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CAP Talk  |  Operations  |  Emergency Services & Operations  |  Topic: 406 Beacon Modulation
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Author Topic: 406 Beacon Modulation  (Read 5767 times)
♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,838
Unit: GLR-WI-204

Timmerman Composite Squadron - WIWG - CAP
« Reply #20 on: February 25, 2009, 05:21:16 PM »

So how feasible is it then for us to homebrew a unit that could do this?  I was considering the audio approach too, but didn't know if that would work considering I can't even find a sound byte of the signal anywhere.
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           Capt. Dan Turkal
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                WI-204/CC
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lordmonar
Too Much Free Time Award

Posts: 10,662

« Reply #21 on: February 25, 2009, 05:27:29 PM »

So if I understand this,

ELTs with external GPS sources should have updated lat/long every minute (presumably because an external source will have its own power and wont compromise beacon longevity?)

If the ELT has its own GPS, it will update every five to twenty minutes.


Do I have that right?
More or less. The 1 minute update is prior to beacon activation. The spec doesn't state what happens after the beacon activates. It might still get a 1 minute update, but the transmission interval of updated position is 5 minutes or greater. There is no upper interval limit, the 20 minutes is the minimum for EPIRBs and PLBs.

Some (many, most?) built-in (fixed) ELTs get their position data from the aircraft navigation system, not an internal GPS. If the aircraft system fails or stops updating after a crash, then the beacon is going to transmit the last location at some interval for 4 hours, at which time the memory is wiped out if the position hasn't changed. The spec I read for one model of ELT stated the 406 transmitter automatically turns off after 24 hours, leaving only the 121.5 homing signal.

If the ELT has its own GPS, the position will be transmitted at the 5 minute or greater interval.

Mike

This makes sense....if the COSPAS satellites have not received and retransmitted the positioning data to control center within 4 hours.....it never will.

This data is not used by any of the DF systems (but there are some systems such as the becker that can recieved and decode it) for actual DFing the signal.

So even if the data gets wiped AFRCC has the data has passed the location to the responding SAR agency...at which point they will use 121.5 or 406 to home in on the signal...even if it does not have any location data in the data message. (please note that the 406 signal will still be going out...it just will not have any location data in it.)
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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
Major Lord
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Posts: 1,817

« Reply #22 on: February 25, 2009, 05:29:08 PM »

I would say your chances of developing decoding software without having access to a  signal to decode are low....Maybe you can go over to your local FBO and see if they will set off the test beacon while you have a recorder running.

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
Too Much Free Time Award

Posts: 10,662

« Reply #23 on: February 25, 2009, 05:30:12 PM »

So how feasible is it then for us to homebrew a unit that could do this?  I was considering the audio approach too, but didn't know if that would work considering I can't even find a sound byte of the signal anywhere.

The 406 signal does not have an audo component.  It is 300mili seconds long!  I understand the the sweep tone on the 121.5 signal is a little different than an old style ELT.
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PATRICK M. HARRIS, SMSgt, CAP
Pacific Region
JoeTomasone
Salty & Seasoned Contributor

Posts: 1,661

« Reply #24 on: February 25, 2009, 05:41:23 PM »

Let us know what you find. Does anyone in your neck have a 406 test beacon to see what we can expect in field use?

No, but I have an audio capture of a data burst from one that I made on a recent mission.   I am planning to put it in a presentation I am making.   


I did find this on the encoding method, haven't found any software yet that will decode it.

Quote

3.2.1 Processing of 406 MHz beacon signals

The structure of the message burst transmitted by the 406 MHz beacon consists of three distinct fields: an unmodulated preamble (for carrier synchronization), a bit/frame sync, and a message field.  The bit rate is 400 bits per second.  The data is encoded using Biphase-L Manchester phase modulation with a peak modulation of 1.10.1 radians.  The bit size may exhibit variation of upto 1%.

The unmodulated preamble lasts for 160 ms, followed by 60 ms sync sequence, and then follows message bits.  In a standard message, the 88 message bits divided into fields, which identify the user and (optionally) give location information.  The total standard format burst lasts 440 ms (112 bits) while the total long format burst lasts 520 ms (144 bits).

