The use of pulse-echo ranging systems for measuring, for example, the depth of a liquid in a tank is well known. In principle, the time elapsed between transmission of a pulse and receipt of an echo can be used to measure the depth of the liquid. In practice, however, a number of problems arise.
One is how to determine the “best” point in the echo profile to be used for timing. Typically either the echo peak or the leading edge is used, and various methods are known. These generally assume a well shaped echo, and select either the peak of the echo or a point on the leading edge. However, not all echoes returned are clean, well-shaped echoes, with the result that the represented distance can change from echo to echo, even when the actual distance is not changing.