Phase Resolved Wave Prediction From Synthetic Radar Images

Abstract

A method is presented for the inversion of images of the sea surface taken by nautical radar into wave elevation that is specifically suitable for the prediction of the wave elevation outside the observation domain covered by the radar. By means of a beam-wise analysis of the image obtained by a scanning radar, the image information is translated into wave elevation. Subsequently a 2D FFT is applied in order to obtain the directional wave components required for a linear propagation of the wave field. Assuming knowledge of the significant wave height, a method to obtain the correct scaling of the wave prediction is proposed. The proposed method is verified using synthetic radar images which are modelled by applying shadowing and tilt effect to synthesised short crested linear waves.

Reconstruction and deterministic prediction of ocean waves from synthetic radar images

“…2.6, does not take into account this decreased visibility at large ranges from the radar and in fact does not even guarantee a correct variance at the radar for τ = 0. An alternative which is supposed to be practical and feasible for real life applications is proposed by Naaijen and Wijaya [2014]: a time history of the wave elevation at the radar position (e.g. by an auxiliary wave buoy or via recorded ship motions) and a time history of the predicted wave elevation can be recorded and used to calculate the variance of the true waves and the prediction.…”

“…tilt and shadowing whereas the hydrodynamic modulation is assumed to have a minor contribution and will be neglected. The generation of the synthetic images will use the same procedure as in Naaijen and Wijaya [2014] for the tilt modulation and in Section 2.2.3 for the shadowing. We summarize the procedure as follows.…”

“…The tilt modulation is simulated by the scalar product σ = u · n where u is the 3D unit vector from the sea surface elevation to the radar antenna and n is the 3D unit normal vector of the sea surface elevation (e.g. Borge et al [2004], Naaijen and Wijaya [2014]), see Fig. 4.1.…”

“…In this paper we will show that this is indeed the case provided one uses the two dimensionless quantities h = Hr Hs and ρ = r λp in the vertical and horizontal direction respectively; then the visibility as function of ρ is indeed a rather robust indicator for H s . This result then gives the possibility to determine H s from the measured visibility (that can be determined directly from the radar images) by comparing it with a visibility database which is obtained by calculating the visibility from simulations of shadowed seas for various values of h. The shadowed seas for the database can be constructed using a specified model spectrum or using the normalized spectrum that is obtained by applying 3DFFT on a reconstructed sea using the method described in Wijaya et al [2015] (or Naaijen and Wijaya [2014] for additional tilt modulation). To improve efficiency, the database is constructed in the main wave direction only, using Monte Carlo simulations to reduce randomness of the seas.…”

“…Synthetic radar images are constructed by taking into account the most dominant effect in the radar mechanism, i.e. the tilt and shadowing (Borge et al [2004] and Naaijen and Wijaya [2014]). We assume that the radar is positioned on a fixed ship.…”