37KHz from Black Box

[vk6zgo]

It is probably a convenient division of  a standard crystal frequency.
If it comes to it,why are receiver I.Fs the weird frequencies they are?

Receiver IF frequencies are chosen to place their image and fundamental frequencies out of the way.

It was a rhetorical question--I know why!

IF frequencies aren't set in stone,though --some older  radios use 465kHz rather than  455kHz--some were 262kHz at one time.
The 10.7MHz  frequency is sometimes replaced by 10.68 MHz,or 11 or 9 MHz.
Australian analog TV sound IF is 5.5Mhz,in the USA it is 4.5MHz,& in the UK,6.5MHz.

[zapta]

I was expecting to hear from Dave by now... I dont see him replying much in the blog... I wonder what is he up to nowdays....

Fixing the Tapatalk plugin of course, or on a beach in Hawaii posting prerecorded videos, or in a dark cell due to a DUI...  This could be an interesting Where Is Dave thread. ;-)

As for the topic at hand, here is an interesting video from Mike

http://youtu.be/mQehX0rVYuY

[mfeinstein]

I was expecting to hear from Dave by now... I dont see him replying much in the blog... I wonder what is he up to nowdays....

Fixing the Tapatalk plugin of course, or on a beach in Hawaii posting prerecorded videos, or in a dark cell due to a DUI...  This could be an interesting Where Is Dave thread. ;-)

As for the topic at hand, here is an interesting video from Mike

http://youtu.be/mQehX0rVYuY

I should have linked mike's video on my first post, really good one Mike!

[bench_knob]

Hey Dave, watching so many of your videos I know you worked for a long time with sonar buoys and other seismography related equipment for the navy, right?

With all the lastest news of the missing malasian flight, I keep hearing that the pinger inside the Black (orange) Box, sends a signal of 37KHz when it's under water...the thing that gets my curiosity is why 37KHz? Is it a "magic" number? Does signals in this frequency go further in the ocean? Does anyone has a knowledge why they chose this particular frequency as a standard?

Thanks!

www.radiantpowercorp.com/dk-series-underwater-locator-beacons.aspx

www.radiantpowercorp.com/dk-180-airframe-locator-beacon.aspx

Thanks for sharing the specifications, but I dont see where it says WHY they chose this particular frequency, and not like 100Hz or 20KHz or 40KHz or 30KHz or even 35Khz or even 37KHz, but 37,5KHz :0 it sounds too magical for me...maybe they flip a coin, but i rather ask then speculate

Yep.

I forgot to mention in my post that this manufacturer is the actual maker of the two ultrasonic-beacons that were mechanically installed on the OUTSIDE of one end of each of the two 'black-boxes'.  Apparently the two black-boxes are located at each end of the airplane, the tail box isn't pinging, which is consistent with the plane depleting its fuel, as apparently, the plane stalls, and goes in tail-first.   Physically the beacons are housed inside cylinders that are mounted on the long-side of the cylinder, so while the emitted wave-shape from the transducer on one end (likely piezocrystal) is nearly lambertian (80%), due to the beacon mechanical mounting arrangement the effective pattern is likely somewhat isotropic.

In addition to all the other dynamic (moving) variables one likely sees in such an environment along with the black-boxes being carried along with the currents, its just another variable that likely affects the detection 'beam-corridor'. Three minutes of signal + 20hrs of silence, 15 seconds of signal + 11hrs of silence, 7 minutes of signal, etc, and then the battery is past the 30 day spec (mininum) and is apparently diminishing as anticipated, yet another anticipated verifiable characteristic. I suspect that its likely that some engineer (if there is any think'n happening) has attempted to verify the anticipated signal level drop rate / signal variance characteristics against the manufacturer's battery depletion curve for the beacon models deployed. Its simply another verification physik that need fit the characteristics of acoustic-beacon unit. Individually each of this seemingly unrelated poor quality data-point...things, are not very good evidence, but collectively, it shows that the aircraft wreckage area has very likely been located. Then couple those few points of which the public is aware (the authorities have many more, hopefully), with the fact that in that area of the world's oceans, there's not much happening, no buoys, no oil-platforms, no research vessels, no islands, AND its the long-way around the world so there's not much aircraft traffic either, so what's the likelihood of finding a non-naturally occurring 37.5KHz emitter source there?  Its another speck of paint from the overall painting, while being unable to see the actual picture.  Those poor people.. . .

in any case, finding that airplane...its a crapshoot in my opinion. Unlike the Titanic, which was very large and essentially went down intact (two big peices), the airplane likely disintegrated upon impact, so there ain't gonna be large descernable chunks lying about and then the ocean currents have been moving all that aircraft grade aluminum, scattering it here and there. Unlike the Titanic, it may never be found. And the bodies? 30 days in an environment where a single eye-dropper of ocean water contains uncountable hungry animals, (radiolarians etc)....yep yep yep.  Its a pipe dream.

