Originally Posted by mm43
iabel; Thanks for your well thought-out analysis of the Emeraude's pinger detection possibilities. The locations you have indicated are ideal possibilities.
Thanks for the reminder that I had not studied the post from
iabel. It is an interesting analysis, pulling together aspects that we have discussed separately. I would caution that the distance from the center of the Navy search box to the mountain range is 12km; add to this the slant distance from the pinger to the mountains. 12-18km is a long way for a weak 37kHz signal, but perhaps not impossible with the "right" focusing and ducting.
Originally Posted by mm43
Hyperveloce; As the Navy have apparently reported that two pinger signals were detected, I wonder how they came to that conclusion. Consider that the Emeraude was moving at say 10 knots (I think its search speed is mentioned in one of the Interim Reports), then there are doppler shift events to be taken into account. These signals could possibly be indentified as separate entities if each had a detectable difference in their precise frequency and the sub's distance and speed in relation to each pinger was the same. But at this stage there is no way of knowing that, and one or both of the pinger signals detected could be multiple path. The fact that they have clearly indentified 37.5kHz pinger bursts is important, and though there is some current disagreement between the BEA and Ministry of Defence, I am sure that when Phase 4 is being drawn up, a prime area will be in the SW quadrant from West to a position WSW and between 30 to 40NM radius of the LKP.
I was thinking that reception of two pings per second would be sufficient evidence of two pingers, but I agree that the effect could be mimicked by multipath. I expect that frequency is the key.
The Doppler shift (if I have the math right), based on 10kt=5m/s, is 5(m/s) / 1500(m/s) * 37.5(kHz) = 125Hz. That is the maximum possible (not counting for currents) and the actual is likely much less if the sub only hears the pinger when the pinger is below or to the side of the sub (range-rate = speed * sin(angle from vertical) * cos(angle from bow)). However, if the sound was ducted or refracted to near horizontal, then the full Doppler could be detected if the target was ahead or astern.
Note, however, that the frequency of the pingers is badly controlled, likely by an RC timer. The spec says the frequency is 37.5 +/-1kHz. That leaves plenty of room to separate one pinger from another, even with Doppler obscuring the result, unless the two pingers happen to be close in frequency.
As for exploiting the Doppler shift for localization, that is certainly possible in principal, but may be difficult with the amount of data they have. The pulse length is only 10ms, which translates to 375 cycles. The maximum possible Doppler shift is 5/1500 = 1:300. Thus the Doppler has to be detected from about 1 cycle or less change in length of a signal buried in noise. If the aspect isn't favorable, that may be hard to do. Someone who has experience with Doppler measurements will know better. Doppler is routinely used to measure water current from the weak echos reflected from particles and turbulence in the water, but that is done at 10-50 times higher frequency, receiving with the same clock that was used to transmit, and averaging over multiple pings.