Of the standard message bits, 61 are known and fixed at the time of beacon manufacture.  The 61 bits in the protected data field, as illustrated in fig.4, consists of a user identity code. These bits are subject to error correction by 21 error correction bits (BCH error correcting code).  There are additional 6 bits, which can be used to transmit either an emergency code or other data. 

The bit synchronization field consists of 15 data "1" bits, while the frame synchronization comprises the bit pattern 000101111. The Fast Fourier Transformation (FFT) provides an effective means of detecting and recognising the presence of one or more ELT signals over the 25 kHz frequency band. 

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JoeTomasone
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Posts: 1,661

« Reply #25 on: February 25, 2009, 05:45:00 PM »

So how feasible is it then for us to homebrew a unit that could do this?  I was considering the audio approach too, but didn't know if that would work considering I can't even find a sound byte of the signal anywhere.

The 406 signal does not have an audo component.  It is 300mili seconds long!  I understand the the sweep tone on the 121.5 signal is a little different than an old style ELT.

Data over radio is always audio as it is transmitted, even if it isn't treated as audio before and after transmission.     Remember the sound a modem would make when you accidentally picked up the telephone way back in dial-up days?   That's what we're talking about. 

If anyone needs a copy of the MP3 I made, email me - first name @ last name.com or first initial and last name @flwg.us.

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♠SARKID♠
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Posts: 1,838
Unit: GLR-WI-204

Timmerman Composite Squadron - WIWG - CAP
« Reply #26 on: February 25, 2009, 11:04:59 PM »

I took that data burst and slowed it down down from .4 seconds to fifty.  You can see/hear the individual pulses but I can't distinguish different pitches in tone.  Perhaps they're outside the human hearing range.
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           Capt. Dan Turkal
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                WI-204/CC
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JoeTomasone
Salty & Seasoned Contributor

Posts: 1,661

« Reply #27 on: February 25, 2009, 11:13:01 PM »

It's binary - tone or no tone in a given time slot in the sequence.

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♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,838
Unit: GLR-WI-204

Timmerman Composite Squadron - WIWG - CAP
« Reply #28 on: February 25, 2009, 11:44:48 PM »

Not with biphase-L.  The binary isn't decoded by "tone" or "no tone", its decoded by "high to low" or "low to high".  I can't hear the two tones as it sounds like a cacophony of different pitches, none of which seem to repeat regularly.  I can pick out the carrier when its unmodulated and that's it.
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           Capt. Dan Turkal
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                WI-204/CC
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Major Lord
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« Reply #29 on: February 26, 2009, 12:00:43 AM »

http://www.interfacebus.com/mil1553-manchester-encoding.html

This kind of coding takes a little bit of mind-resetting for us poor guys used to FSK modulation. The bits are implicit in the transition between states. Crikey!

Nonetheless, if you look at the waveform on the Oscilloscope ( slightly rounded and kind of Fubared by audio filtering and receiver passbands) you can see the data as waveform. My guy did have some preliminary luck treating it as audio, and parsing it out. Someone made the point that it can't be audio, since its only a 300 ms data burst, but we APRS people send a bunch of 1200 Baud data in that time frame using the much more time intensive Bell 202 1200/2200 tones transmission technique.

It may not be realistic to achieve a perfect signal capture with audio, but it should be good enough to confirm that you are at least listening to an ELT/EPIRB instead of a Nintendo Wii with a bad attitude.

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."
♠SARKID♠
Salty & Seasoned Contributor

Posts: 1,838
Unit: GLR-WI-204

Timmerman Composite Squadron - WIWG - CAP
« Reply #30 on: February 26, 2009, 02:10:55 AM »

I understand the coding, thats not too complicated.  What I'm trying to figure out is how I'm gonna make a computer understand it.  This is looking more and more like I would have to go back to college to do this project...
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           Capt. Dan Turkal
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                WI-204/CC
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es_g0d
Seasoned Member

Posts: 301

CAP ES Resources Website
« Reply #31 on: February 26, 2009, 06:29:24 AM »

Each wing should have received a 406 test station (ie practice beacon) in a package deal along with the Seimac DFs and a small AA-powered 121.775 practice beacon.

These were sent out by National about 3 or 4 years ago.  Hunt down the appropriate belly button at your wing...
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Good luck and good hunting,
-Scott
www.CAP-ES.net
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CAP Talk  |  Operations  |  Emergency Services & Operations  |  Topic: 406 Beacon Modulation
 


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