Acoustical Search

[itsmoked]

Let's have a little discussion on the hunt for the Flight 370 black boxes.

There's now a map out of the week's detected pings.
AP Article
Acoustical
What's happening is that as they drag the small skate-shaped microphone platform around they hear the recorder pings for a couple of seconds to over a couple of hours. Once they lose the the signal they turn and weave and try to pick up the signal again, always failing.

From their statments they are trying to "triangulate" the pings to figure out where the black boxes (BB) are.

My two questions:
1) How can they possibly hope to triangulate on the BBs with such sparse data containing NO directional information.

and

2) Why do they not have two, or three, or four microphones on the drag platform so they can instantly get a direction vector from ANY and EVERY ping detected? I'm baffled by this obvious lack of sophistication.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

One reported problem is that the beacon is broadcasting off frequency, i.e. s/b 37.5 kHz, but is actually broadcasting at ~33 kHz, and, it's apparently not broadcasting continuously, like it's suppposed to.

As for location, I'm guess, given the size, is that it's doing it solely based on signal strength, and building up a signal strength map, which would show the direction of maximum signal. I'm surprised someone isn't using submarine towed arrays, which ought to be able to do direction finding within a single pass.

TTFN
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[Skogsgurra]

Yeah, I would also be surprised if they didn't.

We used low-frequency microphones to locate enemy gun locations when I did my military service around years ago. An array with microphones (we used large loudspeakers as microphones) spread out along a 200 - 300 m long baseline, a fast recorder (used UV at that time) and some trigonometry located any gun or haubits to within 20 - 50 meters. Depending on weather and distance to enemy.

Surely, a similar technique is available today? With a lot more precision - even if speed of sound in water is a lot faster than in air.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[OperaHouse]

Water can do strange things. When two water temp layers meet it can divert the signal. Submarines have used this to divert detection. Real life isn't like textbook examples.

 

[VEBill]

Around 1980 as a summer job, I was tasked to design and build a precisely phase-matched 3-channel low-Q bandpass filter for this purpose, getting a relative bearing from an L-shaped array of three hydrophones. Back then, f was 10 kHz.

--

Some wag on the Internet has processed an audio sample from the MHG370 search coverage and he believed he could detect a change in the period indicating increasing range. Perhaps it's a future search technique.

--

These UABs will be good for 90 days, not just 30, in future versions. The transition to the new standard is just starting.

 

[Skogsgurra]

Yes Opera, I think that most of us have some real life experience. We don't make our living from reading text-books. The water temperature layers were a problem in the shallow waters between USSR and Sweden during the searches for Soviet submarines back in the eighties. But, still, there were some useful results.

We are obviously discussing deeper waters here. Some 3000 - 4000 m. So, it should be possible to get below the upper layers and get useful readings.

I admit that I am out on deep waters here. Never done that myself. But the fact that there may be a few problems to solve shouldn't discourage us nerd engineers - should it?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MikeHalloran]

I think the 'pingers' emit bursts of clicks at ~1s intervals.

{
It won't help now, but it should be possible, and relatively simple, for future pingers to just identify themselves directly, by encoding a unique serial number in the burst by suppressing individual clicks in a pattern.
}

The self-imposed requirement to dredge up some identifiable wreckage before committing the UAVS to do side-scan sonar sweeps is sort of self-contradictory, because you need an ROV to grab stuff off the ocean floor, and you're sending it down blind without the side scan map to tell you where to drop it.

OTOH, the side-scan will presumably deafen the TPLs, so I can understand running TPLs as quietly as possible until the pingers die. ... which seems to be happening about now.

Identifiable flotsam would be nice. Either there is no flotsam, or it has blown into a gigantic gyre of garbage, or ... the wreckage is actually on a mountainside in Kazakhstan.
Yeah, I saw the analysis showing better correlation for a straight course intersecting the 'southern arc', but I couldn't help wondering if a curved course would correlate to the northern arc. I'm troubled that that decision to abandon half of a huge search space was based on five-ish inferred data points and the assumption of a straight line course.








Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

Ah, IR a signal strength map.. That's a good point. Except when you have those thermoclines the strength could be really misleading.

-------*--     *
     *   *   *
   *   ----*---------------
 *
BB

A sub makes soOOO much sense. I bet they have crazy good direction abilities. If it was an American flight they'd probably jump right to it.


VE1BLL; Don't leave us hanging!! How'd the L-shaped array and your filters work out?

