A NOVEL FDTD BASED MODEL FOR PREDICTION OF BISTATIC RCS OF S

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A NOVEL FDTD BASED MODEL FOR PREDICTION OF BISTATIC RCS OF SINGLE LEAVES AND TREESR Caldeirinha and M Al-Nuaimi University of Glamorgan, UK Abstract: Accurate modelling of the propagation modes arising from vegetation media depends, on the one hand, on the physical parameters specific to the tree, such as size, shape, orientation and moisture contents of leaves, branches and t u k On the other hand, parameters of rn. the radio link associated with the operational frequency, directions of transmit and receive antennas, their heights, beamwidths and polarisation add extra complexity to the characteristics and understanding of the scattering modes from single trees. In this paper, we describe the development of a novel method, which will enable accurate modelling of leaves, trunk and branches. INTRODUCTION In this paper, a novel method aimed at studying the propagation modes arising from interaction with single trees at 20 GHz, using the Finite Difference Time Domain (FDTD) technique, is presented. The method is proposed as a means of accurately computing electromagnetic scattering by arbitrary-shaped relatively complex metal or dielectric objects excited by an external plane wave. The individual components of the vegetation specimen are modelled using simple geometrical shapes and characterised by their complex material permittivities, effective volume occupied, and by the statistical distribution and orientation of branches, twigs and leaves. The scattering problem is analysed in two stages by treating the complex nearfield region and the far-field region separately. The latter is obtained by a near-to-far field transformation based on electromagnetic equivalents of the scattered near fields[l]. Results expressing the predicted bistatic radar cross section (RCS) from a single tree are compared with measured results conducted on a realistic tree at 20 GHz. THEORECTICAL MODEL BISTATIC SCATTERING the far field of the canopy. Bistatic RCS is a characteristic of the vegetation medium that represents its size as seen by the receiver at a particular position s^, and has the dimensions of area (obi The RCS area is ). not the same as the physical area of the tree. The power re-radiated from the tree in the receiver direction s^, is equivalent to the re-radiation of the power captured by an antenna of area obi(;,;)(the bistatic RCS). Therefore, the power re-radiated in the receiver direction is given by (l), where Pt is the transmitted power, G,(i)and G,(s^) are the gains of the transmitting and receiving antenna in the direction of i (incident) and - s^ (scatter), respectively. RI and R, are, respectively, the distances from the transmit antenna to the tree and from the tree to the receive antenna.

Despite its complex nature, the canopy volume has been treated statistically in the literature[2] as a homogeneous mixture of discrete, randomly distributed and oriented dielectric disks representing leaves, and cylinders representing branches. Analyt

ical solutions of such complex electromagnetic problem are difficult to obtain. With today's fast and high capacity computers, it has become possible to handle such complex structures with arbitrary geometries directly in the time domain by using discrete forms of Maxwell's equations. Motivated by the,complexity of analytical solution identified above, a novel FDTD based model has been developed of for prediction of obi(s^,i) single leaves and trees. The development of the model is discussed in the next sections. FDTD BASED MODEL Prediction of Single Leaf Bistatic RCS

Similar to a receiving antenna, a tree canopy also intercepts a portion of the power incident on the canopy, but reflects (re-radiates) it in the direction of the receiver. The amount of power reflected towards the receiver is determined by the bistatic radar cross section (RCS) of the tree canopy, assuming both antennas are in

Leaves are an important feature of any vegetation canopy. In order to model the scattering of a tree effectively, it is essential to develop an efficient and effective technique for predicting the RCS of a single leaf. Based on this, an individual leaf of the vegetation specimen was modelled in the FDTD space using a

11th lntemational Conference on Antennas and Propagation, 17-20April 2001, Conference Publication No. 480 0 IEE 2001

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simple geometrical shape and characterised by its complex material permittivity and effective volume occupied, as shown in Figure 1. The tree under investigation was a Ficus Benjamina Daniel of the type used mostly indoors.

thickness of the leaf. A total number of about 6 million cells were used in the computation, which included the main grid and subgrid with cell sizes of h/20 and h I 60,respectively. Near/Far Field Analysis. In Figure 3 is presented the normalised near electric field distribution (in V/ m ) transverse sections (cuts) of the dielectric leaf and compared with results obtained from an idealised leaf made of a perfect conductor material. These are expressed in terms of total electric field distribution (E,) and its components in the x-y-z directions, respectively, E, E,, and E,. The electric field, or simply E-field, cuts are presented in the y-z plane in which the leaf is located. The plane wave incident on the leaf is subsequently scattered in all directions. A colour scale was used to display the intensity of the fields in the computational space.

Figure 1: Problem space of the leaf model in the yzplane, with direction of propagation out of the paper. Evaluation of Complex Dielectric Properties. The complex permittivity of the single leaf was first evaluated from appropriate measurements to determine the mass of water in the leaf, on a wet weight basis. This parameter, together with the volume occupied by the leaf, provides the basis for the model proposed by Mutzler[3] to estimate …… 此处隐藏：12290字，全部文档内容请下载后查看。喜欢就下载吧 ……