A method that employs the geometrical characteristics of an object as a source of information with respect to the object, that comprises:
- directing acoustic wave energy upon an object that reflects the same;
- detecting the reflected wave energy and developing a signal .theta..sub.A therefrom, which signal .theta..sub.A bears a relationship to geometrical characteristics of the object, comparing the signal .theta..sub.A with a standard signal .theta..sub.B representative of the same geometrical characteristics of a standard to determine any differential .theta..sub..DELTA. therebetween, and adjusting the frequency of the acoustic wave energy to maintain a constant wavelength of the acoustic wave energy despite changes in a environmental condition which would otherwise effect changes in said wavelength.