It was a very clever idea that recognized that there was essentially a big spherical void surrounding the earth between the surface and the ionosphere about 30 miles up. This big spherical void acted like a massive, but silent, church bell that had a resonant frequency of about 7.5 Hz. A frequency far below what people can hear but strangely the same frequency as human alpha brain waves, and by tinking the “bell” at specific intervals you could determine geolocation.
