TIt's operating principal was basically that a narrowband, low power signal was bounced off a launchplate at the top of a stainless steel or titanium rod, or weighted cable, and a TDR reading taken, this gave a starting time, or top of tank reading. The next largest reflection seen would be that of the liquid level, the next largest would be the end of the sensing element, or bottom of the tank. The time taken between the first two readings or last two readings was proportional to the height of the liquid in the tank. There were other claims too, in that each device used a random gating signal so that each "chirp" sounded sufficiently differently to allow more than one device to be used within the same location. There was a very small dead band at the bottom of the sensing element, but that didn't matter to my application.
rgds