The directional wave observation should be done with the measurements of one vertical and two horizontal components of wave motions. The covariance method was examined for the GPS buoy. The vertical position and horizontal speeds of buoy were dealt as the data. However, the covariance method did not always estimate wave directions correctly because those buoy motions have each delay according to the wave frequencies. The wave direction without a sign defined in 0-180 degrees is possible to be

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... timated with the measurements of two horizontal components of wave motions. We tried to estimate unsigned directions for the GPS buoy with the Longuet-Higgins method and horizontal GPS buoy positions. To change unsigned direction to signed direction defined in 0-360 degrees, the covariance for the unsigned direction is computed and the sign of the covariance is added to the unsigned direction. It is proved that the combined method proposed in this paper gives proper directions.