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Old 18-05-2015, 22:21   #121
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Re: Are EPIRBs dead?

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Originally Posted by Dockhead View Post
You are confusing the beacon signals with homing signals. I guess a lot of people don't know exactly how EPIRBS work, leading to a lot of confusion like what we've seen in this thread.

Modern EPIRBS send their beacon signals including position data on 406 megahertz. This is what goes to the satellites.

But to guide SAR services in for the last few miles, EPIRBS (and PLBs) also send out a homing signal at 121.5 megahertz. This does not go through the satellites, but is directly received on board SAR vessels and aircraft. I actually have one of these homing receivers on board (long story).

Consumer non-GMDSS devices like the InReach do not send out these homing signals, which are a crucial part of rescue procedure.
I understand the difference between a beacon and homing signal.

Since the satellites no longer use the 121.5 MHz beacon signal, and

Quote:
Although these EPIRBs also include a low power 121.5 MHz homing signal, homing on the more powerful 406 MHz frequency has proven to be a significant aid to search and rescue aircraft.
That's from the USCG website.

This sounds to me like they use the 406 MHz signal to home in on for obvious reasons, what exactly is anyone doing with the 121.5 MHz signal? It looks like it's not being used as a beacon or homing signal.
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Old 19-05-2015, 02:54   #122
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Re: Are EPIRBs dead?

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Originally Posted by socaldmax View Post
I understand the difference between a beacon and homing signal.

Since the satellites no longer use the 121.5 MHz beacon signal, and



That's from the USCG website.

This sounds to me like they use the 406 MHz signal to home in on for obvious reasons, what exactly is anyone doing with the 121.5 MHz signal? It looks like it's not being used as a beacon or homing signal.
You make the wrong conclusion.

Modern EPIRBS emit two signals: One burst signal at 406 mhz every approximately 50 seconds (pseudo randomized to vary a little from that in order to prevent accidentally simultaneous bursts from different EPIRBs), and one continuous homing signal at 121.5 mhz.

The 406 mhz signal contains a digital burst, but also contains a short emission of an unmodulated carrier, so it can also be homed on, which the satellites also do besides receiving the digital signal with position data. The old 121.5 hz analogue satellite system did nothing but home -- the old EPIRB signals contained no digital information.

The 406 mhz signal is stronger (5 watts), but is emitted for only half a second at a time. Some later RDF equipment (like the Rockwell-Collins DF500) can home on it. But many SAR aircraft, maybe most of them, cannot home on 406 signals. Typical equipment is the Becker SAR-DF 517. In any case, the intermittent 406 mhz burst signal is not intended for homing in the last miles. That is what the continuous 121.5 mhz signal is for. ALL SAR aircraft can home on the 121.5 mhz homing signal, and in fact, you can even home on it with an ordinary VHF airband radio, if you know how to do it.


You know what "homing" is, right? It's a technique about as old as radio. If you receive a radio signal and have a directional antenna, you can determine the bearing to the transmitter. If two or more receivers do this, you can triangulate two or more bearings to get a position. Even if your antenna is not directional, you can follow the signal as it gets stronger or weaker, if your receiver is mobile.


So in short, the continuous 121.5 mhz homing signal emitted by all modern EPIRBs and PLBs is the primary means of locating the casualty in the last few miles of the rescue operation. Not the 406 mhz signal, which is primarily used for transmission of bursts of digital information to get out the distress signal to the satellites, and to provide an initial position to get the rescue operation started.
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Old 19-05-2015, 03:32   #123
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Re: Are EPIRBs dead?

Socaldmax may be on to something here

Lifted this paragraph from a USCG document
Quote:
The 406 MHz signal is approximately 200 times stronger than the 121.5 MHz “homer” on 406 MHz EPIRBs; as such, CG aviation response assets, at altitude, can easily lock onto this stronger signal from 100+ NM away (S/V Paradox, F/V Silver Wings). Additionally, because of its strength, the 406 MHz signal easily penetrates ship hulls, buildings, even the human body whereas the 121.5 MHz signal is easily attenuated.

That would suggest USCG SAR aircraft have the ability to home on 406Mhz.

I would hazard a guess that 121.5 homing signal is retained as most aircraft can receive this frequency on a direction finder, whereas only a few dedicated aircraft can home on the 406 signal.

What we need is a USCG pilot to step in and either confirm or deny.
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Old 19-05-2015, 03:54   #124
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Re: Are EPIRBs dead?

There is no need for any confusion. The SAR aircraft can home on the 121.5 signal for certain. Regardless of how the SAR teams home in on the EPIRB there is no homing feature of the little satellite comm devices. They simply are not designed for SAR. EPIRB and PLB are full SAR systems supported the world over. They are not obsolete.
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Old 19-05-2015, 04:05   #125
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Re: Are EPIRBs dead?

