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Malaysian Airlines MH370 contact lost

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Malaysian Airlines MH370 contact lost

Old 7th Apr 2014, 14:58
  #9401 (permalink)  
 
Join Date: Sep 2006
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Sound versus radio energy

I've been offline all weekend and a very quick peruse of the last couple of pages spurred me to produce the following "tome". Apologies if it is just repetition. I was a sonar specialist with the UK Nimrod force and offer the following to help those who still mix radio energy with sound energy...

Some people are still getting radio transmissions mixed up with sound transmissions. They are totally different but share some similar characteristics and terminology. The confusion clearly arises from these similarities.

The fundamental difference between sound and radio transmissions is that radio signals travel (as near as dammit) at the speed of light whereas sound travels at a speed some 880,000 times slower.

Radio transmissions are a form of radiation and require no medium, like air or water, in which to propagate (travel). Radio energy can be transmitted across space - think of pictures and telemetry beamed back to earth by Voyager, the Hubble telescope, the International Space Station, etc. Radio energy can propagate happily through the vacuum of space and through our atmosphere.

Sound, on the other hand, is a series of compressions and rarefactions of molecules. No molecules, no sound transmission or reception. In space, no one can hear you scream!

Sound can travel through air, most solids (listening for a train by putting your ear on a rail is NOT recommended - especially not the one carrying the electric current!!). Sound can travel through water. So too can radio energy, but it is so fiendishly attenuated that there are few practical uses for trying.

Speeds:

Radio energy, RADAR energy (derived from RAdio Detection And Ranging), light and other forms of radiation are all part of the electromagnetic spectrum. To most intents and purposes, they all travel at (rounded) 186,000 statute miles per second and we'll call it 300,000 kilometers per second for cash. Pilots are well accustomed to working with a multiplicity of units but forgive me for choosing metric, i.e. metres per second. Radio energy travels at 300,000,000 metres per second.

Sound! For the purpose of this post, I'll stick to air and sea water:

Sound in air, as any ATPL holder will tell you is calculated by: "Thirty-eight point nine four ROOT Tee!" This ancient mantra tells you that if you convert the air temperature from Celsius to Kelvins (i.e ADD 273), you get "Tee", the temperature on the Absolute scale.

Multiply "Tee" by 38.94 and you arrive at the speed of sound in air, in nautical miles per hour. That's knots to you!

e.g. Air at 15 degrees Celsius is 15+273. T = 288Kelvins.

The square root of T is 16.97056274847714

The speed of sound in this air is 38.94 X 16.97056274847714 = 661 KNOTS
= 340 metres per second. Compare this speed with that of radio!

The speed of sound in seawater is a bit more tricky to calculate. Our (Nimrod) equipment used data based on Wilson's Simplified 32 term Equation. Well why wouldn't we?

Whereas the temperature of air (at levels and densities in the atmosphere where conventional flight takes place) is the dominating factor by far, temperature, pressure and salinity of seawater cause significant variations in the speed of sound in that medium. If we accept that the criminally average speed of sound in seawater is 4900 feet per second, this gives us 1493 metres per second.

For what it's worth, to get sound to travel at 1493 m/s in air, the air would have to be 5,281 degrees Celsius or 5,554 degrees Absolute! RAF Kinloss on a sunny day, hey!!

Sound transmissions and radio transmissions share many properties and problems. One important shared characteristic is the general principle that higher frequencies travel shorter distances than lower frequencies. There are particles in the air and there are particles in the water that obscure our ability to see very far. Clouds and dust in the air spring to mind as do shoals of fish and particles of digested and excreted "stuff" in the water. The chief reason for this obscurity is the wavelength-to-particle ratio.

We tend to speak of "frequency" whereas, in the context of a signal's obscurity or inability to penetrate the medium, we should really be thinking in terms of "wavelength". Bear with me, I'll explain why in a short while.

Wavelength and frequency are directly related to each other. But they are GOVERNED by the speed of propagation of their transmitted signal. I say again: the speed of propagation is VASTLY different between radio waves and sound waves. The two should not be mixed up but in the heat of the moment....!?!

Wavelength is calculated from the following triangle. Cover the element you need and the structure gives you what to do with the other two terms. In this case, C = f X λ

C
f λ

Where C = the speed of the transmission in the stated medium. I'm using metres per second.

f = the frequency of the transmission in Hertz (cycles per second).

