Airborne SAR Imaging Algorithm for Ocean Waves Based on Optimum Focus Setting

Abstract

Ocean waves are the richest texture on the sea surface, from which valuable information can be inversed. In general, the Synthetic Aperture Radar (SAR) images of surface waves will inevitably be distorted due to the intricate motion of surface waves. However, commonly used imaging algorithms do not take the motion of surface waves into consideration. Therefore, surface waves on the obtained SAR images are rather blurred. To solve this problem, an airborne SAR imaging algorithm for ocean waves based on optimum focus setting is proposed in this paper. Firstly, in order to obtain the real azimuth phase speed of dominant wave, the geometric and scanning distortion in the blurred SAR image is calibrated. Subsequently, according to the SAR integration time and wavelength of the dominant wave, a proper focus setting variation section is selected. Afterwards, all the focus settings in this variation section are used to refocus the image, which are then compared to decide the optimum focus setting for dominant wave. Finally, by redesigning the azimuth matched filter using this optimum focus setting, a well-focused SAR image for the dominant wave can be obtained. The proposed algorithm is applied to both simulation and field data, and SAR images of surface waves are obtained. Furthermore, the obtained images are compared with those obtained with a zero-focus setting. The comparison shows that the focus of surface waves is significantly improved, which verifies the effectiveness of the proposed algorithm. Finally, how to choose the appropriate focus setting variation section under different parameters and the applicability of the algorithm are analyzed.

Estimating Significant Wave Height from SAR with Long Integration Times

“…The focus setting that makes the target to be best-focused is called the optimum focus setting [20,21]. The contrast of ocean waves can be improved by changing the focus setting, and the optimum focus setting is obtained according to the azimuth phase speed [22]. However, the ocean waves are usually invisible and obtained by the long integration time SAR, which will result in the optimum focus setting being unable to be calculated by the present methods.…”

SAR Imaging Algorithm of Ocean Waves Based on Optimum Subaperture

“…Referring to the literature [ 25 ] for the values of k and Δ v , they can generally take k = 8 and Δ v = 1 m/s .…”

“…To sufficiently illustrate the effectiveness and superiority of the proposed algorithm, the refocusing results of the proposed algorithm are compared and quantitatively analyzed with the original SAR image, the refocusing SAR image based on half of azimuth phase speed [ 16 , 17 , 18 , 19 , 20 , 21 , 22 ] and the refocusing SAR image based on the optimum focus setting [ 25 ] in this section.…”

“…For example, the condition that the SAR integration time is short or that the wave propagation direction is large relative to the azimuth. To solve the problem that the optimum focus setting of ocean waves is not necessarily half of its azimuth phase velocity, Wei [ 25 ] proposed an airborne SAR imaging algorithm for ocean waves based on optimum focus setting. However, the above studies did not consider the impact of ocean surface coherence time on the quality of ocean wave images.…”

“…The proposed algorithm was applied to airborne L-band and P-band SAR data. Then, the processing results of the proposed algorithm were compared with the original SAR image, the refocusing SAR image based on half of azimuth phase speed [ 16 , 17 , 18 , 19 , 20 , 21 , 22 ] and the refocusing SAR image based on the optimum focus setting [ 25 ]. At the same time, quantitative analysis was carried out by calculating the contrast of SAR images, the relative normalized modulation of image gray and the modulation difference of positive and negative gray, which sufficiently demonstrated the effectiveness and superiority of the proposed algorithm.…”