I was not able to directly discern the specific model deployed, but rather found a range of models that basically fit the minutia that has been revealed by the talking-heads of the American news-media.

Re; the 'magic-number'. Quite true, however, my reason for citing the mfr's (marketing) specs, was simply because folks had nothing other than Clueless Network News for information among all of their wild-a.ss guessing (but their marketers sold lots of juicy advertising slots).

Ad rem, here are a few pointers to the answer to your specific question, about the frequency selectionbeing a magic-number or having been selected via a flip-of-the-coin...some juicy tidbits to succulate while sipping a JD with a WD40 spritzer splash in da workshop! Hah!

Underwater Acoustics physics
http://en.wikipedia.org/wiki/Underwater_acoustics

"Acoustic-Impedance is the relationship of how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a given frequency."
 
Transducer matching to the medium:
 http://en.wikipedia.org/wiki/Acoustic_impedance

And then here's a bit about how to transduce the acoustic-vibrations:

Hydrophone
http://en.wikipedia.org/wiki/Hydrophone

bench knob

[mfeinstein]

Excessive quoting removed, see previous post for what was quoted.

Thanks for all the links! I was expecting a more stragiht foward answer than have to read all this technical stuff about sound underwater could you make it short, maybe? I dont have time (at least not in the coming weaks) to read it all, and looking at it quickly, I couldnt find anything related to 37,5KHz.

[bench_knob]

Excessive quoting removed, see previous post for what was quoted.

Thanks for all the links! I was expecting a more stragiht foward answer than have to read all this technical stuff about sound underwater could you make it short, maybe? I dont have time (at least not in the coming weaks) to read it all, and looking at it quickly, I couldnt find anything related to 37,5KHz.

Awwww, ya hurt my feelin's <<snip>> <<snip>> all my yak'n! Heh heh.

However...its all in the physics...the math. You said ya wanna know why they chose that frequency, the only way to know that info definitively, is to ask the Dukane-SeaCom design engineer....info that DSC may not tell ya, or ya can back-track to the likely reason, by doing the physics math.  Its very unlikely that they just flipped a coin.

bench knob

[KedasProbe]

Why isn't it modulated, then it could even send info like how deep it is or the strength of the signal so the distance can be calculated.
or its last (or current if possible) GPS location.
Makes it also a lot easier to verify that the signal is from the black box and not some other source.

[VK5RC]

Isn't it, with a very simple binary modulation, I thought the KISS principle is pretty important.

[eurofox]

Maybe I miss something here 

1/ reception of those beacons is max lets say 3000 m
2/ from surface to bottom of the sea in this area +/- 4000 m
3/ they should be almost on the vertical of the beacon to detect something but they got signal from different spots quite far away from each other
4/ I can imagine that all is according hydrophone and quality of amplifier but this beacon send a quite limited in amplitude signal and there is so much "parasite noise"
5/ they need 6-8 hours to turn this big boat, why not use a small one? Maybe they got a long cable to have the hydrophone already 1000 m or more under water level.

eurofox

[dannyf]

1/ reception of those beacons is max lets say 3000 m
2/ from surface to bottom of the sea in this area +/- 4000 m
3/ they should be almost on the vertical of the beacon to detect something but they got signal from different spots quite far away from each other

Do they have to listen on the surface? Cannot they drop the listening device deep into the water, like maybe 2 - 3000m deep?

there is so much "parasite noise"

I am not so sure.

[David Hess]

3/ they should be almost on the vertical of the beacon to detect something but they got signal from different spots quite far away from each other

Salinity, temperature, and pressure gradients can cause refraction and reflection of the sound waves in water moving the apparent position and bearing of the transmitter.

5/ they need 6-8 hours to turn this big boat, why not use a small one? Maybe they got a long cable to have the hydrophone already 1000 m or more under water level.

The hydrophone is certainly operating below the thermocline.

http://en.wikipedia.org/wiki/Sonar#Sound_propagation

[eurofox]

1/ reception of those beacons is max lets say 3000 m
2/ from surface to bottom of the sea in this area +/- 4000 m
3/ they should be almost on the vertical of the beacon to detect something but they got signal from different spots quite far away from each other

Do they have to listen on the surface? Cannot they drop the listening device deep into the water, like maybe 2 - 3000m deep?

there is so much "parasite noise"

I am not so sure.