I believe that this little gizmo is what is being used by some SAR aircraft.
The DF-500. The blurb does say it can receive the 406 signal.
http://www.rockwellcollins.com/sitec...on_finder.aspx
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Old 19-05-2015, 04:08   #126
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Re: Are EPIRBs dead?

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Originally Posted by nigel1 View Post
Socaldmax may be on to something here

Lifted this paragraph from a USCG document



That would suggest USCG SAR aircraft have the ability to home on 406Mhz.

I would hazard a guess that 121.5 homing signal is retained as most aircraft can receive this frequency on a direction finder, whereas only a few dedicated aircraft can home on the 406 signal.

What we need is a USCG pilot to step in and either confirm or deny.
I would be gobsmacked if most (or at least some) USCG SAR aircraft don't have 406 homing capacity.

Downunder, most states have a few SAR aircraft with 406 homers. Of course there are many here that only have older 121.5 homers.

I think I can be pretty sure of my facts here as I have installed a few of the 406 homers myself.
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Old 19-05-2015, 04:12   #127
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Re: Are EPIRBs dead?

Here is another popular DF unit.
RHOTHETA - Welcome!
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Old 19-05-2015, 04:17   #128
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Re: Are EPIRBs dead?

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Originally Posted by nigel1 View Post
I believe that this little gizmo is what is being used by some SAR aircraft.
The DF-500. The blurb does say it can receive the 406 signal.
DF-500 direction finder
Just an aside tongue in cheek comment...
It has been said by some "Collins, you can buy better avionics but you can't pay more".
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Old 19-05-2015, 04:28   #129
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Re: Are EPIRBs dead?

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Originally Posted by Wotname View Post
I would be gobsmacked if most (or at least some) USCG SAR aircraft don't have 406 homing capacity.

Downunder, most states have a few SAR aircraft with 406 homers. Of course there are many here that only have older 121.5 homers.

I think I can be pretty sure of my facts here as I have installed a few of the 406 homers myself.
I think all the new RDF devices will home on 406 signals. This capability can be important if you're trying to rescue someone who has an older 406 EPIRB without GPS. In that case you won't start out with such an accurate position and you will want to start homing sooner. The 406 signal, although it is intermittent, is stronger, so you can receive it from further away.

But if you're rescuing someone with a modern EPIRB which sends GPS position data, then you'll just go to that position and start looking for the continuous 121.5 signal.

People get confused because the first generation of EPIRBs operated on 121.5 mhz. These emitted a continuous signal without any data. The signal was received by satellites to get an approximate position by triangulation and doppler. This whole system was switched off in 2009 (IIRC).

But that does not mean that 121.5 mhz homing is obsolete! Every modern EPIRB and PLB emits a 121.5 mhz continuous homing signal for use of SAR services for final location of the casualty in the last miles of the rescue operation.


As someone said, consumer satellite messenger devices like InReach have none of this capability and can't be homed in on by SAR aircraft. All they can do is send a distress message with position to some intermediary, who you hope will properly alert SAR services. You can hope that communication to the intermediary and between the intermediary and SAR headquarters and between SAR headquarters and the actual rescue team will be efficient enough to give the actual rescue team a fresh enough position so that they can actually find you. This might or might not work -- do you want to chance it? SAR teams are trained to find you by homing on your 121.5 mhz homing signal, once they get on scene, and also to look for the strobe light which EPIRBS and PLBs have, if it's at night. So when the chips are really down, you will really want to be emitting this signal from a proper EPIRB, which is directly received by the SAR aircraft rather than through a chain of intermediaries, however nice it would be to have, in addition to that, two way communications by some other means, like an InReach.
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Old 19-05-2015, 04:59   #130
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Re: Are EPIRBs dead?

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.....
People get confused because the first generation of EPIRBs operated on 121.5 mhz. These emitted a continuous signal without any data. The signal was received by satellites to get an approximate position by triangulation and doppler. This whole system was switched off in 2009 (IIRC).......
Your total post was well worth reading but the bit above was personally interesting to me.

First, your memory is pretty good , the actual date was Jan 31, 2010 ( IIRC).

Second, the only time I ever used an beacon in anger was in the 70's and this was pre-satellite. The SAR aircraft in that instance had no radio direction finding equipment (in the VHF band) and thus carried out the search simply by listening to the modulated carrier growing from weak to stronger and when it goes weaker again, turn 90 degrees and repeat.

Times have changed
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Old 19-05-2015, 05:16   #131
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Re: Are EPIRBs dead?

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. . the only time I ever used an beacon in anger was in the 70's and this was pre-satellite. The SAR aircraft in that instance had no radio direction finding equipment (in the VHF band) and thus carried out the search simply by listening to the modulated carrier growing from weak to stronger and when it goes weaker again, turn 90 degrees and repeat. . . .
Indeed! I think that's one reason why 121.5 mhz was originally chosen for distress beacons -- that frequency on aircraft VHF is like Channel 16 for us. Every aircraft would receive it, and every aircraft could home in on it using just that technique

Old-fashioned, but not irrelevant!

