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♠SARKID♠
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« on: February 24, 2009, 03:45:32 AM »

While having our after meeting dinner last night, me and one of my squadron's LTs got to thinking about a couple of future ES projects.  One thing we talked about was having a homebrew setup that would allow our ground team to receive 406 data bursts; not for DFing, but to read the data, get the lat-long right then and there.  If the signal is in motion, we could have our own up to date data stream with GPS coordinates without having to wait for RCC updates.  Well...really we're just geeks and want a neat pet project to work on :).

What I want to know is what type of modulation is the data burst in?  I know its a 30ish figure hex code, phase-modulation, bi-phase L, but I can't find any other info on the modulation type (Packet? AX.25? Hellschreiber :P? )  I've scoured the cospas-sarsat specs for the system, and all it gives is info on the hex-code formats, nothing for the modulation type other than "bi-phase L" which didn't get me far.  Before we can go any farther with the project we need to know the modulation type so we know what we have to decode.

I found some nifty devices here that will do the whole thing, even a couple of handheld models.  But we A) don't wanna pay for this and B), as previously stated, are geeks and want a neat pet project.

Any help?
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           Capt. Dan Turkal
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es_g0d
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CAP ES Resources Website
« Reply #1 on: February 24, 2009, 03:50:53 AM »

*sniffle*  You just made me a little misty.  This is the kind of geekery that makes CAP great.

I wish I had an answer; my guess is that you will get into proprietary information rather quickly if you approach companies who have already worked on such projects.

My suggestion would be to go to NOAA.  Write them, call them, whatever it takes.  Do this in the guise of a private citizen, of course.  They are public servants, though, and unless the information is protected they have an obligation to release it to you.  From time to time simply mentioning FOIA (Freedom of Information Act) will get past laziness or minor roadblocks, as its much easier to provide you with what you're asking for than to have to comply with your formal FOIA request.
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SJFedor
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« Reply #2 on: February 24, 2009, 03:56:59 AM »

The software in the Becker DF units is capable of doing that. Maybe read through their stuff and it'll mention something about it?

Other then that, I'd agree going w/ Scott. And no lie, Scott would probably love to know your results so he, too, can make one himself  :P
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Steven Fedor, NREMT-P
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es_g0d
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CAP ES Resources Website
« Reply #3 on: February 24, 2009, 04:00:32 AM »

You've found me out!  You're not a true Jedi until you're build your own DF ... er ... something like that.  :D
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SJFedor
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« Reply #4 on: February 24, 2009, 04:04:51 AM »

You've found me out!  You're not a true Jedi until you're build your own DF ... er ... something like that.  :D

No Scott, you weren't a true Jedi until you created a section of your webpage totally dedicated to cruel ways to hide practice beacons  >:D
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Steven Fedor, NREMT-P
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sardak
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« Reply #5 on: February 24, 2009, 06:04:29 AM »

You can contact NOAA, but they are going to refer you to the Cospas-Sarsat technical documents.  The RTCA spec for ELTs and the RTCM specs for PLBs and EPIRBs all refer the reader to C/S T.001 "Specification for Cospas-Sarsat 406 MHz Distress Beacons" for all aspects of the signal modulation.

C/S T.001 defines the modulation as, take a deep breath, phase modulation with the data encoded as biphase L. Sections 2.3.5 and 2.3.6 show the waveforms for the encoding and modulation. That's all there is. It's not AX.25 or anything else.  If that doesn't make sense, then you'll need references to digital signal processing.

Mike
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JoeTomasone
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« Reply #6 on: February 24, 2009, 02:55:13 PM »


Allow me to toss a minor monkey wrench in here (sorry).


First, any data (including GPS) that is in the burst should have been passed along by AFRCC; I'm not sure what value decoding it in the field adds.

Second, most (all?) 406 units that I have researched only grab GPS coordinates once and then shut down the GPS unit (or stop polling the a/c GPS) to save power.    So even if it's moving, it will (should?) give you the original lat/long anyway. 

