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By Bruce Rule - Apr 29, 2016


This posting describes acoustic signal processing and analysis techniques that are an extreme example of the axiom that “the more carefully you measure something, the more you find out about many other things.”


It is only an apparent anomaly that while time-difference fixing (TDF) techniques have an absolute position derivation accuracy of only about one nautical mile (nm) in broad ocean areas, the relative accuracy of one TDF solution to another in a sequence of acoustic events can have accuracies measured in feet because signals generated in a temporally short periods travel almost precisely the same transmission path to each sensor. and; hence, have precisely the same sound-travel time.

Relative TDF solutions for SCORPION acoustic events that occurred during a 111-second period – based on relatively crude (10-50 ms) time of arrival measurements made from helicorder displays - provided the basis for refuting John Craven's assertion that SCORPION has reversed course to deactivate a torpedo that had become active in its launch tube. Those relative TDF solutions indicated SCORPION's position changed less than 100-feet during that 111-second period. (This change in relative position was made when the range from SCORPION to the involved acoustic sensors was 821 nm to the east and 976 nm and 1021 nm to the west/NW.)

Had those SCORPION signals been tape recorded, relative position – one to another – accuracies of five-feet could have been achieved by measuring the event wavefront arrival times to the millisecond (0.001s)

An under-exploited acoustic vulnerability of nuclear submarines may be low-frequency transient energy: those signals with durations measured in milliseconds.

The linked site describes mechanical problems associated with actually stopping the rotation of BOREY and YASEN Class Russian nuclear submarine main propulsion turbines during turbo-electric mode operations. Even – actually, especially - at very low rotational speeds, those turbines will generate potentially strong low-frequency, transient acoustic energy as the main shaft stops - “grabs” the shaft-support member - and is “released” to regain rotational velocity. i.e., the shaft “slips”and then “sticks.” Those “sticks” produce impact energy that is almost impossible to avoid under the operating conditions associated with TE mode.


If such low-frequency transient signals are detected by multiple acoustic sensors and highly accurate TDFs are derived, it may be possible to track transiting submarines even if they radiate no “conventional” acoustic sources.

Further, if transient energy is TDF-positioned for times separated by less than - or about - five milliseconds – and those positions do not agree within measurement error, the energy may have been radiated by different areas of the submarine; hence an estimate of the length of the radiating hull may be derivable, information that has target classification value. Bottom line: you never know what's there until you look.

All the above information open source.