An Efficient Subswath-Subband Chirp Z Transform Algorithm for Multiple-Receiver SAS Considering the Differential Range Curvature

The interpolation-free chirp z-transform (CZT) algorithm is efficient but requires approximating the accurate 2-D spectrum’s phase. However, this approximation is invalid for data from multiple-receiver synthetic aperture sonar (MRSAS) systems with wide beamwidths, large fractional bandwidths, and wide subswaths. To address the issue above, this article derives a new range history and 2-D spectrum, and proposes a new method to limit the approximation error of the 2-D spectrum’s phase. This article first represents the range history as a combination of basic functions and derives a new 2-D spectrum without any approximation. After the receiver dependence of the 2-D spectrum’s phase is partially reduced, the echo data are divided into multiple subapertures in the receiver domain to ensure that the 2-D spectrum’s phase difference of different receivers within each subaperture can be ignored. A frequency-domain filter is designed based on the 2-D spectrum of the reference receiver to compensate for all receivers’ data in the subaperture. Therefore, only one imaging process is needed in a subaperture, rather than receiver-by-receiver imaging. The 2-D spectrum’s phase is expanded up to the third range frequency and its error is limited by the subswath-subband processing rather than considering the higher order terms in Taylor expansion. The quadratic and cubic terms in each subband are linearized through subband-wise linear approximation (SLA) to facilitate the CZT algorithm. Finally, the high-resolution image of the whole swath is obtained through the subimage fusion. The superiority of the proposed range history and subswath-subband processing has been verified under wide beamwidths, large fractional bandwidths, and wide subswaths through simulation and measured data.