Waves (quite low frequency), dolphins who use ultra sound to navigate ....

I know the technology of sonar's, I build one from scratch a very long time ago, ultrasonic transducer generating a pulse, the echo was visible on a neon that was spinning on a disk spinning with a motor. The "look" of the neon light was reflecting the size of the echo.

eurofox

[bench_knob]

Why isn't it modulated...

It is modulated, according to the manufacturer (Dukane-SeaCom), the beacon 'pulse' is a once per second, 10 ms (10 thousandths of a second) gated (turns it on and then off) 37,500 cycle (37.5 KHz) ultrasonic 'tone'.  That works out to be a short 2.66 cycle 'pulse' at 37.5KHz during the gate 'ON' time, once per second.  The likely reason they chirp it, is to magnify the pulse-power while prolonging the battery life. In other words, doing so, allows the release of a significant tone blast (very short power 'on' time) without rapidly draining the battery.  With a fresh battery, the chirp power is 157 dB.

...then it could even send info like how deep it is

However your point is 'why isn't the beacon transmitting a lot of telemetry data, info such as depth and GPS style position info?' To do the answer justice requires a fairly complicated answer, however, breifly, the DK120 (Dukane-SeaCom) beacon is designed to be survivable during a transient extremely violent event; be active for a relatively long period of time; and be fairly 'loud' over a long range in an incredibly, yet similar type signal, noisy environment, while remaining functional at extreme pressure depth, at low temperatures, and be battery operable inside a relatively small mass container. As one of the fellows here in the EEVblog succinctly said, 'KISS' or more verbosely, 'keep it simple Sue'. And because the beacon works in ocean water, and as I stated before, the environment is incredibly noisy and then the oceanic physics constrain the quantity and type of data that can be reliably recognized and that one point is a **BIGGIE** and then just simply, GPS does not work underwater.  It would be very difficult if not impossible to make such a gadget as you ask about.

or the strength of the signal so the distance can be calculated.

Tough to do in ocean, as the media is so variable and without also transmitting internal health and likely precision time information, the distance info would only be relative at best. But the enviornment is so noisy and the bandwidth is so constrained, that the data transmission would need be long, in order to reliably sequence (and be recognizable) the intra-message symbols, but in of itself renders it unreliable as evidenced by the actual 'ping' durations being detected in the suspected MH370 search area.

However, there is always, seemingly, an alternative possible strategy available.  A retired (US) NTSB investigator/engineer interviewed by US C_lueless N_etwork N_ews, early on, made an interesting suggestion which he emphatically re-embraced several times. His suggestion may in my opinion and obviously his, address several of the points raised. However, from an engineering perspective, some of the desired characteristics involve possibly significant practical engineering challenges in of themselves.  The retired aircraft accident investigator (whose name I do not recall :[ ...), suggested that upon aircraft (assuming water) impact, that a satellite beacon would be released and deployed, that floats on the water, and being microwave enabled (exposed to atmosphere), it would be capable of wide-bandwidth transmission. Taking the idea a bit further, I speculate that such a device could be powered all the time, taking GPS location readings while the aircraft is in normal flight, but not transmitting, but just storing the data, so as not to add any more to the already congested RF spectrum. And then, God Forbid, when impact-trigger-released, the microwave-satellite-beacon would mechanically disconect, and also switch to internal, fully charged, battery power, and having stored a record of waypoints and at least the impact point if not post-impact position data. It would be able to uplink significant useful location data. The battery would not need to endure as long either, as the data would be immediately uplinked and likely received. However, this satcom-beacon would shock-burst the buffered data, periodically uplinking to conserve battery life, in any case. One of the challenges is how to deploy the beacon so that it doesn't become enmeshed in the wreckage tangle, possibly sinking uselessly to the bottom, or be so encumbered to lose line-of-sight with the sky overhead. Another challenge is how to prevent the beacon, floating on the surface, from being too far removed by surface currents from the initial impact point, but this may not actually be an issue, considering the several areas that were searched, each 'block' avg'g around 43,000² Km/(27,000² Mi),
because of the bad data given by the Malaysian 'authorities'.  In comparison ±200 hundred miles is like painting a target-icon on the impact point.

bench knob

[Wytnucls]

The trigger for the ejection of a data and ELT capsule before a crash would be easy. If the landing gear is not down by say, 200 feet (radio altimeter), a launch would happen. Most of the logic is in place already in the GPWS (Ground Proximity Warning System).

[VK5RC]

If the plane hits hard (at full flight speed) say 1000km/Hr vertically (~300m/s) if the ejection were to occur at 200 feet altitude AGL that would give the data system only 0.3 of second to clear the plane and it potential impact site.

