Fast wideband near-field imaging with URAs applied to urban sensing

High resolution wideband near-field imaging requires wideband signals and large array apertures. Hence, a large amount of data needs to be processed in the image formation step. The computational load of the standard delay and sum beamformer stems from the calculation of the focus delays and the evaluation of the sums over the measurements. We propose an approach tackling the problem of calculating the focus delays. Provided a uniform rectangular array (URA) is used, the image can be evaluated on a well tailored discrete grid such that most focus delays are redundant. Now by calculating only the non-redundant values we can reduce the numerical complexity significantly. The above approach is combined with fast beamforming based on the non-equispaced fast Fourier transform (NFFT). We demonstrate the good performance with experimental results from a through-the-wall radar imaging system.