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Old 14-09-2011, 11:55   #16
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Re: What Receives 121.5 Mhz ?

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Originally Posted by Dsanduril View Post
Seems like what you are talking about is something like this:

MOB VHF DSC locator

Have no idea on the cost, haven't looked at them myself.

That link is yet another transmitter. The reciever is a more specialized piece of equipment:

Search and Rescue Direction Finders, Rhotheta High-Precision Direction Finder Systems, RT-202 RT-300, RT-500
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Old 14-09-2011, 16:29   #17
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Re: What Receives 121.5 Mhz ?

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Originally Posted by xymotic View Post
That link is yet another transmitter. The reciever is a more specialized piece of equipment:

Search and Rescue Direction Finders, Rhotheta High-Precision Direction Finder Systems, RT-202 RT-300, RT-500
The VHF DSC locator can apparently be received on Channels 70 or 16 on normal marine VHF sets. The cost, er, is around $850 with batteries probably included...
Anyone who wishes to use one of these on my boat is welcome to bring it along, but I will not be providing..

Michael
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Old 14-09-2011, 19:19   #18
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Re: What Receives 121.5 Mhz ?

For the 121.5 MHz emergency beacon you need a special receiver, and preferably a directional antenna. on VALIS I have the ACR ARX-50 (sadly this model has been discontinued): ARX-50 / Cobham - Beacon Solutions
I can leave this receiver turned on continuously and if any of our EPIRBs or a crewmember's PLB is activated we will know about it immediately, and a the MOB function on the chartplotter is triggered. I installed this unit after we had an accidental activation of a PLB, and I only found out about when the Coast Guard phoned my wife at home (she gave them my satphone #). Now, if there's an emergency, or a false alarm, we know about it.

That's one bad thing about PLBs and EPIRBS: I've been the cause of one false alarm, and I've heard of many others as well, where the people on the boat had no idea that their EPIRB/PLB had activated. Not a good situation.

ACR still has other beacon receivers: Direction Finders / Cobham - Beacon Solutions, as do other manufacturers. The technology is changing rapidly, and there are now other MOB and emergency solutions, but I still like the idea of being able to monitor my EPIRB and PLB.
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Old 15-09-2011, 08:29   #19
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Re: What receives 121.5 Mhz?

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Originally Posted by Hannah on 'Rita T' View Post
the device I'm lookign at is the McMurdo fast find, which has the 406 EPIRB frequency but, I can't for the life of me see how it is going to help me find the captain when he's overboard...
Short answer is, it won't. A PLB (which is what the FastFind is) is made to send a signal to Search and Rescue personnel. It is not intended as a way for the boat to get back to a man overboard.

If you had the proper kind of VHF receiver, with a directional antenna, and you weren't too far away, then yes, you could use the 121.5 signal that it broadcasts to find the person in the water. Obviously, though, you need a whole lot more equipment than just the FastFind and your marine VHF to do this.

If you want to be able to get back to a man overboard then you need to look at dedicated MOB technologies.
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Old 20-09-2011, 04:22   #20
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Re: What Receives 121.5 Mhz ?

It's my understanding that many modern 406mhz PLB's and EPIRB's will also transmit a 121mhz signal purely for the purpose of close-range homing, when SAR assets are already close by.

Confusion arises because that same frequency used to be used for the main signal. As others have said, it is no longer monitored for that purpose.
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Old 20-09-2011, 04:37   #21
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Re: What Receives 121.5 Mhz ?

I dont think very many PLBs tx on 121 as a homing signal, most high end EPIRBs do, and its a requirement for SOLAS units.

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Old 20-09-2011, 04:55   #22
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Re: What Receives 121.5 Mhz ?

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I dont think very many PLBs tx on 121 as a homing signal, most high end EPIRBs do, and its a requirement for SOLAS units.

