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

 

[TheBlacksmith]

itsmoked - you move a receiver several miles on a steady course, the direction the pings are coming from will slowly shift. Now you kind of have a series of lines, like spokes on a wheel. These lines may converge on a single point or may be virtually parallel. Now change the receiver direction of travel significantly, say 90 degrees and repeat. You will get second set of lines. The lines will begin to be seen as originating from a relatively small area. There are also some templates that can be overlaid on the plot that allow a crude approximation of distance.

 

[Comcokid]

The sensor platform they drag through the water is at the end of a very long cable that allows it to get deeper. Part of the problem is that they don't know exactly were the platform is underwater behind the ship. As TheBlacksmith points out, [the ship] keeps a steady course for a long distance as they collect data. However, the currents several thousand meters underwater may be different from surface currents so the sensor platform may be several hundred meters to the side of the ships course, and may be crabbing through the water at a different angle than the ship, creating detected sound direction uncertaintly.

The ship mows-the-lawn in a series of parallel lines. At the end of each line, the process of turning 180 degrees and running the next line with a platform on the end of a long cable may require several hours to perform. Additionally IRstuff points out the slow speed required to keep noise down (from waterflow over the sensor platform). So the process of doing the search is tedious and takes time.

Some of the encouraging news is that on some single lines, pinger noise would be heard continuously, fade away, and heard again about a km apart. This could be due to propagation through the water (temperature inversion, path effects, etc) or could represent hearing the pinger from one of the two data recorders, and then the other.

Some discouraging news is that the recorders could be under a meter of silt if seperated from aircraft structure. So, if the pingers are dead by the time they get a ROV down, finding them may be impossible.

News articles quoting information from the pinger manufacturers have reported the lower frequency of the signals could come from a combination of the water depth (high pressure), water temperature (it's cold down there), and low pinger battery voltage (nearly gone).

 

[itsmoked]

Hi TheBlacksmith. I totally understand that, but you're using the term "direction" and I don't see how you can with the equipment they're presently using. Sort of the point of this discussion - the waste of valuable time and resources due to a lack of directional information.

IR: I had no idea they towed that slowly. That's hideous. It's probably hard to even point the ship at that speed.

Buried in the silt?! Oh, that's a horrid possibility.

A plane dropped hydrophone buoy has heard a ping. If they could get a couple of them to hear the same ping they'd have it in the bag.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

There still seems to be so disjointedness in the search. One the map that was posted in the Pub

http://www.marinetraffic.com/en/ais/home/mh370-search-area

the Ocean Shield and only 4 of the other vessels are searching in a very tiny region, while the majority of the other ships are several hundred km sway in a totally different area. Of course, if they were all in the tiny little area where the Ocean Shield is, the prop noise might swamp out the locator beacon. The bad news is that no new signals have been vetted since Tues, so the batteries may have died completely. Clearly, design margins were tiny. The hydrophone detection was later discounted as not coming from the beacon.

TTFN
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[TheBlacksmith]

I thought they had better directional control over the sensors. We did where and when I learned target motion analysis. Plus it's been a few years.

 

[IRstuff]

I have no doubt that there are sensor hardware that can do direction finding, with high accuracy. For whatever reason, the touted system, being used by the Ocean Shield, appears to be a single omni-direcitonal hydrophone system with no specified directional accuracy. Certainly, the hydrophone arrays used by US subs could determine arrival angle down to the degree.

Interesting, the maritime traffic site from above doesn't show any of the US warships, for good reason, no doubt, but it makes it very unclear where they are, and where they're searching. The search map shown on the site has shrunk somewhat; it showed a large rectangular area that encompassed both of the two rectangular regions to the west, but the Ocean Shield has pretty much been glued to the tiny rectangle closer to Aus. since Saturday. The track history for Ocean Shield shows a portion of a raster pattern.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[itsmoked]

Looks like the batteries are truly dead, dead,dead. Bet they're starting to wish they'd used a more reasonable one ping per 2 or 3 seconds instead of the ridiculous 1 sec. Might've been nice to have sixty or ninety days instead of 30 to work with.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dV8r]

Just too bad they had to "Free Willy" he would have found it for us in short order. Eh!!