Estimation of the wave height from the sea surface velocity measured by Along-Track interferometry SAR
Abstract:NICT developed the along-track interferometry SAR (AT-InSAR) system to detect the cars and ships and measure its speed in 2011. The preliminary experiments for the running truck and ship were performed to check its function and capability of the moving targets detection. As a result, it was confirmed that the system performance was satisfactory to its specifications. In this paper, to estimate the distribution of the wave height from SAR images observed by the AT-InSAR precisely, the relationship between the wave height and the sea surface velocity measured by the AT-InSAR was clarified and a method to estimate the wave height from the measured sea surface velocity was developed. In addition, to check the capability of the ocean waves observation by the AT-InSAR, the preliminary observation for the ocean waves was performed. To estimate the wave height from the sea surface velocity, the relationship between the velocity in the sight direction and the wave height was formulated. From this formulation, it was clarified that the phase of the velocity in the sight direction is shifted to -1/2πfor the phase of the wave height. To estimate the distribution of the wave height, the wave surface velocity was estimated by subtracting the average velocity from the estimated sea surface velocity. The spectra of the wave surface velocity were calculated by 2D FFT and the spectra of the wave height were estimated from the spectra of the wave surface velocity. The distribution of the wave height was calculated from the spectra of the wave height by 2D inverse FFT.
To check the capability of this wave height estimation method, I observed the sea surface by the AT-InSAR. The observation area is the sea surface at 3 km off the coast of Ooarai, north-east to Tokyo, JAPAN. The analysis area is about 2 km square. The incident angle at the center of the SAR image was 55 degrees. The sea state condition was a calm during the observation. The observation was performed in 2 directions and the accuracy of the estimation results were checked. The significant wave height and period measured by the AT-InSAR agreed with it measured by the wave gage located close to this observation area. In addition, there were no irregular wave heights in the distribution of the estimated wave height. As a result, it was confirmed that the AT-InSAR could observe the wave height in a calm sea.