Wind-Direction Estimation from Single X-Band Marine Radar Image Improvement by Utilizing the DWT and Azimuth-Scale Expansion Method

Abstract

In this study, a method based on the discrete wavelet transform (DWT) and azimuth-scale expansion is presented to retrieve the sea-surface wind direction from a single X-band marine radar image. The algorithm first distinguishes rain-free and rain-contaminated radar images based on the occlusion zero-pixel percentage and then discards the rain-contaminated images. The radar image whose occlusion areas have been removed is decomposed into different low-frequency sub-images by the 2D DWT, and the appropriate low-frequency sub-image is selected. Images collected with a standard marine HH-polarized X-band radar operating at grazing incidence display a single intensity peak in the upwind direction. To overcome the influence of the occlusion area, before determining the wind direction, the data near the ship bow are shifted to expand the azimuth scale of the data. Finally, a harmonic function is least-square-fitted to the range-averaged radar return of the low-frequency sub-image as a function of the antenna look azimuth to determine the wind direction. Different from the wind-direction retrieval algorithms previously presented, this method is more suitable for sailing ships, as it functions well even if the radar data are heavily blocked. The results show that compared with the single-curve fitting algorithm, the algorithm based on DWT and azimuth-scale expansion can improve the wind-direction results in sailing ships, showing a reduction of 7.84° in the root-mean-square error with respect to the reference.

Wind Direction Extraction from X-Band Marine Radar Images Based on the Attenuation Horizontal Component

“…Radial noise lines caused by interference from other marine radars appear as highintensity pixel lines distributed along the range direction in radar images [38]. Given that the marine radar data used in this study come from the same equipment as that used in [33], median filtering is applied here to remove the interference caused by radial noise lines. A 3 × 3 sliding template is selected, and each pixel is replaced with the median radar return value of the remaining eight image points within the adjacent window for that point.…”

“…Based on this principle, there are two main types of wind direction extraction. One type uses integrated radar image sequences to obtain the wind direction [29,30], while the other type requires only a single radar image to directly retrieve the wind direction [31][32][33]. The above methods can retrieve wind direction well in the absence of rain, but rainfall causes severe interference in radar imaging [34][35][36].…”