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Stepped frequency radar systems are gaining popularity in ground penetrating radar applications since these can be designed to be phase coherent, thereby taking advantage of coherent processing techniques for target detection and clutter rejection. One such system is the GeoRadar model 1000B GPR system. This system operates over the 100 - 1000 MHz frequency range with a step size of 2 MHz and a modulation frequency of 500 kHz. The magnitudes of the in-phase (I) and the quadrature (Q) channels of the received radar return signal are combined vectorially to yield the amplitude, which is then fast Fourier transformed (FFT-ed) and displayed as the depth profile. The original design of the system used a pair of log-spiral antennas for transmit and receive functions. Although log-spiral antennas are broadband and frequency- independent, their polarization, although linear at each frequency, rotates as the frequency is stepped. The images acquired under these conditions are similar to those obtained by circularly polarized antennas, making it difficult to detect long slender objects, such as pipes and cables, which can be better detected by linearly polarized antennas oriented in the same direction. In order to enhance the performance of the system, a fully polarimetric processing technique has been implemented. A set of four wideband linearly polarized antennas is used, two for transmit and two for receive, each of the two being orthogonally polarized. The system sequentially transmits, orthogonal polarizations, while the receiver simultaneously measures both reflected polarizations. The transmit polarization switching rate is high enough to ensure that the successive pulses are correlated, thereby appearing as if taken from the same location. Reflected data from all four linear combinations of transmit and receive polarizations are then coherently processed to yield the polarimetric image of the target. Sandbox test results indicate that the system is better able to image pipes and utilities.
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