Dave
ACR SarLink and ResQLink PLB's transmit homing signals on 121. See: ResQLink PLB / Cobham - Beacon Solutions

Some models of McMurdo FastFind PLB's also transmit homing signals:

Fast Find 200 / 201

Doesn't seem too unusual, and I would think it is a desirable feature.
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Old 07-10-2011, 03:17   #23
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Re: What Receives 121.5 Mhz ?

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ACR SarLink and ResQLink PLB's transmit homing signals on 121. See: ResQLink PLB / Cobham - Beacon Solutions
$400AU I bought 5 one for my whole family when I go out to sea 100nm out I also have a sat phone, so it someone wanted a swim they could set it off and I would get a phone call from the rescue so I can pickem up. well in the ideal world anyway, that is my solution.
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Old 18-10-2011, 14:43   #24
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Re: What Receives 121.5 MHz ?

Military and Commercial international flights routinely guard 121.5mhz AM and can be useful in an emergency because they have truly huge lines of sight at Flight Level 350 (~35,000 feet). Effectively a very high radio tower.

I've also thought a lot about the 'what if someone goes overboard' scenario. At night, I think a strobe light is far more useful than an EPIRB, even the new 400mhz sat EPIRBS only reduce the search area to around 5km CEP, though that's a major improvement. I don't think it would be at all practical for a cruising couple to RDF the position of someone overboard.

Consider that the person still on board has to handle the boat and visually search for the person in the water. Picture doing that and trying to turn dials and such to get a bearing on a transmitter in the water. Not easy at best and I'd say impractical.
A simple strobe light attached to your lifejacket would be great at night, a smoke flare or two during the day and a simple canned air powered foghorn would be great as well. All low tech and don't require you to be inside trying to read an S meter or a bearing instead of driving the boat.

I'm going to look into making something small and automated that will do something like it. There are plenty of matchbox or smaller sized GPS units around. Sounds silly, but you could attach, say, a waterproofed 'mouse' gps with a serial output to something like a TinyTracker APRS gizmo, and a small transmitter, say, 250mw or so in something like the 400mhz ISM band to a headband or the top of the PFD. Range of about a click most likely. If you want to be tricky, perhaps make it water activated. Some APRS software and you are in business. The person in the water would show up as an APRS position on a 'radar' display and it would be fairly simple to go almost directly to them, without having to do any fiddling with DF (which is not as easy as it appears).
You could do it with DSC or AIS but AX25 1200bps packet is relatively simple to generate, a crappy old scanner with nothing more than a line out from the headphone to the soundcard on a computer (or an old TNC if you can find one). At it's simplest level, a TNC connected to a serial port with Winterm will display the received APRS packets as text, with the position, punch it into the GPS or chartplotter and head for it!

There would be issues, the GPS and and the TX antenna need to be out of the water. So does the person's head, so somewhere on the upper body or head for those bits. Probably not that hard. Waterproof the whole thing. There may be commercial things out there smaller and better engineered and already waterproof, but not everyone can afford commercial equipment with big price tags aimed at the oh so wealthy cruisers of the world (lol).

Aussie Geoff
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Old 26-10-2011, 17:42   #25
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Re: What Receives 121.5 MHz ?

Here is another really powerful solution, especially if you have a DSC radio integrated with your chartplotter.

Use the Standard Horizon HX-851 handheld radio. It has its own GPS receiver and DSC channel in addition to the normal VHF-FM channel receiver. It is waterproof, floats, flashes when in the water and has a glow-in-the-dark gasket.

The MOB pushes the distress button and the radio on the boat participates in the DSC process and can send the Distress position to the chartplotter over the 0183 interface. Then you can navigate to the location. The boat can also do a DSC Position Request to the handheld to update the position or use DSC position polling to enable a steady stream of position reports. Lots of capability for 250 bucks.

You can also test every function between the handheld and the boat without irritating the Coast Guard.

I never do on-air radio checks anymore. I use DSC test to the USCG coastal station national MMSI number or to the operational sector for testing the distance more robustly.
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