Full-Wave Calibration of Time- and Frequency-Domain Ground-Penetrating Radar in Far-Field Conditions

Full-wave modeling of ground-penetrating radar (GPR) data using Green´s functions for wave propagation in planar layered media and antenna characteristic global reflection and transmission functions for describing far-field antenna effects, including antenna-medium interactions, has shown a great potential for nondestructive characterization of soils and materials. The accuracy of the retrieved parameters in the GPR data inversion depends on the accuracy of the GPR external calibration. In this research we studied the stability and the repeatability of two different GPR systems, namely, frequency- and time-domain systems. A combination of a vector network analyzer and 800-5200 MHz horn antenna was used as a frequency-domain GPR (FD-GPR) whereas a GSSI GPR system using a 900 MHz bowtie antenna was used as a time-domain GPR (TD-GPR). Both GPR systems including their antennas were calibrated several times using measurements with the antennas at different heights over a perfect electric conductor (PEC) in the laboratory as well as over a water layer. In addition, measurements were performed over a thin water layer and a relatively thick sandy soil layer as validating medium. The results showed that the FD-GPR is relatively stable while the TD-GPR presents a significant drift which can be accounted for using corrections based on the air direct-coupling waves (free-space measurements). Water- and PEC-based calibrations provided very similar results for the GPR calibration functions. Inversions for the water layer and the sandy soil layer provided reliable results and showed a high degree of the repeatability for both radar systems. The error on the calibration based on inaccurate antenna heights over PEC showed the significant errors on the inversion results for the directive antenna (horn antenna) but less error for the bowtie antenna. This analysis demonstrated the general validity of the proposed far-field radar modeling approach, not only with respect to frequency a- d time domain radars but as well with respect to the calibrating medium.