4 ms·

The potential Doppler shift is mentioned at the end. If there were three towed ping detectors and all of them were moving at a fixed speed, would you be able to

The potential Doppler shift is mentioned at the end. If there were three towed ping detectors and all of them were moving at a fixed speed, would you be able to immediately triangulate the position of the device? How many seconds of ping would you need in order for it to be reliable? I wonder if the software to do that (I guess it must exist) has a very clean UI showing approximate location getting better as more data flows in. (Think the indicator for GPS accuracy in map apps.)

ISL 13y ago
You can do better than Doppler shift. If the clocks on three ping detectors are well-synchronized and their locations known, you can use the ping arrival times to triangulate. Sound propagation in water is nontrivial, but the people who are good at it are good at it. Furthermore, as long as the clock on the pinger is fairly good, the ocean environment fairly stable, and you don't drop many pings, you can drive one ping detector around and triangulate from the ensemble of ping measurements.
- vonmoltke 13y ago
  You can do that with a single moving platform, since the target is not moving. Record the bearings for half an hour, make a 90° turn, and recore another half hour of bearing data. That should give a decent approximate location. Its also basic target motion analysis that a military sonar operator can do in their sleep.
  - toomuchtodo 13y ago
    > Its also basic target motion analysis that a military sonar operator can do in their sleep. Which makes me wonder: Does the US not have military assets in the area? Or do they and they don't want to disclose their capabilities? EDIT: I'm behind the times; I didn't know the US had a sub-hunter in the area.
    - deleted 13y ago
      [deleted]
    - cube13 13y ago
      I think it's safe to assume that someone(US, Russia, China, etc.) has something in the area, but nobody's going to be talking about it to the public. It's not so much about disclosing capabilities as much as it's just simply operational secrecy. The entire point of a submarine is that it's hard to find.
      - trhway 13y ago
        >I think it's safe to assume that someone(US, Russia, China, etc.) has something in the area accidentally, this search may find a lot of that "something"
    - Tloewald 13y ago
      It's in international waters, the Chinese have no authority here and in any event Australia is coordinating the search. Ocean Shield is an Australian Defence Force ship (although not grey like your typical warship). http://www.theguardian.com/world/2014/apr/07/mh370-black-box-type-signals-picked-up-twice-by-australias-ocean-shield http://www.theguardian.com/world/2014/apr/07/mh370-black-box... The US has a specialist sub-hunter in the area, and they've made extensive use of P3 and P8 aircraft. I doubt they're terribly sensitive about their ability to find a deliberately pinging object 6000m below the surface. "There are 11 military aircraft, four civilian aircraft and 14 ships taking part in the search on Wednesday." http://www.theguardian.com/world/2014/apr/09/mh370-search-area-drastically-reduced-after-two-more-pings-heard http://www.theguardian.com/world/2014/apr/09/mh370-search-ar...
  - SixSigma 13y ago
    >since the target is not moving assumption
    - vonmoltke 13y ago
      True, but is very likely a correct one.
      - benjamincburns 13y ago
        Not really - underwater currents can be quite strong. If you were creating a model to accurately position this thing, you'd be best to add a v⃗ component for both the transmitter and receiver sides of the equation.
        vonmoltke 13y ago
        I'm not that familiar with the bathymetry[1] of the area, but I have been seeing reports that the locations are between 3000m and 6000m deep. At that depth, even a strong persistent current is probably only a few cm/sec. For the precision of TMA, that's effectively stationary; the noise in the bearing rate measurement is going to be greater magnitude than the true bearing rate. [1] Why is "bathymetry" not in Chrome's spelling dictionary?
        SixSigma 13y ago
        It's in the 10s of cm per second and up to 1m/s in some places, though not SW Australia http://hypertextbook.com/facts/2002/EugeneStatnikov.shtml http://hypertextbook.com/facts/2002/EugeneStatnikov.shtml
  - thommo101 13y ago