λ(Greek lambda) = wavelength. Your adding stick will churn it out in metres if you used metres per second for C

Radio, RADAR and visible light are all bands of the electromagnetic spectrum. To all intents and porpoises, use 300000000 (3, then eight taps on the zero key). If you do so, you are using metres per second.

For example, typical airborne weather RADARs work between 9GHz and 10GHz. Using 10 gigs (ten thousand million Hertz), the wavelength would be:

C
f λ

C = 300000000 metres per second, divided by 10000000000
λ = 0.03 metre or 3cm

Subwoofer sound at 20 Hertz in air in your living room at 23 degrees Celsius:

C in knots = 38.94 √T = 38.94 √(23+273) = 38.94 X 17.21= 670kts = 345m/s

Wavelength:
C


345
20 = 17.25 metres. Useful for checking your listening room's dimensions versus the space needed for one wavelength of sound! Or finding a null zone where direct sound and reflected sound from ceiling or floor might cancel each other out if in antiphase.


In seawater of average salinity (35 parts per thousand), a sonic locator beacon transmitting at 37.5 kHz:

1. For imperialists:

C = 4900 feet per second

frequency = 37500 Hz

Wavelength = 4900/37500 = 0.13066666666667 foot = 1.568 inches


2. For bass tenors (!) 4900 fps = 1493.52 metres per second

Wavelength = 1493.52/37500 = 0.0398272 metre = 40mm for cash

Signal in seawater at 9.5 kHz = 1493.52/9500 = 0.15721263157895 metre. 6 inches, give or take.

Signal in seawater at 1000 Hz; Wavelength = 1.49352 metres

Signal in seawater at 500 Hz; Wavelength = 2.987 metres

Signal in seawater at 100 Hz; Wavelength = 14.9352 metres

Sound transmissions, like electromagnetic transmissions, are prone to attenuation. The signal power and intensity diminish with range from the source.

Ordinarily an emission spreads from its source in a spherical pattern (i.e. in ALL directions). Transmitted power at the wavefront diminishes rapidly. Power loss = 20 log Range.

If conditions are conducive to the formation of a duct, the signal losses, being constrained in the vertical, spread cylindrically. The power lost at the wavefront is considerably less with spherical spreading than with cylindrical spreading. Power loss = 10 log Range.

The importance of thinking in WAVELENGTH rather than frequency.
Absorption and scattering. Any object roughly equal in size, or larger than the wavelength of the transmission will be in part absorbed by the object as heat energy. In addition, some of the signal will bounce off the object and be reflected from its surface. You see this with RADARs, whose wavelengths become equal to or shorter than large water droplets or large agglomerations of frozen water held aloft by updraughts. To some users these RADAR returns will be annoying because they shield what the user is looking for; this user would call this "clutter". To someone else, this same "clutter" is useful for illustrating weather to be avoided.

In air or water, sound wavelengths may be similarly absorbed and scattered by objects of similar size to the wavelength. If you were to try to cross a busy shopping mall at Christmastime, your progress would be much impeded by other angry shoppers of similar size and mass to you. Drive through those shoppers in a tank and your passage through them is much improved.

For a given patch of sea, absorption of sound in the sea is linear with range. Absorption will be increased by an increase in salinity, a decrease of temperature or an increase of transmitted frequency. Absorption varies approximately with the square of the frequency.

We tend to talk a lot about frequency whereas, in my opinion, the effects can be better conceptualised by reference to wavelength - hence my crude illustration about the shopping mall above!

The scattering of sound in the sea is also frequency (in fact think wavelength) dependent. Surface scattering varies directly with wave height, signal wavelength and angle of incidence of the sound wavefront.

Bottom scattering is dependent upon bottom roughness, particle size versus signal wavelength and angle of incidence of the sound energy.

Volume scattering is the name given to scattering of sound by whatever is in the water of comparable size to the sound's wavelength - or larger. Again this is frequency (wavelength) dependent. It depends on the mass of the object if the object is a solid - like a fish but bubbles are even more efficient scatterers of sound energy.

Layers in the ocean.

Without temperature measurement it will be impossible to state there is or is not a sound channel (duct) in the locality.

The ocean is often conceptualised into a three-layer model: the surface layer, the thermocline and the deep ocean layer.

