Abstract
The cross-spectral correlation approach has been used to estimate the wave spectrum from optical and radar images. This work aims to improve the cross-spectral approach to derive current velocity from the X-band marine radar image sequence, and evaluate the application conditions of the method. To reduce the dependency of gray levels on range and azimuth, radar images are preprocessed by the contrast-limited adaptive histogram equalization. Two-dimensional cross-spectral coherence and phase are derived from neighboring X-band marine radar images, and the phases with large coherences are used to estimate the phase velocity and angular frequency of waves, which are first fitted with the theoretical dispersion relation by different least square models, and then the current velocity can be determined. Compared with the current velocities measured by a current meter, the root-mean-square error, correlation coefficient, bias, and relative error are 0.15 m/s. 0.88, –0.05 m/s, and 7.79% for the north-south velocity, and 0.14 m/s, 0.86, 0.06 m/s, and 10.75% for the east-west velocity in the experimental area, respectively. The preprocessing, critical coherence, and the number of images for applying the cross-spectral approach, are discussed.
Highlights
The sea surface current is important for marine activities and scientific research
Both the NS current velocities and EW current velocities retrieved from the radar image sequences showed a distinct period of about 12 hours, which corresponded well with those measured by the current meter, which was caused by the semidiurnal tidal current
The NS current velocities measured by this current meter changed from –1.5 m/s to 0.8 m/s, which were close to those retrieved from the X-band marine radar image sequences using different methods; large differences between them occurred between the hours of about 1:00–1:30 and 2:30–2:50, when the sea state is low, and the coherence indicator γI < 0.7 at the time (Figure 3c)
Summary
The sea surface current is important for marine activities and scientific research. The current is usually measured using a current meter and an acoustic Doppler current profiler (ADCP), which provide current velocity with high accuracy, but can only be used at fixed positions, and they are both expensive and difficult to deploy [1]. The algorithms used to retrieve the current from these X-band marine radar images are mainly based on a three-dimensional (3D) fast Fourier transform (FFT) [2]. By applying this 3D FFT on the radar image sequence, a 3D wavenumber-frequency spectrum is obtained, and the current can be derived by minimizing the difference between the image spectrum and the theoretical dispersion relation [2,4,5,6]. 3D FFT requires that the global wave field be stationary and homogeneous To overcome this disadvantage, the dispersive surface classificator was developed to analyze inhomogeneous image sequences on a local spatial scale [7]. Gangeskar [8] showed that the 3D FFT algorithms can provide high-accuracy current measurements using the X-band marine radar in deep water environments
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