Research On The Composite Electromagnetic Scattering From Low Altitude Target And The Environment

Research on the composite electromagnetic scattering from low altitude target and theenvironment has been attracting more and more attention in recent years, due to its extensiveapplications to airborne radar observation，target identification and acquisition, etc. Becauseof the interaction between the target and the environment, background and objectives to thesurface of the complex scattering and radiation make the target of the modulation effect of theincident electromagnetic wave with the goal of independence exists very different. The maincontributions of this dissertation include the development of some novel methods forcomputerized scattering calculation of targets and surfaces (flat and rough), the statisticalanalysis on echo signals from low flying radar targets, and integrative modeling between lowaltitude radar targets and the environment.In this dissertation, the specific work and innovation include the following:1. A hybrid method of finite elements method (FEM) and physical optics (PO) method isproposed to calculate the electromagnetic scattering by electrically large conducting targetswith anisotropic medium partially coated. FEM is applied in the region which is coated withcomplicated medium, while PO is used for the other of electrically large conducting target.Considering the coupling of two regions, the shadow regions of the boundary surface areeliminated. The radar cross section (RCS) of aircraft with partially-coated plasma iscalculated by this FEM and PO hybrid method.2. A hybrid double-iterative algorithm is developed，which effectively combine theanalytic KA (Kirchhof Approximation) for most of rough surface scattering computation，andthe numerical MoM (method of moments) for target scattering and the underlying roughsurface scattering．According to the interaction between the target and the environment, therough surfaces are classified into two categories: strong and weak. The external couplingiteration takes account the interactions between KA area and MOM area, while the innercoupling iteration takes account the interactions between the target and the strong interactionarea. With Monte Carlo method，this hybrid KA-MoM iterative algorithm is used forscattering computation from a3D target above a2D rough surface．3. In order to consider the scattering fields of the electrically large PEC targets above flatsurface, the half-space Green’s function is introduced into the graphical-electromagneticcomputing (GRECO) method and the method of equivalent electromagnetic (MEC). Theincident plane wave is used for the first illumination, while the reflected wave from flatsurface for the second illumination. 4. In order to improve the accuracy for bistatic RCS compution of electrically large targrtabove flat surface, a new improved high frequency method is developed. Based on magneticfield integral equation, this method is deduced by introducing the current marching techniqueinto monostatic technique. Combined with graphical electromagnetic computing method thatextracted the illuminated and shadow facets in accordance with the direction of the incidentsort iteration, the currents of illuminated and shadow facets can be calculate accurately.5. The statistical analysis on echo signals from low flying radar targets based on actualenvironment is presented. The phase factors of different paths are modified for fluctuation inmultipath based on the radar equation. The statistic peculiarity of echo is calculated with theMonte Carlo method, and the numerical results show that this method is efficient andaccurate.6. Focusing on the difficulty in integrative modeling between the low altitude radartargets, the ground clutter and the environment, a new coherent method is proposed to analyzethe echo signal from radar targets in actual landform or sea conditions environment. Theground and sea clutter is produced by using coherent clutter model. Based on airborne PulsedDoppler (PD) radar, the scattering of the ground clutter cell is computed with Shuttle RadarTopography Mission (SRTM) and Digital Feature Analysis Data (DFAD), and the sea clutteris calculated with sea conditions. The signals are sent into the appropriate range gates. Finallythe efficient echo signals with the target peculiarity and clutter peculiarity are simulated.