The surface layer is isothermal - the same temperature. It is isothermal due to wave action mixing the water in the layer.

The deep ocean layer is also isothermal and in most oceans, is around 4 degrees celsius.

Separating the surface layer and the deep ocean is a layer called the thermocline, where the temperature decreases from that of the surface layer to that of the deep ocean layer.

The heat of the day can cause a transient thermocline in the surface layer and the seasons impose their own alterations.

The importance of knowing the thermal profile of the water is in determining how a sound may behave between the source and the receiver.

An increase in any of the following will produce an increase in the speed of sound in the water: temperature, pressure, salinity.

It's late and I can't be bothered giving you the mathematics of Snell's Law. But sound exhibits some properties of waves and some properties of rays in the way it propagates in a medium. It is subjected to changes in behaviour as it encounters boundaries: the sea surface, the boundary between the bottom of the surface layer and the thermocline, the boundary between the bottom of the thermocline and the top of the deep ocean layer and the seabed are all boundaries.

The signal path the sound takes may reflect off a boundary, pass through unimpeded or be refracted (i.e. change angle). The angle it meets a boundary is quite important.

Sound rays tend to be refracted away from depths of maximum sound velocity. As a result there often exists at these depths a region into which very little acoustic energy penetrates. Such a region is called a shadow zone.

The limiting ray is that ray which just grazes, or is tangent to, a boundary as just described. Any sound ray approaching the boundary at a more perpendicular angle than that of the limiting ray will be reflected, or pass through the boundary.

The Critical Angle ray is that which strikes a boundary at the steepest (most perpendicular) angle and still exhibits almost 100% reflection. Any sound rays striking the boundary at a more perpendicular angle than that of the Critical Angle ray will lose some significant reflection losses.

A sound, from its source, may undergo many alterations of course. But don't forget, all the while the sound is losing power with distance and being scattered and reflected by particles and objects in its path. And the higher the frequency, the shorter its wavelength and the greater the number of things in the water at or above its wavelength dimension to absorb the sound and/or reflect it away from the path it needs to travel to the receiver!

Alterations in the sound signal path may also occur near river mouths where the salinity is reduced by fresh water. There are also ocean fronts which can cause density discontinuities and also alter the signal path.

Another layer worth considering is the "Deep Scattering Layer". Like the ionospheric changes that occur in the D, E, F1 and F2 layers, the Deep Scattering Layer is on the move at sunrise and sunset. It rises to near the surface at sunset and descends to the deep at sunrise. Plankton rise as the sun sets. So too do the fish etc that feed on them and they all go back to the deep as the sun comes up. In the average ocean, the Deep Scattering Layer will be about 600 feet at night and 3000 feet by day.

Pontius Navigator has described sound channels and a peculiar type of transmission path called Convergence Zone in earlier posts. The difficulty I see in the path sound takes in the case of this suspected sonic locator beacon, is that the beacon is on the seabed, whereas any receiver could well be at least two boundary layers up. What with refraction and reflection of the ray, this adds a further difficulty in homing in on any such signal. Getting source and receiver in the same layer would be a huge help.
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Old 7th Apr 2014, 15:00
  #9402 (permalink)  
 
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Shielding and insulating an electrical device to contain a fire is all well and good. However, it should not be forgotten that electrical fires can be initiated not only in the end device being powered, but also in the wires of the power circuit leading up to it. (SwissAir).
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Old 7th Apr 2014, 15:04
  #9403 (permalink)  
 
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Corrected Satellite Data

Hi,

I see many news agencies reporting that Angus Houston on Sunday said:
"The hunt for the jet was refocused on the southern end of the search zone Sunday after corrected satellite data showed it was more likely the plane entered the water there."

Anybody here have any information about the "corrected satellite data" ?
Was the original values released by Inmarsat wrong?

The media seem to be in "BB pinger story" mode and do not seem puzzled by this, so maybe we will never know the answer to this.

If someone has updated doppler/latency figures, please post it here.

Thanks.
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Old 7th Apr 2014, 15:05
  #9404 (permalink)  
I don't own this space under my name. I should have leased it while I still could
 
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Kooljack, if this is the paper to which you refer:

https://fenix.tecnico.ulisboa.pt/dow...651/resumo.pdf

There is a diagram that appears to show multi-path propagation, is convergency zones. If that be the case it is possible that both ships indeed detected the same signal source.

