A Directional Wave Array Using Ultrasonic Sensors

A typical approach to determining wave direction is to assume that the sea surface is made up of several sinusoidal waves of various frequencies and directions. One method to determine wave direction as a function of frequency is to use an array of time-series point measurements of water elevation. These multi-element arrays can either be linear or polygonal, and utilize phase, time and path differences to determine wave direction. Typically, pressure gages or capacitance wave probes are used in a directional wave array, however, recently at the Naval Surface Warfare Center, Carderock Division, a directional wave array was employed using five ultrasonic level sensors in an array to quantify wave direction in the Maneuvering and Seakeeping basin (MASK). Two methods were then used to calculate wave direction, a phase/path/time difference method of Esteva which yields a mean direction for each frequency bin, and the Maximum Likelihood Method (MLM), which yields a directional spectrum for each frequency bin. Testing in the MASK was performed to assess the feasibility of using the array on a moving vessel to measure directional seas in the field. The sensors' sampling rate was set at 20 Hz and the five sensors were set up in "slave-master" mode, with the “master” driving the four “slaves” to sample concurrently. This method helped to reduce cross-talk between the sensors and their subsequent dropouts and spikes. Data was collected using LabView software with custom written real-time analysis in MATLAB. Wave direction was measured with regular and irregular waves, with unidirectional and bi-directional systems ninety degrees apart. Tests were performed with the array in a stationary position, as well as with forward motion and simulated pitch and roll motions to assess the potential of using the array on a moving vessel. Results with the stationary array from the basin are good, with the array correctly measuring regular waves of a single frequency from two directions, as well as irregular waves from two directions. Results from the system undergoing motions have increased variability.