Variable focusing in range from a transmit array can be accomplished by a f
requency shift in an acoustic waveguide [1]. In this paper, a general algor
ithm is derived in free space that can move the focal spot of a linear tran
smit array both transversely and horizontally. In particular, for variable
range focusing, the prescribed frequency shift is the same for all elements
of the array, whereas, for transverse focal shifts, the frequency shift va
ries with element location in the array. The method described in this paper
is especially suitable for varying the location of the focal spot of a tim
e-reversal mirror in the presence of an aberrating layer in that no knowled
ge of the medium properties is necessary. Simulation results demonstrating
the proposed method are presented in comparison with those from a conventio
nal steering method.