An inverse method for estimating air volume fraction of sea foam from emissivity data

The air volume fraction (AVF) of a sea foam layer is a key parameter that affects the microwave emissivity and brightness temperature of the sea surface. To extract AVF of a sea foam layer from the measured emissivity data at microwave frequencies, a novel retrieval method is developed based on the spectral representation of effective dielectric constant of sea foam composites media, with the volume fraction of constituents and the geometry of foam microstructure included in the spectral function. In the retrieval method, the cost function of emissivity is constructed with two constraints: one is the first two moments of the spectral function and the other is derived from the spectral representation of coated spherical composites. We tested the inverse method using the simulation data of a known emissivity model with Gaussian noise. Good agreement is obtained between the inverse results and the known AVF of sea foam from the simulated emissivity data with varying sea surface temperature, salinity, and incident angle. Finally, the proposed method was used to estimate the AVF of the sea foam layer from the measured emissivity data at frequencies 1.4, 6.8, 10.8, 35, and 36.5 GHz. Generally, the retrieved values of the AVF fall in the range of 0.85–0.98. This study implies that our method can be extended to retrieve the volume fraction of constituents of other composites media from the passive microwave remote-sensing data.