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Black boxes put out a sonar ping at about 32kHz. From what I've read, depending on sea conditions the ping will be audible at the surface out to about 16 km, leaving a search area of around 1024 square km. By adding a little complexity to the ping circuit, you could encode within it the last received GPS position of the box, before it hits the water. With average sea depth and average sink angles of less than 45° that information would cut the search area down to about 10 square km. Is there some reason this tech is not used, or is it still in the rigorous testing pipeline these devices must go through?

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under sea currents would still cause a large drift, also directional detectors (and just going a mile in one direction and detecting differences in the ping strength) will do more than an inaccurate GPS lock –  ratchet freak Apr 8 at 10:44
    
What is the black box's "last received GPS position"? The black box is inside the plane: it can't receive GPS because it doesn't have line-of-sight to anything, including the constellation of GPS satellites. –  David Richerby Apr 8 at 13:15

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Current underwater locator beacons (ULBs) have a detection range of only a few km. 16 km would be under exceptionally favourable conditions. So in most cases the GPS data would not be of any additional help. It might tell you where the aircraft was before it hit the water, not exactly where the debris ended up after falling through several Km of water.

An advantage of current ULBs is their extreme simplicity and small size. There's no need for external data-feed and power connections. There are no issues of openings in the casing for waterproof connectors that also need to withstand the g-forces and pressures expected to be endured by a ULB. They are easy to test regularly and relatively easy to maintain.


From what I have read, current proposals for improvements to ULBs mostly focus on

  • increase duration from 30 days to 90 days
  • increase range by changing frequency from 37.5 kHz to ~10KHz

There are trade-offs here - longer duration means bigger batteries. changing the frequency may make it harder to distinguish from other sources of sound in the ocean.


An interesting 2011 dissertation describes the difficulties involved and discusses modulating the beacon signal to include data.

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Main reason those signals can be distinguished for what they are is because they have a specific signature. If the signals were to change all the time because you mix in extra information, they'd be impossible to recognise and thus useless.
Combined with the short range of the signal (inevitable consequence of the need to be low power, anything more would mean heavier equipment and/or shorter lifespan), there's just no benefit.

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That doesn't follow. For example, radio receivers can only pick up radio waves that "have a specific signature" but those radio waves can be modulated to encode data and, obviously, this can be done without breaking the "specific signature". –  David Richerby Apr 8 at 13:18
    
@DavidRicherby such would mean replacing every single receiver out there, just like you now need AM, FM, LW, etc. etc. receivers for every single type of radio signal. And all for nothing. No, modulation is not the answer, it only adds complexity for no reason whatsoever (but then, that's politics, doing things for no reason whatsoever so I'm sure you'll get it mandated by the FAA if you just demand "they do something NOW!"). –  jwenting Apr 8 at 13:21
    
You would only need a new receiver if you actually want to decode the data. A simple receiver could still detect the fact that there is a signal, just as it would now. –  fooot Apr 8 at 14:44
    
@fooot Indeed: receiver records signal, sends to HQ, HQ decodes, HQ says "The message was xyz". I'm not saying this is a good idea, just that jwentings objections to is aren't well-founded. –  David Richerby Apr 8 at 17:11

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