mr10+@andrew.cmu.edu (Michael Reed) (09/01/89)
From: Michael Reed <mr10+@andrew.cmu.edu> Speaking of Thomas Clancy [a few weeks back], I was rereading _The Hunt for Red October_ last night and found myself wondering what passive sonar is. I know the basics of active sonar, and can fully understand the perils involved. But what is "passive?" I assume that particulars are highly classified, but would appreciate any non-classified info. Thanks in advance, -- Lestrade <mr10+@andrew.cmu.edu> "Do not come to this University."
broman@trout.nosc.mil (Vincent Broman) (09/02/89)
From: broman@trout.nosc.mil (Vincent Broman)
Michael Reed <mr10+@andrew.cmu.edu> asks:
> I know the basics of active sonar, ...but what is "passive?"
Passive sonar systems "listen" with very sensitive hydrophones to the
sounds that distant ships and subs make themselves: sounds of their
engines and propellors, their internal machinery, disturbances of the
water being passed through, etc. Their main advantage over active
sonar lies in the fact that you need not reveal yourself to others by
making loud pinging noises in the water. Of course, the usefulness of
passive sonar depends on the sounds of interest being either louder
than or significantly different from the background noises in the
ocean. The advance of Soviet technology is making their subs quieter.
The hydrophones used are either attached to the hull of a vessel under
the water-line, or else dragged behind the vessel on a long tow cable.
By comparing the signals arriving at each individual hydrophone, one
can estimate useful parameters, e.g. the azimuthal angle of arrival of
the sound rays relative to the sensor. "SOund NAvigation and Ranging"
is a slight misnomer for such a sensor, because the range to the
target cannot usually be estimated by means of one passive detection,
although under certain conditions the range, course, and speed of the
target can be estimated from many repeated measurements made over
time, just as Gauss did in calculating the orbits of the planets.
The basic ideas of passive sonar are not classified, only details
like: detection sensitivity and range, measurement accuracy, signal
processing algorithms used, frequencies of signals of interest, etc.
Vincent Broman, code 632, Naval Ocean Systems Center, San Diego, CA 92152, USA
Phone: +1 619 553 1641 Internet: broman@nosc.mil Uucp: sdcsvax!nosc!broman