Gunnar; It's interesting that they're hearing the pings even though it seems they're greatly exceeding the reported distance limits they were saying earlier. It's probably why the hearing is so erratic. Pipes of clear conduction stretching the distance considerably when things are lined up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

@Mike - the 'clicks' are short pulses of 37 kHz. The acoustic amplitude is off the scale at something like 160 dB. Based on instinct of how they'd have to be designed (the piezo transducer must be part of the oscillator), it might be challenging to modulated the emission, given how short the pulses are.

If it could be modulated, then it might be better to encode a Barker code or similar SNR improving waveform than an ID. They're unfortunately short range and rarely used, so there's not too much business case for an ID.

 

[IRstuff]

The pingers bursts a 37.5 kHz carrier for > 9ms at ~1 Hz rate, per spec, but the signals that were reported earlier were at ~33 kHz, so there's some concern that it may be man-made, but not the actual locator beacon. Additionally, the pings are occurring intermittently, the first ping sequence was on the order of an hour, while the subsequent pings have lasted minutes or seconds

TTFN
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[VEBill]

@Keith - the filter worked hot-d@mn, but I went back to school and never heard how the larger project multi-year worked out. I have no doubt that the system could tell the bearing of the signal, but the bearing of the source may be impacted be layers and reflections / refractions. Of course if one uses it for homing (following the signal back to the source), you'd get there eventually.

 

[IRstuff]

Interestingly, the Teledyne ELB doesn't list operating temperature as a TSO qualification test. The aircraft is probably below freezing, where it is, so without a temperature stabilized or compensated oscillator, the frequency could possibly be lower.

TTFN
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[MikeHalloran]

I'd expect the pinger frequency to drop a bit, given two and a half miles of water on top of the diaphragm.

They're acting like there could be an infinity of pinger-type devices active at any given time in that desolate ocean; I'm having a hard time with that.


Okay, if the piezo is part of the oscillator, gating individual cycles gets harder. It's still possible to modulate the oscillator gate duration to encode numbers; add or subtract a little time, etc.

Mike Halloran
Pembroke Pines, FL, USA

 

[OperaHouse]

Years ago I worked on an ultrasonic whistle project. It was supposed to work in liquid sodium to detect flow. Water was used because it closely matched. The whistle would work for about 30 seconds and then fade away. Took us a while to figure out that it was making billions of small bubbles and that was absorbing the sound.

So when they first talked about the pinging they said they had to be within three miles. Later the reports talked about picking up signals 100 miles away. Then I remembered the experiment. At great depths there are very few dissolved gases.
Might be the reason for extended travel. Also the reason they want to drop the microphones down to pick up.

 

[Pud]

It's still possible to modulate the oscillator gate duration to encode numbers; add or subtract a little time, etc.

Could it not just send the aircraft callsign in Morse code? (If anyone can still read it!)

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[MikeHalloran]

Yeah, it could send the a/c call sign, but programming that requires opening or programming or modifying the pinger when it's installed in the a/c, which is a bad idea, not least because a slack maintenance staff would never bother to change the default.

If the pinger had its own globally unique serial number, not unlike the MAC address of each and every network adapter, then the a/c could be identified by searching paper-ish records, after picking a number out of an audio recording or scope trace, where say a 9ms burst was a binary 0, and a 15 ms burst was a binary 1, and taking it from there.

If you wanted to get fancy, you could move the trailing edge of the burst around in 1ms increments to encode more than one bit per burst, but there's something to be said for not requiring anything more special than an oscilloscope or laptop screen, and given the demonstrated capture times of whatever they've found, the data rate is not terribly important.







Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

"Could it not just send the aircraft callsign in Morse code? (If anyone can still read it!)"

That would make it substantially more complicated than it is, which is currently and oscillator, a gating circuit, and a water pressure activated switch. In order to send anything else, there would need to be a programming interface, flash memory, etc.

TTFN
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[TheBlacksmith]

Target motion analysis is a technique for passive locating; by moving your receiver in a known direction targential to the noise source you can develop lines of probability, then travel in a different direction and develop several lines, the lines should begin to cross. The intersection should be the approximate location of the noise source. It's just celestial navigation in reverse, where you sight several stars at differant angles and record the time and elevation. There is only one circular path one earth where you'll get that angle at that time, do at least three stars and you're located where the circles intersect.

 

[itsmoked]

TheBlacksmith: I'm not seeing how "lines of probability" are going to give any kind of directional info unless something else is added.. Like amplitude?

Any chance a Doppler shift could be detected based on a 10knot tow speed that would give directional info?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Ocean Shield's reported speed is only 2 kt, probably to minimize the motion noise for the locator, which is consistent with

https://www.supsalv.org/00c2_tpl.asp?destPage=00c2

which list speed as between 1-5 kt.



TTFN
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