-batterying, liferaft service due and probably beyond economic repair) so I look forward to your further reports.
Thanks ribrage.
 
ribrage said:
http://mrtsos.com/products/sMRT-v100-vhf-dsc-ais-maritime-survivor-locating-device

What I like best is that the range is greatly increased and any vessel nearby also gets to know I'm in the water and come bloody find me ! No special kit required to become a SAR vessel , no additional base stations to fit to the rescue boats or workboats, Ill update you when Ive fitted and tested them if your interested ?

ian
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Thanks for this, exactly what I was trying to find. No need for the 121.5MHz DF

Very costly though for leisure use at £550 per unit, but hey, what's it worth to ensure you can locate an MOB
 
A little theory may help. The wavelength of a signal at 121.5 MHz is approximately 2.5 metres. This suggests that you won't get much signal if the transmitter and receiver are both at the bottom of a trough in waves more than a metre or so in height; the wavelengths are wrong to get much diffraction. Of course, you'll get a signal when both are on a crest. So, it really boils down to asking how clever are the electronics at dealing with an intermittent signal? The signal will certainly be intermittent in anything of a sea.
 
charles_reed said:
Super information - meets some of the questions I'm having to answer, single-handing in the Med (EPIRB needs re-batterying, liferaft service due and probably beyond economic repair) so I look forward to your further reports.
Thanks ribrage.
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No problem , will be 6/7 weeks min before I will be doing anything as im off here soon and don't have the pouches to fit these to the life jackets in country hence the delay
 
apward said:
Very costly though for leisure use at £550 per unit, but hey, what's it worth to ensure you can locate an MOB
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Oh - were you asking about DF because you weren't aware of AIS SARTs?

In that case the Kannad R10 (and a McMurdo unit which is the same thing with a different badge, I forget the name) are well-regarded and available for just under £200. There are others too.

Answering AntarcticPilot's point, the R10's signal timing is apparently specifically designed to ensure that transmissions are regularly made from the top of a wave. There's a youtube video somewhere (filmed at a US trade show) where they explain it.

One thing to make sure of is that your AIS kit on the boat behaves appropriately when it receives the signal. The beacon will always appear, but on many displays out there it looks just like a boat and there is no special alarm. This is changing; my dedicated AIS display sounds an alarm and automatically enters a special MOB mode with directions back to the casualty.

Pete
 
The advantage of AIS over DSC is the MOBs position is continuously updated.

DSC presumably doesn't re-send the position, BUT has the advantage that more people have DSC on board and it will always go biserk when there is an alert on it.
 
The range of any VHF based transmission is proportional (ish) to the heights of the two aerials. A mob system will work much better for a large seismic vessel than a yacht, or even lifeboat - the aerial is going to be 100'+ Compared with 50' or so

I would love to know how effective consumer plb's are since they have small aerials which you have to hold above water, not easy if you are trying hold your head above water.

It is great there is such a range of electronic options to help the location of a mob, which will make location by all sorts of assets easier, but it surprises me that perhaps the most effective (and cheapest) means of being seen by your own crew at the ranges of most yachting mobs is not used - a simple day/night pyro.
 
apward said:
Given the use of PLB's which some crew are now carrying I was wondering how effective onboard direction finding equipment might be in locating an MOB transmitting on 121.5 MHz.

Having spoken to one manufacturer of such equipment who claims a 2 to 3 mile distance can be expected in calm waters (as a result of their testing), I was interested in how well these devices might perform in a 3 - 4 meter swell.

Their response was surprisingly that they were unsure as they've never tested their equipment in such conditions, and that I should ask the search and rescue operators who will have used such equipment in those conditions.

I'm trying to determine if DF an MOB transmitting on 121.5MHz is any easier / better than locating an MOB transmitting an AIS signal.

Anybody got any experience of locating a target by these methods in rough conditions
Click to expand...


To DF something you need a minimum of three directions to the signal source and the result is a cocked hat, similar to taking three compass bearings to get a fix. Unless they are carrying something that transmits their lat/lon from GPS you've no chance. As AntarcticPilot #11 says, you will only get sporadic signal and there is no way you will have time to get a bearing using a directional antenna. Also you need a sophisticated antenna to get within 10deg of where the signal came from and that type of antenna ain't gonna fit on the average yacht.
 
SimonFa said:
To DF something you need a minimum of three directions to the signal source and the result is a cocked hat, similar to taking three compass bearings to get a fix. Unless they are carrying something that transmits their lat/lon from GPS you've no chance. As AntarcticPilot #11 says, you will only get sporadic signal and there is no way you will have time to get a bearing using a directional antenna. Also you need a sophisticated antenna to get within 10deg of where the signal came from and that type of antenna ain't gonna fit on the average yacht.
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And if you have ever looked at a the antenna rig on all weather RNLI lifeboat it has an antenna array with 8 antennae - and with some signal processing they know the direction of which two of these the casualty lies between - and being some 6m above seal level it has sufficient range once they are in the vicinity of the incident.
 
boomerangben said:
I would love to know how effective consumer plb's are since they have small aerials which you have to hold above water, not easy if you are trying hold your head above water.
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Chap crossing single handed from Wales to Ireland in a 6m RHIB during the summer, on his way to the Round Ireland RHIB Challenge, went overboard. He was late leaving Wales so no other RHIBs in company. He had a VHF clipped to his LJ which was at the point of contact as he hit the water and was destroyed. He had a dry suit on, kill cord stopped the boat but he ended up too far away to swim back. He had a PLB. Pressed the red button and it worked.

Some debate if it worked as quick as you'd hope but he did get a sea king out to pick him up (3hours). (He got them to put him back on the RHIB and was going to carry on over to Ireland!! But his console was damaged so he was persuaded to return to Wales, undertook repairs and joined the fleet the following night!

So they DO work. Advice is to lie the device on you life jacket between the two bladders, with the little aerial pointing vertical.
 
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