Interpreting directional data from large pitch-roll-heave buoys

Abstract Surface-following buoys are commonly used to measure directional wave spectra. At each frequency, four directional parameters can be calculated from the auto- and cross-spectra of pitch, roll and heave. It is shown that for many buoys, particularly large data buoys, the response of the buoys must be taken into account when calculating the spread parameter s 1 or related parameters such as the r.m.s. angular spread σ. The detection of noise in the records is discussed, and some likely sources identified. These include the effects of compass tilt. It is shown that the mean direction θ M 1 is robust against the presence of noise, but that s 1 and σ are very sensitive to it. The theoretical results are illustrated with data from the U.K. data buoys DB1 and DB2.