A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Sep 15, 10:14 -0700
Lu Abel, you wrote:
"I do remember taking a coastal navigation course pre-GPS where we did do one exercise of taking a hypothetical run of soundings over a period of time, plotting them spaced by distance run between soundings on a sheet of transparent paper, and then overlaying it on a chart and trying to estimate our position. Pretty low on the accuracy scale, but if nothing else is available, it's better than nothing."
With the right bottom profile, this can be remarkably accurate for navigation. It was especially popularized before acoustic sounders by Lord Kelvin who patented a "piano wire" sounder in the 1870s. More on this in a book by RIchard Dunn (who posts NavList messages now and then, too).
Back to John Howard's query, yes, it is "widely reported" in the gossip around classified methods of navigation that USN vessels, especially submarines, navigate when over the continental shelf by acoustic imaging of the bottom. Apparently the key to this process is trash, including wrecks. They provide strong acoustic reflections and they tend to be small targets with fixed positions. This supposedly became feasible for navigation when computing power became cheap because it requires the computationally-intensive analysis of passive sonar. Obviously you can ping along with active sonar to find these bottom targets, but that destroys the stealth of a submarine --which is basically the whole point of having a submarine. The trick is to use passive sonar, deciphering the echoes and reflections in ambient ocean noise. That is, you listen to the hiss of great shoals of shrimp, the calls of whales, and also the periodic reverberations of the propellers from nearby ship traffic, and in that cacophany, you detect the patterns that indicate reflection from hard bottom targets, and then by tracking how those change over time, you map the bottom debris and then match against charts of the same. Simple! Supposedly this charting of bottom debris for navigation was a primary function of various "oceanographic vessels" operated by the USN and contractors decades ago. Bear in mind that this is "informed speculation" (not mine) from sources who may or may not know what they are talking about!