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Author Topic: Aircraft Crash Pingers Use 37.5 KHz  (Read 14118 times)
K0OD
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« on: April 05, 2014, 08:02:36 AM »

Just Out:

"A Chinese ship searching for the missing Malaysia Airlines plane in the southern Indian Ocean has picked up a pulse signal, Chinese media say.

They say the signal has a frequency of 37.5kHz - the same as those emitted by the flight recorders.

However there is no evidence so far that it is linked to MH370. "

http://www.bbc.com/news/world-asia-26902127
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WA4053SWL
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« Reply #1 on: April 05, 2014, 09:17:25 AM »

Maybe if these VLF pulses are received at the surface, perhaps other ships could have helped search long before.
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AA4PB
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« Reply #2 on: April 05, 2014, 10:46:19 AM »

Those 37.5KHz pings are not received at the surface. A hydrophone must be towed at a depth below the thermocline level which would reflect sound back down and prevent it from coming to the surface (sort of like an ionosphere in water). A passing ship would have virtually no possibility of detecting the pings unless is was towing a hydrophone at 1000 feet or so above the ocean floor.

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K0OD
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« Reply #3 on: April 05, 2014, 10:56:21 AM »

AA4PB, interesting.

Is the signal just a ping or is some identification sent? Such frequencies can propagate worldwide with megawatt transmitters and huge antennas. I've picked up strong digital signals from Hawaii and Australia down around 25 KHz with simple equipment.

I presume battery operated pingers just use about a watt.  Antenna would be dreadful for 37.5 kHz.
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AA4PB
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« Reply #4 on: April 05, 2014, 11:02:36 AM »

The pingers are ultrasonic acoustic transducers operating at 37.5KHz. They just "ping" - no data modulation. I think they apply about 1W of power.
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K0OD
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« Reply #5 on: April 05, 2014, 11:07:14 AM »

Quote
acoustic transducers operating at 37.5KHz

So it's audio not RF?
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AA4PB
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« Reply #6 on: April 05, 2014, 11:43:12 AM »

Quote
acoustic transducers operating at 37.5KHz

So it's audio not RF?

Yes, it's acoustic - not RF. Uses a hydrophone transducer rather than an antenna. It's above the normal audio hearing range.

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G3NOQ
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« Reply #7 on: April 06, 2014, 02:24:50 AM »

Yes, it's acoustic - not RF. Uses a hydrophone transducer rather than an antenna. It's above the normal audio hearing range.
RF only has a short range at 37.5 kHz, about 5-10 metres in sea water.
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W3RSW
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« Reply #8 on: April 06, 2014, 10:51:56 AM »

Apparently the Aussis have picked up the same signal a/c recent news.
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K0OD
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« Reply #9 on: April 06, 2014, 11:44:28 AM »

An interesting primer related to this thread:
http://en.wikipedia.org/wiki/Communication_with_submarines

Also was surprised to learn that the speed of sound is several times faster in water than in air.
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AA4PB
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« Reply #10 on: April 06, 2014, 12:10:22 PM »

Here's some technical info:

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

http://en.wikipedia.org/wiki/Towed_pinger_locator
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WA7PRC
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« Reply #11 on: April 06, 2014, 09:22:29 PM »

They'd be located a lot faster if they were buoyant, and released upon impact.  Considering the surface of Earth is over 70% water, that would seem to make sense.  I understand that idea has been "floated" about 10 years ago.  However, due to the cost, it has never been implemented. Meanwhile, we spend exhorbitant amounts of resources, time, and expense to try to locate black boxes from seabeds.

vy 73,
Bryan WA7PRC
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NA4IT
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« Reply #12 on: April 07, 2014, 02:57:15 AM »

Sad thing is some of the technology used on airliners is so antiquated it isn't funny. That airplane should have been able to be tracked to the exact point where it went down by GPS technology. Google can do a sat pic of your house, but the gov can't do a pic that is clear of a crash scene.

We are so far behind it is pitiful.
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AA4PB
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« Reply #13 on: April 07, 2014, 05:39:30 AM »

There are deployable beacons that are ejected upon impact and picked up by satellite. That doesn't locate the recorders but it identifies where the plane went down. Unfortunately many of the airlines don't use them yet (cost).

GPS positions require a real-time constant transmission of the GPS position from the aircraft. Remember, the GPS satellites don't know where the GPS receiver is located, only the GPS receiver knows. You've got to get that information back via some type of communications system (a satellite if you want world-wide coverage). If you want to know where the plane went down you'd have to transmit position updates every few seconds. Multiply that by all the planes flying around the world at any given time and you have quite a large bandwidth requirement. Not that it can't be done, but it's very expensive. Then of course if you have a fire that destroys cable bundles you could loose power to the transmitter quite some time before the plane goes down.
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AA4PB
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« Reply #14 on: April 07, 2014, 06:00:06 AM »

Funny, one of the "talking heads" on TV the other day was proclaiming how ridiculous it was that we can track a cell phone all around the city but can't find a giant airliner. I would have responded that I'll bet that you can't track your cell phone 2 miles down in the ocean either  Wink
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