    Sadly the towed pinger locator (TPL-25) is omni directional. But by analysing the inter-ping interval you can determine a CPA point. Something below I wrote on reddit about this technique: Due to the pulse length (10ms) there is an inherent 100Hz bandwidth in the signal, which is equivalent to the frequency shift you would expect if moving at 4m/s. In other words, Doppler measurements of a single pulse is NOT going to be useful in attempting to localise. However, because the pinger has a relatively stable repetition rate (about 1.106s from what I could see) you can do a cool trick to localise it (assuming the source is stationary): - Perform a pass over the area of interest (ie where the pinger is) - measure the ping times of arrival (either manually with waveform inspection or write an envelope detector) - plot the ping times of arrival versus the time of arrival modulo ping interval. ie in MATLAB: plot(toa, mod(toa, 1.106), '.'); - The plot indicates the relative distance from the source / receiver. Hopefully the plot should contain a local minimum representing a closest point of approach (ie like http://imgur.com/v1ZclaX http://imgur.com/v1ZclaX ). This means that the source lies in a line perpendicular to the vessels course centred at the location of the receiver (ie TPL-25) at CPA time - Perform another pass over the area of interest on a course perpendicular to the first - measure the ping times of arrival and look for CPA as before - You should now have two intersecting lines. Guess what should be at the intersection? :) This relies on having a stable ping repetition rate. BTW this is the author of the linked post here :) Rodney Edit: @ISL has hit the nail on the head there. Hyperbolic trilateration using multiple simultaneous receivers is the way to go. RAAF will be trying this with sonobuoys deployed from P3-C Orions
- benjamincburns 13y ago
  You can do better than complete waveform time of arrival. Sound propagation in water is dispersive (higher frequencies travel faster, but lower frequencies travel farther). With an accurate model if the chirp waveform, you can determine distance more accurately by looking at the amount of signal "spreadding" and the amount of attenuation of specific signal components due to this shifting. You can do better than that, too. Theoretical accuracy of time of arrival distance approximation is directly related to the wavelength of the transmission signal. If you can model the transmitted signal and its transmission path accurately enough that you can get a strong correlation between the received signal and the modified signal (in this case taking into account Doppler effect and wave dispersion), then you can accurately measure both the time of arrival and the phase of the signal. Assuming a perfect model (HA!) the accuracy of the distance calculation is soon dominated by the various noise components (typically your timer noise, power supply noise, decimation noise from your ADC, and various thermal noise sources in both the transmission path and your measurement equipment).
dannypgh 13y ago
Triangulation requires an angle and only two observers. What you're proposing is trilateration, http://en.wikipedia.org/wiki/Trilateration http://en.wikipedia.org/wiki/Trilateration I suspect it'd be challenging to get reliable and precise enough measurements from multiple detectors as close to each other as they'd have to be on a single ship. Three ships, perhaps a different story.
rtkwe 13y ago
I was dubious at first that the movement of the ship would be fast enough for a 4ms delta in the click timing but it comes out to just 6.24 m/s speed if they're moving directly away from the object (this assumes a 1560 m/s speed of sound in water which isn't constant but it's the best I had).
brc 13y ago
A friend of mine works on ships that do towed array for seismic exploration. The software and systems they have behind this sort of thing is very comprehensive. In that case they are looking for things below the ocean floor. They're able to pick up seismic responses from interesting geological formations in the rock under the ocean while moving along towing a large string of lines behind the boat. I would think finding a pinging box would probably be simple in comparison, if you knew where to look for the thing.
CPAhem 13y ago
The change in interval of a few ms could also be because of mp3 binning. Extracting a file of Youtube means you have already been through at least three codecs.
- thommo101 13y ago
  Audio codecs are very good at retaining signal phase as the human ear is very sensitive to errors in this. Whilst I'm not saying it is impossible, I think it would be very unlikely that the interval change is due to transcoding. Also, as it is a moving receiver this effect is almost expected.

