Research On The Calibration Of Frequency By Skywave Over-the- Horizon Radar

This thesis is aiming at solving the problems of actual project under the background of sky-wave over-the-horizon radar(OTHR).It mainly analyzes the OTHR sea clutter and studies the method of sea clutter frequency calibration for skywave over-the-horizon radar.Sky-wave over-the-horizon radar works in the high frequency range(3-30MHz),and have capacity of target detection beyond visual sight by utilizing ionosphere reflected signal.Because of its advantages of long-range detection(800-3500km),large coverage,early warning,and anti-stealth,it plays an important role in national defense construction,national economy,and scientific research.However,influenced by the special propagation medium,which caused there is an error between the true velocity and the measured one,and even reverse,which has a serious impact on practical applications.Therefore,this thesis makes a detailed analysis of the characteristics of sea clutter and the difficulties in the correction of sky-wave over-the-horizon radar.According to various manifestations of sky-wave over-the-horizon radar sea clutter,in order to solve the problem that the target velocity does not match the actual velocity.Aiming to improve the accuracy of the target radial velocity estimation,an adaptive method of sea clutter frequency calibration is proposed in this paper.It mainly includes the analysis of the interference characteristics of the sky-wave over-thehorizon radar working high frequency band,the feature extraction and classification of the different representations of the Bragg peaks in the Range-Doppler spectrum,and the improvement of calibration by studying different sea clutter correction methods.The specific research content of this article is as follows.Firstly,the mechanism of sea clutter generation based on Barrick's Bragg scattering theory and the influence of ionosphere on the echo spectrum are introduced in detail.By observing the measured data and combining the theoretical knowledge of sea clutter,this paper analyzes the properties of the sea clutter spectrum of the skywave over-the-horizon radar.It is the foundation of the classification of sea clutter as a follow-up study.Then,the data is preprocessed before recognition to determine,if there is RF interference in the high frequency band.And the analysis of radio frequency interference,the method based on KL divergence combined with Otsu method is used to segment the background of the measured data.At the same time,the presence or absence of interference is judged by using information such as distance-correlation of interference and other feature.The situation determines which Doppler unit it occupies.It is used to provide a priori information knowledge for subsequent recognition of sea clutter.After that,the histogram of gradient feature(HOG)and SVM-KNN semisupervised learning algorithm are introduced.Due to the influence of the ionosphere and the actual conditions on the sea surface,sea clutter has been broadening and multimode,and cannot be corrected by a single method.Through the feature extraction of sea clutter and the use of SVM-KNN semi-supervised learning algorithm to classify the sea clutter manifestation,it is possible to effectively distinguish the different modes of sea clutter with few label samples.Finally,based on the classification results of sea clutter,an algorithm based on centroid method is proposed.It is suitable for the case where sea clutter is consistent with the frequency shift of the range unit in the RD spectrum,and adaptive correction technology is realized by considering the mode and offset of different beams.The semi-supervised classification method to identify the sea clutter was used to self-adaptive calibration method proposed in this paper,and then the centroid and the improved double filter method has been used to calibrate the sea clutter,which improve the accuracy of the target radial velocity perfectly.