The WS Directional Wave Analysis Module calculates a directional spectrum using the maximum entropy method (MEM) as described in Nwogu (1989). Basically, the requirements for the MEM analysis are auto-spectra and co-spectra (real part of the complex cross-spectrum) for one surface elevation and two orthogonal velocity components take at the same position.
For further introduction to directional analysis, reference is made to Nwogu (1989) and Hashimoto (1997).