As a nondestructive testing technology, ground penetrating radar (GPR) is widely used in the detection and recognition of high-speed railway subgrade diseases. Rough-surface direct wave suppression caused by locomotive vibration and subgrade irregularity is a challenge in the GPR signal process. Flat-surface assumption makes traditional method difficult to apply directly to the rough terrain environment. To remove rough-surface direct wave and improve the signal-to-noise ratio, a new adaptive algorithm is proposed in this paper. First, according to the characteristic of electromagnetic wave propagation, the echo model of GPR was constructed, and the composition of ground radar echo signal was analyzed. Next, the eigenvalue and eigenvector of the echo signal and ground direct wave were studied. With the increase of relative permittivity, the energy eigenvector of direct wave and echo signal converged to the same vector space. On this basis, a rough-surface direct wave suppression framework based on energy feature adaptive analysis was proposed. Finally, a numerical simulation and field experiment were carried out to verify the feasibility of the algorithm. Results showed that the algorithm effectively suppresses the rough ground direct wave, and the processing result preserves the target echo more completely. Without intervention, this method can be applied in automatic direct wave suppression, which will promote the development of automatic identification technology in the detection of railway subgrade diseases.
In DS/SS receiver, correlation peak power is square of sine-function of Doppler shift. By taking this priori knowledge into account, the paper put forward a Doppler shift estimation algorithm using nonlinear least squares estimation method. Estimation performance was evaluated by computer simulation and compared with four-quadrant frequency discriminator algorithm (FQFD) used in GPS receivers. I. INTRODUCTIONSpread spectrum technology is widely adopted in low orbit satellite communication systems, the third generation mobile communication system, and Global Navigation Satellite Systems, etc. Quick and accurate spreading code phase and frequency synchronization are key problems to the DS/SS receiver. Due to oscillator instability and relative motion of receiver, the carrier frequency undergoes a great uncertainty, which called Doppler shift in brief. And signal propagation delay causes the uncertainty of spreading code phase.The Doppler shift ups to ±40KHz in high dynamic GPS receivers. To achieve carrier synchronize in such a large Doppler environment, the receiver searches the carrier frequency by dividing the whole uncertainty range into small frequency bins, and employs a 20 joint search method in the code-phase and frequency planar to acquisite the signal. After the acquisition completed, a Doppler estimator is applied to further reduce the frequency error, which is called frequency pulling. GPS receiver based on GEC Plessey's GP2000 chipset uses FQFD during this pulling procedure. After a certain iterations of pulling, the frequency error falls into the narrow range of AFC loop. Then AFC loop will keep tracking its variation.Estimation method for frequency pulling after initial carrier acquisition plays an important role in the carrier synchronization by filling the gap between the working ranges of frequency bin and AFC loop. During the past years, Doppler shift estimators had been widely discussed in the literatures. Vilnrotter [1] presented a comparison of four different estimators: an approximate maximum likelihood estimator, an extended Kalman filter, a cross-product automatic frequency control loop and a phase-locked loop.
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