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Author Topic: 121.5 vs 406  (Read 4517 times)
rroberts
Recruit

Posts: 22

« on: January 05, 2010, 02:53:16 AM »

I know I should probably search the forum first but.......

The Direction Finders we use I believe are based on the 2m frequency 144mHz DF gear.  Since we use 121.5 (121.75 for testing) would our gear work better if we used 2.5meter (300/Freq=AntennaSize, 300/121.5Mhz=2.46m) antennas?

As more aircraft begin to use 406mHz ELT's do we need to switch to the 70cm Antenna's
(300/406mHz=.74m)?  Would we be better off switching to dopplar based systems since the dopplar systems tend to keep track of frequencies that are not a constant signal?  i.e. 121.5 stays active, but 406 cycles.

Also, I thought this was cool... in ARRL's Handbook.  They mention that individuals who are experienced in DF are highly desired by CAP and Coast Guard Aux.  I was surprised to see us mentioned.

Last question... for now :)  Does anyone have any ideas that have shown to be successfull in getting a long range rough direction of an ELT.  Once we would get to a midrange (203 miles) we could then switch over to the regular antennas that we use.
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lordmonar
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« Reply #1 on: January 05, 2010, 03:34:39 AM »

No...you don't have to switch.

The new ELTs still transmit on 121.5.

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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
Nick
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Posts: 468
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« Reply #2 on: January 05, 2010, 10:28:18 AM »

But the 121.5 signal is fairly low power, so you'll need a relatively good fix either off the 406 MHz signal or by GPS to get into the area and then switch over to 121.5.
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Nicholas McLarty, Lt Col, CAP
Texas Wing Staff Guy
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sardak
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Posts: 1,135

« Reply #3 on: January 05, 2010, 11:04:23 AM »

I don't know what kind of DF you're using, but the L-Pers were designed for 121.5 by a CAP member specifically for finding ELTs. There were two models of the older "blue box" L-Pers with the wooden mast. Single tuner ones for 121.5 and dual tuner for 243.0. The units came with one or two sets of antennas matched to the tuner(s), one for 121.5 and one for 243.0, and the instructions emphasized using the correct antenna for the correct frequency. 

Custom L-Pers could be ordered with one or both tuner sections designed for other frequency ranges. Often the custom ones were 121.5/air band on one and marine or ham on the second (with the antennas tuned for the two bands). The newer "cheese block" L-Pers have digital tuners to cover more frequencies, but their primary design is still for 121.5.

In general, any radio will have better reception/transmission if the antenna is tuned to the desired frequency.

As for 406, you can work on a 406 DF if you want, but you need to do a search here to find others who have already asked the same questions, including using Doppler.

Why are you worried about DFing at 203 miles? I'd like to hear from someone that's worked an ELT from that distance. The Coast Guard has a presentation on long range DFing of 406. The best they've done is 160 nautical miles and per their chart, the average lock for a search plane between 15 and 25,000 ft is 116 NM.

What we need in CAP are members who can DF well on 121.5 from 10 miles down to 0 feet.

Mike
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Major Lord
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« Reply #4 on: January 05, 2010, 11:49:05 AM »

You want to DF 121.5 over the horizon? Easy!  Just get a satellite in orbit high enough above the earth to......wait, .......never mind.

Major Lord
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"The path of the righteous man is beset on all sides by the iniquities of the selfish and the tyranny of evil men. Blessed is he, who in the name of charity and good will, shepherds the weak through the valley of darkness, for he is truly his brother's keeper and the finder of lost children. And I will strike down upon thee with great vengeance and furious anger those who would attempt to poison and destroy my brothers. And you will know my name is the Lord when I lay my vengeance upon thee."
Nick
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Posts: 468
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« Reply #5 on: January 05, 2010, 12:58:54 PM »

What we need in CAP are members who can DF well on 121.5 from 10 miles down to 0 feet.
Again, with the new ELT's only spitting out a low power signal on 121.5, I'm worried about what kind of effective distance where you can even detect a signal let alone DF it.  Anyone have field experience with these new ELT's (airborne or ground DF) that can speak to it?

You want to DF 121.5 over the horizon? Easy!  Just get a satellite in orbit high enough above the earth to......wait, .......never mind.

Major Lord
Touché.  Chalk up another one to government effectiveness. :)
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Nicholas McLarty, Lt Col, CAP
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lordmonar
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« Reply #6 on: January 05, 2010, 01:33:02 PM »

The new system gives you a better initial fix.

Even with the lower power 121.5 signal you start closer to the target.

The Becker DF can DF the 406 signal as well as the 121.5.

If the ELT has the GPS option (and it is working) you will be given the exact location of the target to within something like 3 meters.
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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
rroberts
Recruit

Posts: 22

« Reply #7 on: January 05, 2010, 02:29:19 PM »


Why are you worried about DFing at 203 miles?

