A compressor for compressing a gas comprises an impeller wheel including a plurality of vanes rotatably mounted within a housing. When this occurs, the first R bits are output and any remaining S bits in excess of R are stored and additional groups of M bits added, with the process continuing until all of a packet has been received. The formation of each raw image includes: temporal Fourier transform of radio frequency echo signal from each element; phase rotation; spatial Fourier transform; complex interpolation; and an inverse spatial-temporal Fourier transform. The fine delay may be selected from an overlapping set of fine delay intervals, based on the selected coarse delay.