ISR ocean wave spectrum measurement system
These images represent displays under final development for a marine radar method of measuring directional ocean wave spectra, by Imaging Science Research, Inc. The effort represents a Co-operative Research and Development Agreement (CRADA) between ISR and ERDC, U.S. Army Corps of Engineers, being conducted at the Field Research Facility, Duck, NC. ERDC provides ocean wave surface truth for radar comparison as part of the agreement. In addition to the derived products, processed imagery and other sub-products are available to the user for conducting further research.
(A) The first plot represents a real-time analysis presented every half an hour. It is a summary plot of the following components:
(1) Upper-right: A single rotation image, of the 640 collected that constitutes one data acquisition period. This image is 1120 pixels in the cross-wise dimension, at 3-m sampling resolution, thus covering more than three kilometers of shoreline.
(2) Lower-right: A Summed-Image of all 640 rotations, where wave trains disappear and areas of enhanced wave breaking standout, such as due to the offshore bar, rip currents, and other areas of strong wave-current interactions that force small scale wave breaking and provides a strong radar echo.
(3) Upper Left: 12 of 32 user-selectable wave number spectra are displayed, showing the radar spectral energy increase as bright areas in color, and the direction from which the ocean wave component energy is coming. These are derived from three dimensional FFT processing of 64x64 pixel windows position-selectable by the user from each single rotation image of the 640 shown at right.
(4) Lower-left: Frequency spectra time series covering a 13-day period, current time farthest right. Spectral energy values derived from all 32 plots of the type shown in (3) are used to produce this plot. The brightness corresponds to ocean wave height and is derived from a Modulation Transfer Function approach. That applied here does not currently take into account wind direction and wind speed, and is most reliable under winds above 6 m/s and in the same direction as the wave propagation. A MTF model incorporating full wind speed and wind direction is under development.
(B) The lower plot is a GIF movie that can be downloaded and played to show a time series of several days of plots similar to those derived above in (A). Such a presentation provides the user with a comprehensive summary of active periods and is useful for reviewing catalogued data. In addition, in the Summed-Image plots, fluid structures that are thought to represent rip current signatures can be see meandering about nearly perpendicular to the shore line starting at the offshore bar. These are seen to occur under active wave conditions on a regular basis.