Edit:

Lonewolf below: I wouldn't disagree with that and I am not advocating that it is definitely a CZ detection just that it is just possibly conceivable and should not be discounted.

Last edited by Pontius Navigator; 7th Apr 2014 at 15:16.
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Old 7th Apr 2014, 15:08
  #9405 (permalink)  
 
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Perhaps referring to it as "a known, T7-specific issue" is going a bit over the top.
I assume what he meant by 'known' was that it has happened before on a triple-seven therefore you can't rule out it happening again.

re. 'tamper proof' communications devices, an onboard battery would solve the problem of needing to remove bus power from the device in an emercency. In fact it could be further refined with an algorithm that sent out a warning if the CB was pulled in the absence of any related EICAS warning.
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Old 7th Apr 2014, 15:12
  #9406 (permalink)  
 
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Dai_Farr: nice explanation ... almost makes me nostalgic for the old ASW days.
Originally Posted by Dai_Farr View Post
Pontius Navigator has described sound channels and a peculiar type of transmission path called Convergence Zone in earlier posts. The difficulty I see in the path sound takes in the case of this suspected sonic locator beacon, is that the beacon is on the seabed, whereas any receiver could well be at least two boundary layers up. What with refraction and reflection of the ray, this adds a further difficulty in homing in on any such signal. Getting source and receiver in the same layer would be a huge help.
I think that if the receiver were below the axis of the deep sound channel, best hopes for detection would be realized. A few thousand feet deep, maybe, depending on the BT drop that day.

For Pontius: my opinion is that there isn't enough acoustic energy for the wavelength used to get a CZ path. I'll accept correction on that if I've forgotten a few more bits of "Acoustics for ASW Folks" than I had realized.

EDIT: Agree with Pontius about not ruling out anything, Bill S's point about direction of propagation considered.

Last edited by Lonewolf_50; 7th Apr 2014 at 15:45.
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Old 7th Apr 2014, 15:15
  #9407 (permalink)  
 
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@pontius
Afaik that diagram is more specific to lower frequencies.
37.5 kHz tracks will be near vertical.
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Old 7th Apr 2014, 15:27
  #9408 (permalink)  
 
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wasn't there a problem with design or installation of the media equip in swissair, something like it being hard wired.

In a correctly designed system overloading caused by a box should trip the breaker long before the wiring catches fire, but accepted if the fire spreads from the box to the wiring it will burn whether power tripped ot not.
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Old 7th Apr 2014, 15:49
  #9409 (permalink)  
 
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It seems a pity they do not operate a TPL pair.

"Flying" two locators, perhaps 1000m separation, would be manageable when cable lengths are over 10,000m and they only each require a co-ax pair.
That would provide wider search sweep and lateral location.
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Old 7th Apr 2014, 15:50
  #9410 (permalink)  
 
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lucille:-
The Chinese, with a couple of guys sitting in a rubber dinghy claimed to have heard the 37.5 KHz signal using hydrophones and a shared iPhone earpiece between them
Interestingly, I could have sworn I saw a freq of 87.1 kHz on their display....
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Old 7th Apr 2014, 16:14
  #9411 (permalink)  
 
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transponder always on

Quote:
If you can't turn 'em off, what you gonna do when they catch fire! DOH!
Sadly I fear the answer to that question will be an outcry that nothing could have been done to stop a potentially stoppable fire.

You put a CB down in the E&E bay, where nobody can touch it. Just like they do in Airbuses.
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Old 7th Apr 2014, 16:47
  #9412 (permalink)  
 
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BillS, #9512:

In wonder if they have the capability to use sar techniques to get a better idea of position. I know the pinger is one way, but it should be possible to correlate reception over time, especially if sweeps are done orthoganally...
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Old 7th Apr 2014, 16:50
  #9413 (permalink)  
 
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Looking at the cascade plot posted by Cloudbase4812 there appears to be only one clear signal repeating at set intervals.

This would seem to signify that either the CVR and FDR have been separated or that one has stopped pinging.

