Abstract
In this paper, a novel current inversion algorithm from X-band marine radar images is proposed. The routine, for which deep water is assumed, begins with 3-D FFT of the radar image sequence, followed by the extraction of the dispersion shell from the 3-D image spectrum. Next, the dispersion shell is converted to a polar current shell (PCS) using a polar coordinate transformation. After removing outliers along each radial direction of the PCS, a robust sinusoidal curve fitting is applied to the data points along each circumferential direction of the PCS. The angle corresponding to the maximum of the estimated sinusoid function is determined to be the current direction, and the amplitude of this sinusoidal function is the current speed. For validation, the algorithm is tested against both simulated radar images and field data collected by a vertically-polarized X-band system and ground-truthed with measurements from an acoustic Doppler current profiler (ADCP). From the field data, it is observed that when the current speed is less than 0.5 m/s, the root mean square differences between the radar-derived and the ADCP-measured current speed and direction are 7.3 cm/s and 32.7°, respectively. The results indicate that the proposed procedure, unlike most existing current inversion schemes, is not susceptible to high current speeds and circumvents the need to consider aliasing. Meanwhile, the relatively low computational cost makes it an excellent choice in practical marine applications.
Highlights
Sea surface current information is useful for off-shore activities, maritime safety and coastal protection
To reconstruct the true surface elevation profile, an inversion scheme is required [1]: the radar image sequence is first transformed into the wavenumber-frequency domain using a 3-D fast Fourier transform (3-D FFT), and the surface current speed and direction can be determined by analysing the obtained image spectrum
The energy associated with the ocean waves is separated from background noise through a frequency filter based on the linear dispersion relationship incorporating the estimated current velocity
Summary
Sea surface current information is useful for off-shore activities, maritime safety and coastal protection. By applying a modulation transfer function (MTF) to the image spectrum, the corresponding wave height spectrum can be obtained It is apparent from the above procedure that an inaccurate current estimation may cause deviation in the dispersion relation filter, and the overall inversion accuracy may be significantly reduced. To this end, several current inversion algorithms have been developed, including the basic least-squares (LS) fitting technique [1], the weighted LS method [7], the iterative LS (ILS) approach [8], the dispersive surface classificator (DiSC) method [9] and the normalized scalar product (NSP) procedure [10].
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