00:05 Mar-18-2002 M. Schickert R & D, Institute for Materials Research and Testing, Germany, Joined Jan 2000 18
Re: SAFT SAFT/SAR: SAFT has been developed in analogy to SAR (also adopting principles from optical holography and geophysics), so the algorithms are similiar except that in SAR usually far field approximations are made to accelerate computation. For synthetic focusing, all ultrasonic imaging techniques (SAFT, SAR, holography, tomography) require coherent superposition of received signals, although special incoherent methods exist.
Coherent Pulses: The term "coherent" applies to continuous waves in the first place. When using short pulses as a container for a broad frequency spectrum, the temporal relation of pulses determines the phase relation of the waves and therefore their coherence. So all one has to do in an imaging experiment is to assure fixed temoral relationship between the transmitted and each received signal. This can be done by triggering A/D-conversion by the transmitted pulse. For increased accuracy, the instruments clocks involved should be coupled. It should be noted that coherent imaging also requires coherent propagation of pulses in the medium imaged. Problems might occur in, e.g., fluctuating and strongly scattering media.