An empirical method to derive ocean waves from second-order Bragg scattering: Prospects and limitations

High-frequency (HF) radar wave processing is often based on the inversion of the Barrick-Weber equations, introduced in 1977. This theory reaches its limitations if the length of the Bragg-scattering wave raises to the order of the significant waveheight, because some assumptions are no longer met. In this case, the only solution is moving to lower radar frequencies, which is not possible or desirable in all cases. This paper describes work on an empirical solution which intends to overcome this limitation. However, during high sea state, the first-order Bragg peaks sometimes could not be clearly identified which avoids the access to the second-order sidebands. These cases cause problems to the algorithm which have not been solved yet and currently limit the maximum significant waveheight to about the same values as reported for the integral inversion method. The regression parameters of the empirical solution calibrated from the European Radar Ocean Sensing (EuroROSE) data set are constant values for the complete experiment and when applied to the HF radar data they reconstruct the measurements by a colocated wave buoy quite well. When including a radar-frequency-dependent scaling factor to the regression parameters, the new algorithm can also be used at different radar frequencies. The second-order frequency bands used for the empirical solution are sometimes disturbed by radio interference and ship echoes. Investigations are presented to identify and solve these situations.