ISL 13y ago

You can do better than Doppler shift. If the clocks on three ping detectors are well-synchronized and their locations known, you can use the ping arrival times to triangulate. Sound propagation in water is nontrivial, but the people who are good at it are good at it. Furthermore, as long as the clock on the pinger is fairly good, the ocean environment fairly stable, and you don't drop many pings, you can drive one ping detector around and triangulate from the ensemble of ping measurements.

vonmoltke 13y ago
You can do that with a single moving platform, since the target is not moving. Record the bearings for half an hour, make a 90° turn, and recore another half hour of bearing data. That should give a decent approximate location. Its also basic target motion analysis that a military sonar operator can do in their sleep.
- toomuchtodo 13y ago
  > Its also basic target motion analysis that a military sonar operator can do in their sleep. Which makes me wonder: Does the US not have military assets in the area? Or do they and they don't want to disclose their capabilities? EDIT: I'm behind the times; I didn't know the US had a sub-hunter in the area.
  - deleted 13y ago
    [deleted]
  - cube13 13y ago
    I think it's safe to assume that someone(US, Russia, China, etc.) has something in the area, but nobody's going to be talking about it to the public. It's not so much about disclosing capabilities as much as it's just simply operational secrecy. The entire point of a submarine is that it's hard to find.
    - trhway 13y ago
      >I think it's safe to assume that someone(US, Russia, China, etc.) has something in the area accidentally, this search may find a lot of that "something"
  - Tloewald 13y ago
    It's in international waters, the Chinese have no authority here and in any event Australia is coordinating the search. Ocean Shield is an Australian Defence Force ship (although not grey like your typical warship). http://www.theguardian.com/world/2014/apr/07/mh370-black-box-type-signals-picked-up-twice-by-australias-ocean-shield http://www.theguardian.com/world/2014/apr/07/mh370-black-box... The US has a specialist sub-hunter in the area, and they've made extensive use of P3 and P8 aircraft. I doubt they're terribly sensitive about their ability to find a deliberately pinging object 6000m below the surface. "There are 11 military aircraft, four civilian aircraft and 14 ships taking part in the search on Wednesday." http://www.theguardian.com/world/2014/apr/09/mh370-search-area-drastically-reduced-after-two-more-pings-heard http://www.theguardian.com/world/2014/apr/09/mh370-search-ar...
- SixSigma 13y ago
  >since the target is not moving assumption
  - vonmoltke 13y ago
    True, but is very likely a correct one.
    - benjamincburns 13y ago
      Not really - underwater currents can be quite strong. If you were creating a model to accurately position this thing, you'd be best to add a v⃗ component for both the transmitter and receiver sides of the equation.
      - vonmoltke 13y ago
        I'm not that familiar with the bathymetry[1] of the area, but I have been seeing reports that the locations are between 3000m and 6000m deep. At that depth, even a strong persistent current is probably only a few cm/sec. For the precision of TMA, that's effectively stationary; the noise in the bearing rate measurement is going to be greater magnitude than the true bearing rate. [1] Why is "bathymetry" not in Chrome's spelling dictionary?
        SixSigma 13y ago
        It's in the 10s of cm per second and up to 1m/s in some places, though not SW Australia http://hypertextbook.com/facts/2002/EugeneStatnikov.shtml http://hypertextbook.com/facts/2002/EugeneStatnikov.shtml
- thommo101 13y ago
  Sadly the towed pinger locator (TPL-25) is omni directional. But by analysing the inter-ping interval you can determine a CPA point. Something below I wrote on reddit about this technique: Due to the pulse length (10ms) there is an inherent 100Hz bandwidth in the signal, which is equivalent to the frequency shift you would expect if moving at 4m/s. In other words, Doppler measurements of a single pulse is NOT going to be useful in attempting to localise. However, because the pinger has a relatively stable repetition rate (about 1.106s from what I could see) you can do a cool trick to localise it (assuming the source is stationary): - Perform a pass over the area of interest (ie where the pinger is) - measure the ping times of arrival (either manually with waveform inspection or write an envelope detector) - plot the ping times of arrival versus the time of arrival modulo ping interval. ie in MATLAB: plot(toa, mod(toa, 1.106), '.'); - The plot indicates the relative distance from the source / receiver. Hopefully the plot should contain a local minimum representing a closest point of approach (ie like http://imgur.com/v1ZclaX http://imgur.com/v1ZclaX ). This means that the source lies in a line perpendicular to the vessels course centred at the location of the receiver (ie TPL-25) at CPA time - Perform another pass over the area of interest on a course perpendicular to the first - measure the ping times of arrival and look for CPA as before - You should now have two intersecting lines. Guess what should be at the intersection? :) This relies on having a stable ping repetition rate. BTW this is the author of the linked post here :) Rodney Edit: @ISL has hit the nail on the head there. Hyperbolic trilateration using multiple simultaneous receivers is the way to go. RAAF will be trying this with sonobuoys deployed from P3-C Orions
benjamincburns 13y ago
You can do better than complete waveform time of arrival. Sound propagation in water is dispersive (higher frequencies travel faster, but lower frequencies travel farther). With an accurate model if the chirp waveform, you can determine distance more accurately by looking at the amount of signal "spreadding" and the amount of attenuation of specific signal components due to this shifting. You can do better than that, too. Theoretical accuracy of time of arrival distance approximation is directly related to the wavelength of the transmission signal. If you can model the transmitted signal and its transmission path accurately enough that you can get a strong correlation between the received signal and the modified signal (in this case taking into account Doppler effect and wave dispersion), then you can accurately measure both the time of arrival and the phase of the signal. Assuming a perfect model (HA!) the accuracy of the distance calculation is soon dominated by the various noise components (typically your timer noise, power supply noise, decimation noise from your ADC, and various thermal noise sources in both the transmission path and your measurement equipment).

dannypgh 13y ago

Triangulation requires an angle and only two observers. What you're proposing is trilateration, http://en.wikipedia.org/wiki/Trilateration http://en.wikipedia.org/wiki/Trilateration I suspect it'd be challenging to get reliable and precise enough measurements from multiple detectors as close to each other as they'd have to be on a single ship. Three ships, perhaps a different story.

rtkwe 13y ago

I was dubious at first that the movement of the ship would be fast enough for a 4ms delta in the click timing but it comes out to just 6.24 m/s speed if they're moving directly away from the object (this assumes a 1560 m/s speed of sound in water which isn't constant but it's the best I had).

brc 13y ago

A friend of mine works on ships that do towed array for seismic exploration. The software and systems they have behind this sort of thing is very comprehensive. In that case they are looking for things below the ocean floor. They're able to pick up seismic responses from interesting geological formations in the rock under the ocean while moving along towing a large string of lines behind the boat. I would think finding a pinging box would probably be simple in comparison, if you knew where to look for the thing.

CPAhem 13y ago

The change in interval of a few ms could also be because of mp3 binning. Extracting a file of Youtube means you have already been through at least three codecs.

thommo101 13y ago
Audio codecs are very good at retaining signal phase as the human ear is very sensitive to errors in this. Whilst I'm not saying it is impossible, I think it would be very unlikely that the interval change is due to transcoding. Also, as it is a moving receiver this effect is almost expected.