HOWEVER, having something to monitor 406 while hunting DOES make sense since you may very well pick up the data burst further away than you would with the 121.5 signal. 


Major Lord knows quite a bit about this; in fact I recall reading something that his Tiny Track 4 can decode 406 data...     Also recall reading about some open-source software, might be time to test my Google-fu.
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lordmonar
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« Reply #7 on: February 24, 2009, 04:06:45 PM »

That does not sound right.

A lot of the 406 beacons are for marine operations...that means the beacon WILL move over time.   Granted a lot of the aviation beacon may not have this issue.
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PATRICK M. HARRIS, SMSgt, CAP
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♠SARKID♠
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« Reply #8 on: February 24, 2009, 07:56:13 PM »

Well, as for necessity, its not really.  Like I said, we just want to do it because we can and it would be a neat learning project.  Two, if the beacons do provide updated GPS tracks, a decoding program could output the coordinates into a text file which can be inputted as a Google Earth map overlay (nifty nifty).

Okay, so what I think I've figured out is that Biphase-L is a form of binary coding.  Each pulse is a 1 or 0.  Moving the tone from high to low is 1, moving the tone from low to high is 0.  Here's where my logic circuits take over.  In binary, you have four digits with two options (1 or 0).  Mathematics says that means there are sixteen different combinations of binary codes (2x2x2x2=16).  406 beacon registrations and info are displayed in hexadecimal after decoding (0 through 9 and A through F = 16 notations).  Hexadecimal is just a friendly way of reading binary.  So, what I would need to decode a 406 data burst is software that can...

1)  Listen to the burst, take each individual pulse and determine if it is a 1 or 0
2)  Group those 1s and 0s into binary groups
3)  Convert those binaurals into hex code (Beacon ID)
4)  Process the hex code into useful data, easily done with the Cospas-Sarsat online hex decoder

   For Example
Lets say that the currently fictional software decodes the pulses into this binary stream
   001011011100110000110011010111100111100001100011110111000000

That would convert to
   0010 1101 1100 1100 0011 0011 0101 1110 0111 1000 0110 0011 1101 1100 0000

Which converts to this hex code (Beacon ID)
   2DCC335E7863DDE

Which when processed yields this
  

Online 406 beacon registering allows AFRCC to use that beacon ID from step three to access more information not contained in the beacon (Owner, phone number, address, etc).



At least... I think thats how it goes  :-\
« Last Edit: February 24, 2009, 08:00:53 PM by ♠SARKID♠ » Logged

           Capt. Dan Turkal
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openmind
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« Reply #9 on: February 24, 2009, 10:57:51 PM »

That does not sound right.

A lot of the 406 beacons are for marine operations...that means the beacon WILL move over time.   Granted a lot of the aviation beacon may not have this issue.

There are different standards for ELTs, EPIRBs, and PLBs.  In particular, the PLB standard was just updated.

All of those standards include criteria for LIMITING HOW OFTEN the GPS in the Beacon updates its position and encodes that new position into the Beacon Signal.  The goal of this is to limit the expenditure of battery life by not having the GPS in the Beacon running unnecessarily.

As you say, a Marine EPIRB may very well drift, and we want updated GPS fix info, as compared to PLBs and ELTs which should remain relatively motionless after activation.  Even so, we only need an updated GPS fix every 30 minutes - 4 hours, not every 5 minutes.  This allows the GPS chipset to be powered on only a fraction of the time, greatly extending battery life.

The Cospas-Sarsat folks have lots of fun reading on all of these standards.  Some of it even makes the encoding sections seem like light reading!

openmind
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JoeTomasone
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« Reply #10 on: February 24, 2009, 11:29:58 PM »

That does not sound right.

A lot of the 406 beacons are for marine operations...that means the beacon WILL move over time.   Granted a lot of the aviation beacon may not have this issue.

