Satellite or aircraft-based L-Band interferometry can detect boat wakes in the open ocean.
ESA launched a bird a few years back that could do this (and did), among other things, but I understand the resolution wasn't as good as was expected prior to orbit.
Still... you have to admit it's a pretty cool use of technology.
The point being that you have a known triangular template shape for the wake of a hidden submarine that can be correlated against the oceanic images. They do it with synthetic aperture radar from satellites. eg
(clearly machine translated from Chinese(?))
Mostly though they still rely on deep ocean microphones and other passive phased acoustic arrays to detect and locate submarines. They used to compare notes with radio astronomers back during the CW era.
-- Regards, Martin Brown
Discuss tracking submerged submarines in an open forum? Look! Kittehs!
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