Oops - misprint.  I meant 2 to 3 miles.  It seamed the way we were doing the search, we were barely getting a signal beyond 5 miles.  I would admit that in real life, we would probably be getting dispatched from calls received from residents and those calls would give us the rough location anyways.
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Nick
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« Reply #8 on: January 05, 2010, 03:10:17 PM »

The new system gives you a better initial fix.

Even with the lower power 121.5 signal you start closer to the target.

The Becker DF can DF the 406 signal as well as the 121.5.

If the ELT has the GPS option (and it is working) you will be given the exact location of the target to within something like 3 meters.
Yeah, I knew you'd start closer with the GPS position ... I was curious about in situations like, oh say, planes in hangars where there was no GPS fix possible.  You already have shielding affecting the 121.5 signal so you'll be banging away on 406 MHz with 5 watts but the data burst won't have an accurate location.

How many handheld 406 MHz DF units are out there?  I've seen one since the changeover.

Edit: I just read some USCG documentation that's saying the 121.5 carrier on 406 MHz ELT's is only 25 mW!?
« Last Edit: January 05, 2010, 03:37:14 PM by McLarty » Logged
Nicholas McLarty, Lt Col, CAP
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Major Lord
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« Reply #9 on: January 05, 2010, 05:25:20 PM »

The general idea here is that the beacon will produce a position report as a burst encoded 406.25 MHZ data. Hopefully, the data burst will contain an up to date and accurate set of GPS coordinates. In theory, a great idea. Time will tell how this works in actual crashes. Once a team has been dispatched to a very narrow area defined by the coordinates, they can began working the 121.5 beacon to walk their way in to the beacon. You can be right on top of a crash in dense forest and not see it sometimes, so the added 121.5 lets us bull-dog our way onto the scene. Of course, aircraft monitoring 121.5 can still be a tremendous asset even for the very low powered beacon, and detect even a few milliwatts from many miles at altitude. Very few aircraft will be equipped with the godawful expensive 406 DF'ing gear any time soon, but all aircraft can and should monitor 121.5. Since RF path losses are very high at UHF frequencies (like 406) its actually much easier and practical to DF a lower powered VHF signal ( Theoretically, up to about 6 times less powerful for the same detection range) especially since the 121.5 beacon in continuous, whereas the 406 data burst is intermittent.

I am glad to hear that you won't be signing on to missions and trying to DF them from 203 miles.....you might be disappointed!

Major Lord
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"The path of the righteous man is beset on all sides by the iniquities of the selfish and the tyranny of evil men. Blessed is he, who in the name of charity and good will, shepherds the weak through the valley of darkness, for he is truly his brother's keeper and the finder of lost children. And I will strike down upon thee with great vengeance and furious anger those who would attempt to poison and destroy my brothers. And you will know my name is the Lord when I lay my vengeance upon thee."
lordmonar
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« Reply #10 on: January 05, 2010, 05:46:20 PM »

In a comparison of the two systems.

The old system took over 90 minutes to generate a search area with a 20 Nautical Mile radius.  The new system gets a fix in under 45 minutes with a search radius of 5Km (100 m if they have GPS).

The old system only sent out a beacon signal.  The new system encodes data about the target so you know right away who and what you are looking for (assuming they have properly registered).

The old system is required not only for the signal to be picked up by the satellite but the ground station had to be tracking the satellite at the same time for an alert to be generated.  The new system stores the ELT hit until the next ground stations communicates with the satellite further decreasing the response time.

So...from a SAR point of view no new ground equipment is needed.  Hand held 406 recievers are not really needed.   If you have gotten it traced to an airport...you should be getting a strong enough signal even with the 75 mW signal to find the ELT.  But even then you should already know the Tail Number of the beacon so all you have to do is ask the airport manager where Nxxxx is parked and go right to the hard stand and confirm it with your ELPR.
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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
Nick
Seasoned Member

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« Reply #11 on: January 05, 2010, 06:24:37 PM »

But even then you should already know the Tail Number of the beacon so all you have to do is ask the airport manager where Nxxxx is parked and go right to the hard stand and confirm it with your ELPR.
That's actually the best point of the whole deal -- you already know what you're looking for.
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Nicholas McLarty, Lt Col, CAP
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RogueLeader
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« Reply #12 on: January 05, 2010, 07:11:35 PM »

What if its not on the tarmac?  Say in trees or disintigrated mid air, where you aren't sure of its exact location?
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lordmonar
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« Reply #13 on: January 05, 2010, 07:42:22 PM »

What if its not on the tarmac?  Say in trees or disintigrated mid air, where you aren't sure of its exact location?

Then you won't have the problems of hangar masking of the signal.