[dannyf]

The trigger for the ejection of a data and ELT capsule before a crash would be easy.

Indeed, as long as we don't have to engineer an actual solution for it.

[EEVblog]

Why isn't it modulated, then it could even send info like how deep it is or the strength of the signal so the distance can be calculated.
or its last (or current if possible) GPS location.

As someone who has also worked on ocean depth sensors, that is not something you want to integrate into such a product where the priorities are simplicity and reliability.

Makes it also a lot easier to verify that the signal is from the black box and not some other source.

It's already practically guaranteed to be not from another source, the official public uncertainly is just politics.

[KedasProbe]

Maybe we are concentrating to much on the black box, if we just make it impossible for an airplane to hide its location while flying then you know where the black box will be. (last received position will always be very close too the crash site)
How hard can it be to activate an isolated standalone transmitter when it can't detect its normal position broadcast info anymore.

[Wytnucls]

If the plane hits hard (at full flight speed) say 1000km/Hr vertically (~300m/s) if the ejection were to occur at 200 feet altitude AGL that would give the data system only 0.3 of second to clear the plane and it potential impact site.

Commercial aeroplanes can't fly that fast at low altitude. 350kts (650 km/hr or 180m/s) is about the maximum before they start falling apart. In a stall, they come down at about 50m/s. Electrical signals travel fast enough to initiate a small explosive charge in the unpressurized part of the tail and eject in time, at a safe distance, whatever needs to be recovered later for analysis.

[dannyf]

if we just make it impossible for an airplane to hide its location while flying then you know where the black box will be.

Bingo!

Like mandatory ACARS - like services at all times. If we can isolate the last known position to even 10s of miles, it would have greatly aided search and rescue efforts.

The other solutions discussed (explosive charges in an airplane?) are great, as long as those airplanes are not meant to fly.

[Wytnucls]

Martin Baker has done it successfully with explosive charges for many years, to the relief of many pilots in distress.
Compressed air would probably do the job too. We'll see what the FAA and ICAO come up with.

ACARS is always on and already installed in commercial aeroplanes, but a failure of the system is always possible.
CPDLC and ADS-C are also mandatory in most remote airspaces, but can malfunction too.
Adding another layer is fine, but failure of that extra communication system is also possible.

'Always on' systems, with no means of disconnection from the electrics are bringing a host of safety problems by themselves and are unlikely to be approved.

[dannyf]

Martin Baker has done it successfully with explosive charges for many years,

Are any of those Martin Baker systems automatically triggered?

ACARS is always on ...

'Always on' systems, with no means of disconnection from the electrics are bringing a host of safety problems by themselves and are unlikely to be approved.

so being always on, ACARS is not approved to use on airplanes?

a failure of the system is always possible.

If the possibility of failure is sufficient to ban the use of a system, we should use no system of any kind, and the only way for humans to fly is to jump off a cliff.

[mfeinstein]

Why isn't it modulated, then it could even send info like how deep it is or the strength of the signal so the distance can be calculated.
or its last (or current if possible) GPS location.

As someone who has also worked on ocean depth sensors, that is not something you want to integrate into such a product where the priorities are simplicity and reliability.

Makes it also a lot easier to verify that the signal is from the black box and not some other source.

It's already practically guaranteed to be not from another source, the official public uncertainly is just politics.

And here he is! this solves one of the puzzles hahaha

[mfeinstein]

Maybe we are concentrating to much on the black box, if we just make it impossible for an airplane to hide its location while flying then you know where the black box will be. (last received position will always be very close too the crash site)
How hard can it be to activate an isolated standalone transmitter when it can't detect its normal position broadcast info anymore.

I was watching CNN the other day, and they asked the same question....apparently this idea was raised after 9/11 because the black boxes were ruined in the fire, so there is no data of the airplane to help in the investigations...so people starting to talk about continuos streaming to a ground location...but then came the politics guys and shut the idea off, concerned with "invasion of privacy", since there will be too much information being transmitted to the ground (let's remember black boxes do record audio).... I remember this was explained by some US transportation autority, and she said she has hopes that now this ideia will come back to life, with just maybe less data to dont step into privacy issues.

I think a floating beacon is cool, but straming of GPS and Telemetry is a lot simpler and easier to do, specially because it wont need some new device outside the airplane, connected to the airplane's main computer... AFAIK, this can genarate loooots of trouble (reliability, costs, installation) and goes away of our good and classic KISS

[mfeinstein]

Out of curiosity, Wytnucls do you work in the Aviation Industry?