Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!sun-barr!newstop!sun!amdahl!amdcad!military From: broman@trout.nosc.mil (Vincent Broman) Newsgroups: sci.military Subject: Re: What is "passive sonar?" Message-ID: <27084@amdcad.AMD.COM> Date: 2 Sep 89 07:29:01 GMT References: <27070@amdcad.AMD.COM> Sender: cdr@amdcad.AMD.COM Organization: Naval Ocean Systems Center, San Diego Lines: 33 Approved: military@amdcad.amd.com From: broman@trout.nosc.mil (Vincent Broman) Michael Reed 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