Fully-Polarimetric GPR Acquisition System And Calibration Technique Preliminary Study

Ground-penetrating Radar (GPR) is an effective shallow geophysical detection technique. It has the advantages of bring convenient, operation simply, anti-interference, site adaptability and high detection resolution. It has broad applications in the fields of pavement thickness and subgrade detection, cracks investigation, engineering geological survey, the city pipe and tunnel construction, bridge construction quality testing, prevention of geological disasters, metal mineralization investigation, surface water resources investigation, archaeological investigation, agricultural monitoring, etc.Though GPR is used widely, many basic theory and technical problems have not solved, there are also have some shortcoming, such as shallow detection depth, clutter interference, uneven medium can not obtain necessary speed data. The data collection method of a single transmiting and receiving just can provide limited information, pulse radar signals instability, antenna coupling, ground clutter interference, low signal to noise ratio.Polarization describes the electromagnetic field vector endpoint as a function of time formed the direction and amplitude of space locus. It plays an important role in acquisition of target information , radar data processing and interpretation. It is important information of radar scattering signal, it has a great potential in anti-interference and anti-clutter, target signal filtering and enhancement, target detection and recognition (surface roughness, symmetry, orientation).Based on the development of radar polarization theory and polarization technique playing important role in data information processing, in this paper, we have applicated the polarimetric technology in the GPR detection, developped a fully-polarimetric radar acquisition system to improve the capability of radar target classification and recognition, we have made a preliminary study on system polarimetric calibration. Duing to the Vivaldi antenna performance testing, Using this system,we have done some experiments for detecting the polarimetric properties of typical targets, have calculated the polarization scattering matrix of typical targets, applied radar cross section and radar scattering matrix to calibrate typical targets. FDTD method was proposed by the K.S Yee (1966), using finite differece time domain to replace Maxwell curl equations differential type. Based on the FDTD method, we have used different polarization antenna combinations to simulate the polarimetric characteristics of typical targets, obtained the different polarization characteristics of targets under different rotation angle. for the vertical dihedral, as the angle changed ,its polarimetric characteristics is law changed. in 0°～45°, as the angle increasing, the strength of co-polarization reflection was decreasing, in 45°～90°reflection signal strength reinforced; Opposite, in 0°～45°,cross-polarization reflection intensity increased, in 45°～90°, as the angle increasing, the cross-polarization reflection intensity decreased. As for the plate reflector, trihedral corner reflector, the angle changing can not affect its polarization response, no significant changes.In this work , based on radar measurement principle and the polarimetric theory, we are trying to import the fully polarization technique into the GPR system and developing a full polarimetric GPR acquisition system, which consists of computer (PC control), vector network analyzer (E5071B), three-dimension rectangular coordinate control device, Vivaldi antenna etc. By using VB language, we put the controls of vector Network Analyzer and three-dimensional Cartesian coordinate system in the computer. Implement the entire system automatic operation. with the advantages of this system, we can set the positions of the measuring line and points accurately, realize multiple parameters testing at the same time, storage a variety of data automatically . Antenna is a key component of Radar acquisition system,the merits of their performance is directly related to the accuracy of radar acquisition system. Therefore, after the completion of system integration, we used the radar acquisition system to tested the performance of Vivaldi antenna, Including return loss, VSWR, pattern, transfer function, polarization etc. parameters testing, The results showed that the Vivaldi antenna possesses a wide bandwidth, energy radiation focused, better polarimetric balance, a high cross-polarization ratio, and it may use polarization measurements, and can be used for polarization measurements. Based on the Vivaldi antenna performance, we have applied the fully polarimetric GPR system to test polarimetric properties of typical targets.The measured targets consistent with the forward models. We have tested HH, HV, VV polarimetric of typical targets, respectively, including polarimetric performance measurements of different angles. We used measured frequency domain data, calculated the polarization scattering matrix, and compared its time domain polarimetric curves under different angles. Combination of the polarization scattering matrix and polarization curves, we found that Combination of the polarization scattering matrix and polarization curves, we found that co- polarimetric reflection of the vertical dihedral in 45°was weakest, opposite, cross-polarization response was strongest. As for the plate reflector, trihedral corner reflector, the angle changing can not affect its polarization response, no significant changes. Comparing the theoretical value and measured value of the typical targets, we found the difference and polarimetric imbalance existence between the theoretical value and the measured value, the difference came from the measurement system and the environment interference, so we need calibrate. Based on the extensive application of dihedral, vertical dihedral as a calibrator was used for the system calibration. In this paper, using the radar cross section formula, we have calculated the radar cross section of the. Observed the theoretical value of radar cross section, we found that the radar cross section of vertical dihedral had maximum value in 0°, 45°, this result provided a basic site for the calibrator placed. Using the polarization scattering matrix and calibration formula, employing 0°vertical dihedral, 45°vertical dihedral as the calibrators during polarization calibration. Application of their theoretical value and measured value to calculate, we determined the six calibration factors. Using these coefficients and the calibration formula, we have obtained the scattering matrix calibration value of typical targets.In short, the main content of this thesis are fully-polarimetric GPR acquisition system construction and calibration technique preliminary study, we are trying to import the fully polarization technique into the GPR system, Use the effects of polarized information in anti clutter interference target recognition to enhance the sensitivity, stability and object identification and classification ability of GPR.