The towed pinger locator used to search for the black boxes of Flight 370 was the U.S. Navy's TPL-25. To locate the position of a pinger it is towed behind a vessel along a constant heading and detects ping intensity along a traverse. What one can do is look for a minimum intensity along track. At this point the pinger position will be on a line normal to the track. Repeating this process for a number of different headings one can use the intersection of the lines of position as an estimate of the pinger's location. To illustrate how this works I assumed that the pinger was at a point within a triangle determined by three vertices. If one plots the inverse intensity of the detected pings for the points along a track one will get curves similar to the ones below that one can fitted to find their minimum points.
The inverse intensity was a better match to the quadratic functions used for the fit and as a result gave a better estimate of the lines of position. Ten points along each side of the triangle were used to make estimate of the minimum position. The three normal lines cross at a point.
If one is not very close to the pinger one may still get a line of position from the triangle and a rough estimate of how far away it is.
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