Hah, makes a lot of sense. Basically you're shining a light and looking for light bouncing back (well, sound, not light, but on your display it's shown as light), so regardless of frequency any active would be like using a big floodlight at night.
You don't even need specialized intercept equipment - that's a funny thing to realize - the sonar equipment itself is already made to get that signal back, so...
Unless it's set to a very specific frequency, in which case active intercept-specialized equipment could cover a large range of frequencies.
It's really neat to see it all as a sort of physics problem.
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