I actually have a 121.5 mhz directional homing receiver on board my boat. Never used in anger. It was used with old-fashioned commercial-type MOB devices, which emitted a 121.5 mhz homing signal. Could still be used to find a casualty with an active PLB or EPIRB.
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Old 19-05-2015, 05:29   #132
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Re: Are EPIRBs dead?

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Indeed! I think that's one reason why 121.5 mhz was originally chosen for distress beacons -- that frequency on aircraft VHF is like Channel 16 for us. Every aircraft would receive it, and every aircraft could home in on it using just that technique

Old-fashioned, but not irrelevant!

I actually have a 121.5 mhz directional homing receiver on board my boat. Never used in anger. It was used with old-fashioned commercial-type MOB devices, which emitted a 121.5 mhz homing signal. Could still be used to find a casualty with an active PLB or EPIRB.
Also 121.5 was a harmonic of 243 which was the military equivalent back in the day when military and civilian aircraft didn't share the same bands. This made the design of the beacon transmitter and antennas much simpler and the same beacon be used for both services.
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Old 19-05-2015, 05:43   #133
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Re: Are EPIRBs dead?

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............ and also to look for the strobe light which EPIRBS and PLBs have, if it's at night.........
Indeed, a fact that is often overlooked. Half of every day is night .
The strobe is a very effective device, especially so if the SAR guys are night vision equipped (pretty standard these days). Unless the cloud base is really low, they are going to see you 10 or 20 miles out and makes a radio search redunant.

Heck, they will see the light of a cell phone screen on a dark clear night 10 miles away.
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Old 19-05-2015, 09:05   #134
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Re: Are EPIRBs dead?

"go ahead, figure out where your sky wave is going to land."
Not necessary or relevant. With HF radio you can be fairly sure that someone, somewhere, is going to receive the signal. Often many people, making for a robust and redundant signal reception, with whoever gets the message being able and willing to relay it over one of the many redundant telephone or radio systems. Not guaranteed, but probable. And agencies like the USCG are very well aware that HF radio moves in strange ways, a phone call patched in from ten thousand miles away is just as good as a local one, from their point of view.


I've called a local USCG station, who patched my call to district HQ, who patched it to another entire district (Caribbean) who patched it to...all without any more fuss than an inter-office call in any business office.


"While I just push this little red button. " Which depends on private, and tenuously, financed terribly fragile assets that are easily disrupted. Yes, incredibly simple and streamlined, but in this day and age? Terribly easy to disrupt, for either business or political reasons. And also terribly vulnerable to solar flares, which we KNOW will happen, and which historically DO happen on a scale that can destroy these assets. Sure, you can wait for that to happen and then buy an EPIRB (if the armored satellites are still up or being replaced) but then you've still got nothing till that happens.
The private companies have done a marvelous job, from all accounts, and I suspect they are aware that dropping the ball just ONCE could permanently put them out of business. Still, they're private companies, in the tech industry. Consider if you will INMARSAT, a relatively old and established player. They're trying to launch a third bird for "worldwide" coverage right now. Nope. Can't do it, maybe six months or a year late at best, because the only source of heavy lift boosters is Russia, and they're on hold until they figure out why those keep blowing up. (OOopsie.)


These are very fragile "systems", even with the biggest and best of players.


HF radio? Millions of the damned things. Some insomniac is always listening, somewhere. And if the world as we know it ends--odds are another insomniac will have a battery backup system as well. Maybe not as reliable, but also maybe more robust.
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Old 19-05-2015, 09:34   #135
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Re: Are EPIRBs dead?

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"go ahead, figure out where your sky wave is going to land."
Not necessary or relevant. With HF radio you can be fairly sure that someone, somewhere, is going to receive the signal. Often many people, making for a robust and redundant signal reception, with whoever gets the message being able and willing to relay it over one of the many redundant telephone or radio systems. Not guaranteed, but probable. And agencies like the USCG are very well aware that HF radio moves in strange ways, a phone call patched in from ten thousand miles away is just as good as a local one, from their point of view.
You are not serious ? Please don't leave the impression that HF radio is a modern solution to SAR. A erupting sunspot routinely renders HF useless for any distance on all frequencies likely to be aboard a small boat.

"These systems are fragile" - again I will state the world is very heavily invested in EPIRB tech. There are three full satellite clusters monitoring the frequency and auto linked to world wide ground stations, and that is just the U.S. investment ! Geostationary, Mid altitude sats (MEO), and low earth orbit (LEO), make the coverage bullet proof. The hand held units are made to very high standards, and the USCG is committed to a 95% response rate within 4 minutes of a ping from an EPIRB/PLB. If you push the little red button you can bet the USCG will react immediately with the highest priority. Did you know that in the San Francisco area they have rotors spinning on helo's within 7 min of a received EPIRB signal ?

As for Iridium - great system - and if it wasn't for the US Gov't investing heavily via the state department it would not exist. THat was an example of private enterprise not understanding their own market, a total fail.

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