How does the ACK E-04 handle the GPS data input?
The E-04 receives the data input from your GPS at the same rate your GPS outputs the data.
(Typically once per second) The ELT uses the last data received from your GPS as its location
when the ELT is activated either manually or by the crash sensor.

When the AquaFix™ 406 GPS I/O is turned ON, the GPS Onboard receiver is also immediately turned ON and will immediately begin acquiring data. As soon as the GPS receiver acquires good positioning data the red LED will stop blinking and the green LED will begin flashing once every 2 seconds. Once good global positioning data has been obtained, the GPS receiver waits for 20 minutes before looking for new positioning data again.

The AeroFix™ 406 GPS I and GPS I/O are equipped with a GPS Interface. Prior to activating your P-ELT you can download your GPS LAT/LON into the P-ELT using an external GPS receiver and the GPS Interface cable provided with the P-ELT. Once your external GPS has acquired good global positioning data, press the GPS test button for at least ½ second and no longer than 5 seconds and your GPS data will download into the P-ELT (for full instructions see Section 4.7.2). Once you activate your P-ELT, your GPS coordinates are included in the distress signal. The green LED will immediately start flashing every 2 seconds to alert you that your P-ELT is activated and transmitting your GPS coordinates with the 406 MHz signal.

(With ACR, it appears that if the unit has a GPS built in, it checks every 20 mins - ELT or EPIRB.   If it's an external GPS, it appears to be a one-time deal.
« Last Edit: February 25, 2009, 12:42:09 AM by MIKE » Logged
sardak
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« Reply #11 on: February 25, 2009, 01:34:36 AM »

From the Cospas/Sarsat system specs:

- If a beacon is designed to accept position data from an external navigation device prior to beacon activation, navigation data input should be provided [and stored in the beacon memory] at intervals not longer than 20 minutes for EPIRBs and PLBs, or 1 minute for ELTs. (C/S T.001 para. 4.5.5.4, C/S T.007 para. A.3.8.5)
RTCM specs for PLBs and EPIRBs, called out in the FCC regs, match this.

- If the beacon has the capability to provide updated position data, subsequent transmissions of the updated position shall not occur more frequently than every 5 minutes. (C/S T.001 para. 4.5.5.2, C/S T.007 para. A.3.8.3)
RTCM specs state this as 20 minutes, which is the old C/S requirement, but makes sense given the 20 minutes in the preceding requirement.

- If, after providing valid data, the navigation input fails or is not available, the beacon message shall retain the last valid position for 4 hours after the last valid position data input. After 4 hours the encoded position shall be set to the default values. (The default value is NO GPS POSITION, even if something is stored in the beacon.) (C/S T.001 para. 4.5.5.2, C/S T.007 para A.3.8.6)
RTCM specs match this.

The RTCM PLB spec is the only one that includes a "sleep mode" for the internal navigation device.
- After the first valid location fix, or after 2 hours:
- - for the first 6 hours the device shall attempt at least one fix/update every 30 minutes;

- - from 6 to 12 hours, a fix/update shall be attempted every 2 hours;

- - from 12 to 20 hours, a fix/update shall be attempted every 4 hours.

None of the PLB specs I've read mention anything about a sleep mode.

For ELTs, the FCC regs cite an RTCA standard, which just references the C/S specs for these navigation device requirements.

Mike
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♠SARKID♠
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« Reply #12 on: February 25, 2009, 01:41:03 AM »

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?
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           Capt. Dan Turkal
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MikeD
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« Reply #13 on: February 25, 2009, 01:42:32 AM »

Excellent, let me chime in as appreactive of geeky engineering projects. 
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sardak
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« Reply #14 on: February 25, 2009, 02:29:37 AM »

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
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♠SARKID♠
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« Reply #15 on: February 25, 2009, 02:35:06 AM »

Okay, well good to know.  Thanks for the research.  I'm still stuck with the problem of finding the software that is going to make this project possible.  The Beacon Tester that I found on the link in my first post is way way way out of the question.  I found it being sold for over four grand.
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           Capt. Dan Turkal
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es_g0d
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« Reply #16 on: February 25, 2009, 03:14:01 AM »

