A Study Of Electromagnetic Composite Scattering And Doppler Spectra Of A Target At Time-evolving Sea Surface

The study on the characteristic of radar returns at higher microwave frequenciesfrom the time-evolving maritime scene has always been one of the topics that attract themost attention in the community. The problem will be much more complex andchallenging especially for the composite maritime scene contaminated by a movingship-like target. It mainly displays in three aspects. First, the motion of sea wavesalways has a certain amount of randomness and polytropy, which makes the predictionon the scattering from time-evolving sea a very difficult task. The major difficulties arethe computational burden arises from the numerical simulation on the nonlinear ortwo-dimensional time-evolving sea waves, and the complex interactions between theelectromagnetic waves and sea waves. At higher microwave frequencies, the problem isstill beyond a sufficient solution and comprehensive explanation. Second, the target(mainly the ship-like target) on the sea surface is always with super large electrical sizeand has many refined structures, which also bring great challenge on the fast calculationon the target scattering. Third, the coupling interaction between the scattering wavesfrom the target and sea surface is also very complicated. Besides, the ship motion inseaway makes the posture of target continuously change with time. Under thecircumstances, the problem becomes much more awkward.This dissertation presents the research of the electromagnetic scattering from thesea surface and the composite scattering from the electrically large moving ship-liketargets at sea surface, which is with emphasis on the characteristic analysis of Dopplerspectra and mean levels of the radar echo at higher microwave frequencies. The mainworks are as follows:1. Based on the facetized treatment of the scattering contribution, the“Semi-deterministic facet scattering model, SDFSM” is developed to implement the fastprediction on the spatial distributions and mean levels of the electromagnetic scatteringfrom sea surface for arbitrary bistatic configurations and various sea states. Meanwhile,the model is used to further simulations on the synthetic aperture radar (SAR) imageryof two-dimensional sea scene.2. The capillary waves that detected by radar is represented by a simplified waveaccording to the Bragg resonant mechanism, which is the main term of the interactionbetween the electromagnetic waves and sea surface. The simplifications above allow usto derive a simple asymptotic expression of the integration in the Fuks’ formula, whichfacilitates the development of a compact asymptotic representation of the instantaneous complex reflective function of a deterministic two-dimensional sea profile. Theproposed model is named the “capillary wave modification facet scattering model,CWMFSM”, which is then employed for the fast calculation on the scattering andDoppler of two-dimensional sea surface at higher microwave frequencies (X Band).3. After a careful examination on the traditional multi-path model and the modifiedmulti-path model, the author propose a new approach for dealing with the couplingscattering interaction according to the distribution characteristic of thespecular-reflection scattering elements on the deterministic rough profiles. A so-called“Weighted multi-path model, WMPM” is developed based on the calculation of themulti-path coupling scattering contributions that weighted by the probabilitydistribution function of the specular-reflection scattering elements. In comparison withthe aforementioned two models, the proposed WMPM treats the statistical affect of thedeterministic surface model to the coupling scattering in a more reasonable term. Thus,the WMPM is more suitable for the calculation of the composite scattering from a targetat a deterministic rough sea surface.4. With the long term goal of fast prediction on the electromagnetic compositescattering from the moving ship-like target at a time-evolving sea surface, the authorfirstly examine the characteristics of the sea-keeping motions of a Series-60ship model,then combine the CWMFSM and the WMFM to develop an efficient simulator of theradar return signals from the “fully dynamic” ship-sea scene at higher microwavefrequencies. The simulator allows us qualitative analysis on the influence of thetranslation and rotation motions of ship-like target to the composite scattering andDoppler spectra.