Office of Naval Research
The objective of this experiment is to evaluate the accuracy of a developmental capability to estimate ocean surface parameters such as the wave and wind fields by remotely sensing the surface from large standoff distance with a high resolution airborne EO/IR imaging sensor having temporal dwell. Specific parameters to be evaluated are the directional surface wave spectrum, water currents, water depth, and wind speed and direction. Emphasis is placed on evaluating the accuracy of these derived environmental parameters using data from extensive in situ instrumentation. While the emphasis will be placed on the region from deep water to the edge of the surf zone, the surf and beach will be imaged as well.
Long term objectives are to use this measurement and analysis technology to provide improved environmental measurements for research into basic oceanographic phenomena (wave transformation in shallow water, wave-current interactions, air-sea interactions, turbulence dynamics in ocean and atmospheric boundary layers) as well as improved realtime monitoring techniques for the US Navy and Marines.
Space-time images (2-D spatial images collected over a short period of time) will be Fourier transformed to provide 3-D contrast spectra. These frequency-wavenumber spectra will be used to identify specific dispersion surfaces and these, in turn, will be used to provide estimates of the directional surface wave spectrum, water currents, water depth, and the wind speed and direction.
An available airborne IR system being developed by the Advanced Research Projects Agency of the DoD will be flown several days (or nights) during the October phase of DUCK94. Specific flight plans are expected to include long stretches parallel to the beach, and shorter ones perpendicular to it, covering the locations of in situ instrumentation including the planned buoys and pressure array.
The sensor has effective registration to construct a time series of images that will be navigated to real space-time coordinates. These will be Fourier transformed to calculate the 3-D contrast modulation spectrum. This will be analyzed for energy components on the surface wave dispersion surface, the wind (Taylor) surface, and the surfactant (nearly frozen) surface. The wave energy will be transformed to line of sight slope units using an estimate of the background sky gradient, and this in turn transformed to a directional amplitude spectrum using gravity wave kinematics.
In addition, the contrast spectrum will be analyzed for distortions of the surface wave dispersion surface caused by finite water depth and Doppler shift due to currents (exponentially weighted for each wavenumber), so as to estimate these parameters. Also, energy on the Taylor surface will be analyzed to estimate the wind speed and direction across the scene and, if there is adequate signal-to-norise-ratio, to measure the space-time structure of the turbulence in the atmospheric surface layer.
A target having controlled temperature will be set up on the beach to calibrate the atmospheric transmission path. In addition, collaborations will be fostered with other PIs where the parameters estimated by this technique are good enough to make detailed comparisons with in situ data, namely directional wave spectra, currents, water depth, and wind measurements.
For a list of addresses of the investigators involved in Duck 94 click here.