I was thinking of throwing the fact that the 406 (5 Watt) transmitter shuts down on certain models (ACR?), but I was going to check my facts before sardak corrected me!  I'm glad he's on top of things (and will likely correct my 406 output power listed above).  :D

This was of significance in the missing King Air in Guyana.  There was a 121.5 signal heard for a short time--SEVERAL DAYS after the aircraft was missing, but no one was able to track it (with a 406 ELT on board).  Nothing was received by SARSAT.  The wonder was that if the antenna had been masked during the 24-hour period, and then subsequently come uncovered (after the 406 stopped transmitting), that could have caused the scenario.

The group http://internetsar.org/ is now involved.  You may remember that they made headlines during the Fosset Search.  You can read a little about the aircraft, crew, and search there.  The aircraft has now been missing for several months.  Its a pity.
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Major Lord
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« Reply #17 on: February 25, 2009, 10:27:30 AM »

Don't be phased by the people who say it can't or should not be done! I am even planning on including 406 decoding in my next generation RDF unit.

The Data are transmitted in Manchester coding format, so you may have a bit of a problem decoding it down to the zeros and ones level. Our approach using the TinyTrak 4 essentially is treating the incoming data stream as audio. I don't have any black boxes that receive 406.xx and spit it out as decoded manchested bi-phase data......Even good receivers in this range are scarce.

People have always assumed that a crashed AC with a 406 unit on board will be able to hit the satellites, and that it will have a relatively fresh GPS fix. These are, in my opinion, questionable assumptions. The higher power burst encoded position reports could be DF'd in a pinch, but you would have to have special DF gear to make it happen. Still, being able to receive a 406 signal that is malfuntioning, not able to be localised by the SARSAT with a high degree of accuracy, or caused by intentional jamming seems like a good idea to me. GPS jamming is already here.

Hey that Becker unit is only $20,000 dollars right?

We have not continued work on the decoding software, since it is not a big part of what we do (mostly APRS) but eventually, we will get back to it. Byon was able to decode samples from recordings on CD's in the lab, so it is very possible. I think just knowing that you can receive the 406 signal is an excellent piece of additional information when conducting an OTG search.

Major Lord
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JoeTomasone
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« Reply #18 on: February 25, 2009, 12:42:46 PM »

I think just knowing that you can receive the 406 signal is an excellent piece of additional information when conducting an OTG search.


Agreed.   I'm currently shopping around for a mobile (not portable) scanner to do just that - and to monitor 121.5 as I'm driving.

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Major Lord
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« Reply #19 on: February 25, 2009, 12:48:16 PM »

Joe,

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?

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♠
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« Reply #20 on: February 25, 2009, 01:21:16 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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lordmonar
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« Reply #21 on: February 25, 2009, 01:27:29 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
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Major Lord
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« Reply #22 on: February 25, 2009, 01:29:08 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
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« Reply #23 on: February 25, 2009, 01:30:12 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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JoeTomasone
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« Reply #24 on: February 25, 2009, 01:41:23 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 1.1±0.1 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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« Reply #25 on: February 25, 2009, 01:45:00 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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« Reply #26 on: February 25, 2009, 07:04:59 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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JoeTomasone
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« Reply #27 on: February 25, 2009, 07:13:01 PM »

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

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♠SARKID♠
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« Reply #28 on: February 25, 2009, 07:44:48 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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Major Lord
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« Reply #29 on: February 25, 2009, 08:00:43 PM »

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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♠SARKID♠
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Unit: GLR-WI-002

Timmerman Composite Squadron - WIWG - CAP
« Reply #30 on: February 25, 2009, 10:10:55 PM »

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
.
es_g0d
Seasoned Member

Posts: 302

CAP ES Resources Website
« Reply #31 on: February 26, 2009, 02:29:24 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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