Study of Techniques for Improving Resolution of Stepped Frequency Continuous Wave GPR

Abstract

Ground Penetrating Radar (GPR) is a non-destructive method which uses electromagnetic waves to image the earth’s surface. The principle of operation of GPR is similar to that of airborne Radar. The difference is that, here the medium is soil, ice, concrete, instead of free space or air. Due to the complex and stochastic nature of the medium, GPR system needs to be designed very carefully. At the same time signal processing of the reflected signals must be done precisely. GPR systems have been used in a wide variety of applications due to many advantages. One of the most important advantage is that GPR is able to detect metallic as well as non-metallic targets. Inspite of many advantages, its main drawback is the complexity of the received data and the difficulties to interpret them. Therefore, to reduce the complexity of GPR data and simplify data analysis techniques, various signal processing techniques are developed. One of the applications of GPR systems is pavement thickness measurement which requires high-resolution in order to identify various layers of stratified medium. Usually IFFT is used to obtain the time domain response of the scattered signal, which can be used to reconstruct the geometry of layered media. However, IFFT cannot resolve targets, when they are closely spaced. Therefore, there is a need to improve the resolution of GPR system. This can be done either by increasing the operating bandwidth or by improving the time-resolution by applying various signal processing techniques. Increasing operating bandwidth is not feasible because of hardware limitations. It is possible to improve the resolution using suitable signal processing techniques. Some of the signal processing methods which increase the resolution of a GPR system are MUSIC, ESPRIT etc and are termed as super-resolution techniques. In some scenarios, small targets are masked due to the presence of large nearby targets in the vicinity. To unmask the small targets Adaptive Pulse Compression (APC) algorithm based on RMMSE criteria is used. Therefore, MUSIC and ESPRIT are used to improve the resolution while APC is used to improve the SNR.