Is there any way to differentiate which unit is radiating?
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Old 7th Apr 2014, 16:55
  #9414 (permalink)  
 
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silvertate:-
You put a CB down in the E&E bay, where nobody can touch it. Just like they do in Airbuses
....except, it is still accessable to the pilots.....
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Old 7th Apr 2014, 17:09
  #9415 (permalink)  
 
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One reason to recover the hull is that the pax may have recorded their experience of the flight, in forms that can still be read.
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Old 7th Apr 2014, 17:27
  #9416 (permalink)  
 
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Shielding and insulating an electrical device to contain a fire is all well and good. However, it should not be forgotten that electrical fires can be initiated not only in the end device being powered, but also in the wires of the power circuit leading up to it. (SwissAir).
Isn't that what arc-fault breakers are supposed to fix?
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Old 7th Apr 2014, 17:36
  #9417 (permalink)  
 
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Slight thread divergence here but could someone tell me exactly what starts the black box transmitter off? Would a heavy landing do it for example or conversely would a very soft landing on water not do it?
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Old 7th Apr 2014, 17:39
  #9418 (permalink)  
 
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AIS Marine Traffic

With Marine Traffic, you can see the ships, and by clicking on them, show their tracks...

(I manually added the ship names as you can only view one at a time...)

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Old 7th Apr 2014, 17:40
  #9419 (permalink)  
 
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Sonar Pulse Range in Water

Dai Farr - A fine summary

Long ago in another job I designed the electronics for hydrographic survey work. Things have changed a lot since then.

Those who have followed this thread have heard that the range of the ULB is likely to be 600 or 800 or 1500 or 2000 metres. We have seen the photo of the ULB locator on the small Chinese boat - with headphone monitoring.

Typically those receivers are tunable and have a bandwidth of about 1 to 3 KHz. The range of detection that has been bandied about is probably about right: .6 to 2 km for that sort of detector.

But with modern technology and a pulsed carrier ultrasonic transmission with reasonable stability one can use a very narrow band detection scheme.

Reducing the received bandwidth to 10 Hz could increase the detection range by a factor of 10 or more (without benefit of any ducting contributions). If the signal from the ULB is sufficiently stable in frequency, going to a coherent detection scheme and slightly narrower bandwidth and gating the receiver at the sonar pulse rate could possibly achieve a detection range of 10-30 km. The major limitation on this sort of scheme - it would only work with a non-moving vessel due to doppler shift becoming a major contribution to the frequency. But that can easily be addressed with modern DSP techniques. Having 500 or 1000 virtual receivers (it's only software) focused on detecting carriers every 2 Hz across the spectrum would be easy. And once the carrier is found, the doppler shift would become a viable cue for deciding which direction to move in search of the ULB.

Pure speculation: this technique is being used right now. I am wondering if the Benthos ULB locator was shown on the small Chinese rubber boat for show only... It would be a nice way to get the report out without disclosing some more serious sonar analysis capabilities.

By complete coincidence my work this last week has been focused on analysis of sonar reflective returns at 40.0 KHz
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Old 7th Apr 2014, 17:59
  #9420 (permalink)  
 
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Originally Posted by DocRohan View Post
@slats.
I am not sure that there would be much value in determining carboxyhaemoglobin or lactate levels in blood from bodies immersed in hypertonic salt water...Likely the hypertonicity would significantly alter biochemistry.
Examination of lungs and detecting diatoms could give an indication of whether people were alive or deceased prior to immersion, although this is still not 100% conclusive at autopsy.
(apologies for graphic content)
Hypertonic or not, salt water is still basically water and electrolytes, I don't think that it would mess with lactate, except through simple diffusion.

I'm skeptical about these tests too but for a different reason.

CO poisoning is one of the less likely things to kill you in a fire, it usually happens during prolonged exposure to gases from a poorly ventilated furnace or an internal combustion engine. CO only forms in the presence of insufficient supply of oxygen. CO poisonings are common in general aviation, and almost invariably occur when piston engine malfunction results in the exhaust gas being pumped into the cabin. The only large jet incident I could find was caused by a malfunctioning air conditioner. A fire that gets to the point where hazardous levels of CO are produced would pose a high risk to structural integrity of the aircraft, and there would be ample direct evidence of that.

CO could be one of the things to look for, but it would indicate a mechanical problem, like the aforementioned air conditioner, rather than a fire.

I don't think that lactate forms in significant quantities during hypoxia if the person is stationary.

In both cases, there should be more obvious physical signs, e.g. soot in the lungs to indicate a fire.
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