Estimation of ocean wave height from grazing incidence microwave backscatter

When waves on the ocean surface are viewed with a ship-borne marine radar, their height is not directly related to either the magnitude of the microwave backscatter field or the magnitude of the modulation of this field in this grazing incidence regime. The authors have performed some validation tests of a model, L. B. Wetzel (1990), which theoretically related the statistics of the ratio of illuminated to shadowed areas through the viewing geometry to the significant height of the waves causing the shadowing. Using a subset of the data collected by two different digital marine radars on the Sea Truth and Radar Systems Experiment of December, 1994, the present authors calibrated the model against measured wave heights, and then validated it using the remaining data from the radars and other independent wave height estimations. Significant wave heights in these experiments ranged from less than one to greater than 5 metres. They found reasonable agreement between their inferred wave height extracted from radar imagery and heights estimated by other means when the sea conditions were moderate and not too complex. Scatter in the results was not inconsistent with the statistical variability expected from the sampling statistics. The model failed under conditions of crossing seas, where it overpredicted the wave height, and when the measured wave height was a significant fraction of the radar scanner height, where the model exhibited asymptotic behaviour and underpredicted the actual wave height