U. Oguz

Three-dimensional FDTD modeling of a ground-penetrating radar

The finite-difference time-domain (FDTD) method is used to simulate three-dimensional (3-D) geometries of realistic ground-penetrating radar (GPR) scenarios. The radar unit is modeled with two transmitters and a receiver in order to cancel the direct signals emitted by the two transmitters at the receiver. The transmitting and receiving antennas are allowed to have arbitrary polarizations. Single or multiple dielectric and conducting buried targets are simulated. The buried objects are modeled as rectangular prisms and cylindrical disk. Perfectly-matched layer absorbing boundary conditions are adapted and used to terminate the FDTD computational domain, which contains a layered medium due to the ground-air interface.

Simulations of ground-penetrating radars over lossy and heterogeneous grounds

The versatility of the three-dimensional (3D) finite-difference time-domain (FDTD) method to model arbitrarily inhomogeneous geometries is exploited to simulate realistic ground-penetrating radar (GPR) scenarios for the purpose of assisting the subsequent designs of high-performance GPR hardware and software. The buried targets are modeled by conducting and dielectric prisms and disks. The ground model is implemented as lossy with surface roughness, and containing numerous inhomogeneities of arbitrary permittivities, conductivities, sizes, and locations. The impact of such an inhomogeneous ground model on the GPR signal is demonstrated. A simple detection algorithm is introduced and used to process these GPR signals. In addition to the transmitting and receiving antennas, the GPR unit is modeled with conducting and absorbing shield walls, which are employed to reduce the direct coupling to the receiver. Perfectly matched layer absorbing boundary condition is used for both simulating the physical absorbers inside the FDTD computational domain and terminating the lossy and layered background medium at the borders.

An efficient and accurate technique for the incident-wave excitations in the FDTD method

An efficient technique to improve the accuracy of the finite-difference time-domain (FDTD) solutions employing incident-wave excitations is developed. In the separate-field formulation of the FDTD method, any incident wave may be efficiently introduced to the three-dimensional (3-D) computational domain by interpolating from a one-dimensional (1-D) incident-field array (IFA), which is a 1-D FDTD grid simulating the propagation of the incident wave. By considering the FDTD computational domain as a sampled system and the interpolation operation as a decimation process, signal-processing techniques are used to identify and ameliorate the errors due to aliasing. The reduction in the error is demonstrated for various cases. This technique can be used for the excitation of the FDTD grid by any incident wave. A fast technique is used to extract the amplitude and the phase of a sampled sinusoidal signal.

Interpolation techniques to improve the accuracy of the plane wave excitations in the finite difference time domain method

The importance of matching the phase velocity of the incident plane wave to the numerical phase velocity imposed by the numerical dispersion of the three-dimensional (3-D) finite difference time domain (FDTD) grid is demonstrated. In separate-field formulation of the FDTD method, a plane wave may be introduced to the 3-D computational domain either by evaluating closed-form incident-field expressions or by interpolating from a 1-D incident-field array (IFA), which is a 1-D FDTD grid simulating the propagation of the plane wave. The relative accuracies and efficiencies of these two excitation schemes are compared, and it has been shown that higher-order interpolation techniques can be used to improve the accuracy of the IFA scheme, which is already quite efficient.

Frequency responses of ground-penetrating radars operating over highly lossy grounds

The finite-difference time-domain (FDTD) method is used to investigate the effects of highly lossy grounds and the frequency-band selection on ground-penetrating-radar (GPR) signals. The ground is modeled as a heterogeneous half space with arbitrary background permittivity and conductivity. The heterogeneities encompass both embedded scatterers and surface holes, which model the surface roughness. The decay of the waves in relation to the conductivity of the ground is demonstrated. The detectability of the buried targets is investigated with respect to the operating frequency of the GPR, the background conductivity of the ground, the density of the conducting inhomogeneities in the ground, and the surface roughness. The GPR is modeled as transmitting and receiving antennas isolated by conducting shields, whose inner walls are coated with absorbers simulated by perfectly matched layers (PML). The feed of the transmitter is modeled by a single-cell dipole with constant current density in its volume. The time variation of the current density is selected as a smooth pulse with arbitrary center frequency, which is referred to as the operating frequency of the GPR.

Signal-processing techniques to reduce the sinusoidal steady-state error in the FDTD method

Techniques to improve the accuracy of the finite-difference time-domain (FDTD) solutions employing sinusoidal excitations are developed. The FDTD computational domain is considered as a sampled system and analyzed with respect to the aliasing error using the Nyquist sampling theorem. After a careful examination of how the high-frequency components in the excitation cause sinusoidal steady-state errors in the FDTD solutions, the use of smoothing windows and digital low-pass filters is suggested to reduce the error. The reduction in the error is demonstrated for various cases.

Optimization of the transmitter-receiver separation in the ground-penetrating radar

The finite-difference time-domain method is applied to simulate three-dimensional subsurface-scattering problems, involving a ground-penetrating radar (GPR) model consisting of two transmitters and a receiver. The receiving antenna is located in the middle of the two identical transmitters, which are fed 180/spl deg/ out of phase. This configuration implies the existence of a symmetry plane in the middle of two transmitters and the cancellation of the direct signals coupled from the transmitters at the receiver location. The antenna polarizations and their separations are arbitrary. The transmitter-receiver-transmitter configured GPR model is optimized in terms of the scattered energy observed at the receiver by varying the antenna separation. Many simulation results are used to demonstrate the effects of the antenna separation and the optimal separation encountered for a specific target and GPR scenario.

Modeling of ground-penetrating-radar antennas with shields and simulated absorbers

A three-dimensional (3-D) finite-difference time-domain (FDTD) scheme is employed to simulate ground-penetrating radars. Conducting shield walls and absorbers are used to reduce the direct coupling to the receiver. Perfectly matched layer (PML) absorbing boundary conditions are used for matching the multilayered media and simulating physical absorbers inside the FDTD computational domain. Targets are modeled by rectangular prisms of arbitrary permittivity and conductivity. The ground is modeled by homogeneous and lossless dielectric media.

Subsurface-scattering calculations via the 3D FDTD method employing PML ABC for layered media

The finite-difference time-domain (FDTD) method is suitable for solving scattering problems that contain several inhomogeneities such as multiple objects of different material properties buried in a layered medium. The advantage of the FDTD method is that the number of unknowns remains the same and a small amount of extra modeling effort is needed for these problems. We have developed a three-dimensional (3D) FDTD computer program that employs pure scattered-field formulation and perfectly matched layers (PML) as the absorbing boundary condition (ABC) of choice. The purpose of this study is to model a subsurface radar and to observe and distinguish between the fields scattered from various buried objects with different parameters such as the size, depth, number, etc.

Employing PML absorbers in the design and simulation of ground penetrating radars

The finite-difference time-domain (FDTD) method in conjunction with the perfectly matched layers (PML) absorbing boundary condition (ABC) has been a popular choice for the simulation of ground penetrating radars (GPRs) due to the following reasons: FDTD is suitable for modeling inhomogeneities. FDTD is also suitable for the computation of time-dependent radar signals. PML is a very effective ABC that renders the FDTD method quite efficient. In this paper, in addition to the conventional usage of PML to truncate the FDTD computational domain, we report on how the PML can also be used to simulate an absorbing material inside the FDTD computational domain.