I've found the some very usefull information .
The first principle consists in measuring the phase-shift
between continuously transmitted and received signals. The
measured phase-shift [ch952] is proportional to distance d, and
these quantities are related by 360·d = [ch952]·[ch955], where [ch955] is the
wavelength. The limitation in this principle is that the
measurable distance range, without ambiguity, is only from
0 to one wavelength. For instance, at 20 ºC, a system
operating at 40 kHz would only measure, without
ambiguity, distances up to 8.6 mm
This is were I was so with the new knowledge I will push on.
Hope it helps someone else.