The spatial/time spectrum of short sea waves and radar observed signals are locally modulated by the presence of longer waves or currents. There are two different modulations: tilt modulation and hydrodynamic modulation. Variations in the short sea waves spectrum are described by the "hydrodynamic modulation transfer function" (HMTF). The nonlinear interaction between short sea waves and longer waves makes such modulation. Variations of radar signals are described by the "radar modulation transfer function" (RMTF). In this study, new numerical methods based on numerical nonlinear hydrodynamics and computational electromagnetics are developed to examine modulation in sea surface scattering and to examine the accuracy of existing analytical models.
Electromagnetic scattering from sea surface at low-grazing-angles (LGA) is studied by comparing analytical scattering models. The two-scale model (TSM) is found to yield the most reasonable performance among these models. Ocean surface profile retrieval based on the TSM is also shown to have an acceptable accuracy.
Numerical methods are developed to calculate the HMTF, and RMTF by use of the fast nonlinear hydrodynamics, and by use of the fast computational electromagnetics techniques. These techniques allow us to study the scattering from a stochastic "Pierson-Moskowitz" like surface with Monte-Carlo simulation.
HMTF values obtained from the simulations are compared to those from a first order wave action solution, and found to be in reasonable agreement, although differences on the order of 10% are observed. A numerical evaluation of long wave effects on the short wave dispersion relation is also provided.
The numerical method provides a quantitative way to examine the "third-scale" effect in the two-scale model. The results demonstrate that the intermediate waves influence the RMTF and are modulated by longer waves. This effect explains the RHMTF polarization dependence. Numerical results of the "third-scale" effect match well with empirical and analytical results.
A new analytical Doppler formula is derived from the nonlinear hydrodynamics. The solution is validated by numerical solutions and supported by radar Doppler simulation. Ocean surface profile retrieval based on Doppler information is shown to have a very good accuracy.
|School:||The Ohio State University|
|School Location:||United States -- Ohio|
|Source:||DAI-B 78/11(E), Dissertation Abstracts International|
|Keywords:||Doppler, Electromagnetic scattering, Hydrodynamic modulation transfer function, Radar modulation transfer function, Third-scale effect|
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