Either way your search radius will be 5Km vice 20NM. 
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PATRICK M. HARRIS, SMSgt, CAP
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RiverAux
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« Reply #14 on: January 05, 2010, 07:51:42 PM »

Didn't one of our radio geniuses do a lot of research to find out that the 121.5 transmitters in the new 406 ELTs were actually transmitting just as strong as the old 121.5 only ELTs?
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sardak
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« Reply #15 on: January 05, 2010, 08:49:31 PM »

I don't know about radio geniuses, but the research has been done and posted here. The 121.5 homing signals on 406 beacons sold in the US are (spec values):

PLBs and EPIRBs - 25 mW to 100 mW

ELTs - 50 mW to 400 mW

A Pointer practice beacon is rated at 100 mW and 121.5 MHz ELTs are rated at up to 500 mW (and some might be higher). So yes, the homing signal on 406 MHz ELTs can be as strong as "old" Generation 1 and 2 ELTs.

As for position, 406 MHz beacons aren't required to have GPS capability.  A couple of years ago NOAA published stats that showed only about 50% of registered 406 MHz ELTs had GPS capability while almost 100% of PLBs did. EPIRBs were somewhere in between. Without GPS, position is Doppler determined. With GPS, both the Doppler and GPS position is available.  The geostationary satellites in the Sarsat system have the potential for alerting with GPS position within a few minutes of beacon activation. Without a GPS equipped beacon, the GEO sats can only report the alert, no position.

Mike
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JoeTomasone
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« Reply #16 on: January 06, 2010, 07:07:14 AM »

I have found that the 406 ELTs/EPIRBs are triangulated (sans GPS) much better than the 121.5 - by probably an order of magnitude.   If you plot the lat/long pairs on a map and draw a square that encloses them, you're almost guaranteed to hear the 121.5 homer once you get in that box.   Can't even fantasize about that with old 121.5 units.

As far as the reduced signal level - it's true, but that also keeps it running longer.   Ever hunt for an old 121.5 with weak batteries?  Same deal, largely.
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Nick
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« Reply #17 on: January 06, 2010, 10:50:39 AM »

I have found that the 406 ELTs/EPIRBs are triangulated (sans GPS) much better than the 121.5 - by probably an order of magnitude.   If you plot the lat/long pairs on a map and draw a square that encloses them, you're almost guaranteed to hear the 121.5 homer once you get in that box.   Can't even fantasize about that with old 121.5 units.

As far as the reduced signal level - it's true, but that also keeps it running longer.   Ever hunt for an old 121.5 with weak batteries?  Same deal, largely.
I can see that -- I imagine the signal burst from the 406 (without GPS) offers the satellite processing a better chance of computing a triangulated location per pass than just a continuous carrier.  So yes, if the narrowed search area is small enough to detect a 121.5 signal at the lower power, you got my vote. :)

Edit: I have been advised my assumptions of 406 MHz satellite processing are misguided (based on knowledge of 121.5 MHz satellite processing), so stand by on all the "you're wrong!" flames until I get additional info.
« Last Edit: January 06, 2010, 11:08:08 AM by McLarty » Logged
Nicholas McLarty, Lt Col, CAP
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billford1
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« Reply #18 on: February 04, 2010, 06:54:23 PM »

Didn't one of our radio geniuses do a lot of research to find out that the 121.5 transmitters in the new 406 ELTs were actually transmitting just as strong as the old 121.5 only ELTs?
It wasn't me but the information I have is that the transmitted power from the 121.500 Mhz part of the new 406 is about 50% the power out of the old 121.5 ELTs. We'll now have to get much closer with the DF equipment we've got.
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sardak
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« Reply #19 on: February 05, 2010, 12:15:24 AM »

The answer is posted three posts before yours. Some are less powerful, some are as powerful as older ELTs.

Mike
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Gunner C
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« Reply #20 on: February 05, 2010, 03:02:34 PM »


Last question... for now :)  Does anyone have any ideas that have shown to be successfull in getting a long range rough direction of an ELT.  Once we would get to a midrange (203 miles) we could then switch over to the regular antennas that we use.
I've gotten a strong, usable signal at over 100 miles - probably close to 130 @ 5000'. 
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MikeD
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« Reply #21 on: February 05, 2010, 10:03:52 PM »

I got a call for an unregistered 406 a few months ago, and we got a signal roughly 4-5 miles away though some urban areas and also with it bouncing off of some very large hangars. 
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Eclipse
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« Reply #22 on: February 05, 2010, 10:10:45 PM »

You guys are getting DF'able signal on 406 or just hearing the burst?
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MikeD
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« Reply #23 on: February 06, 2010, 01:40:34 AM »

You guys are getting DF'able signal on 406 or just hearing the burst?


My bad, I was getting the 121.5 from the ELT, we were using the classic LPer.  We didn't have anything capable of picking up 406, and we didn't need to for this mission. 
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