Tropospheric Scattering Reconnaissance Technology

The scattering phenomenon occurs when ultrashort wave signal and microwave signal travel to the troposphere.Making use of this feature in the troposphere,the problem that radar signal beyond visual range could not be detected can be solved.Firstly,the theory and characteristic of the tropospheric scatter are analyzed.A certain type of ship-borne early-warning radar in foreign army is chosen as the research target,whose characteristic of transmitted signal is analyzed.Also,the loss of target signal scattering to radar receiver through troposphere and the signal waveform when multi-path effect occurs are simulated.Secondly,digital channelized receiver is used to solved the problem that the loss of signal is very large after scattering.For providing frequency measurement result with high precision to the following Direction of Arrival(DOA)estimation,the key technologies of digital channelized receiver such as polyphase filtering and Short Time Fourier Transform(STFT)are researched and simulated.Thirdly,according to the signal fading caused by multi-path effect,two methods to overcome the multi-path effect are proposed.The first method is applying the singularity of wavelet transform to distinguish multi-path signals,however,this method requires a relatively high SNR,and it is only applicable for single frequency signals.The second method is to use the super-resolution wave-direction estimation technique firstly and to estimate the direction of the multi-path signals,and then using the Digital Beam Forming(DBF)technique to solely receive one of them and suppressing the other multi-path signals at the same time.Two methods of DOA estimation algorithm for analyzing coherent signals are studied in this paper,one of them is spatial smoothing algorithm,which bases on the traditional DOA estimation algorithm(Multiple Signal Classification,estimation of signal parameters via rotational invariance techniques).For the number of multi-path signals is unknown and the correlation coefficient is high,this traditional algorithm could not accurately estimate the arrival direction of the multi-path signals.Therefore,the Weighted Subspace Fitting algorithm(WSF)is adapted in this article,and the implementing method of WSF algorithm is studied.The simulation result shows that this algorithm possesses good robustness of estimating the arrival direction of coherent signals in the case of the signal source number is unknown.Finally,as with the beam formation technique,this paper studies the traditional phase-shifted DBF technique and the adaptive DBF technique.The characteristic namely,the high coefficient of correlation of the multi-path signals,brings difficulty to the self-adaptive DBF.The beam-former based on the Minimum Variance Distortionless Response criterion(MVDR)and the Linear Constrained Minimum Variance criterion(LCMV)is not sensitive to the signal coefficient of correlation.The simulation result shows that the MVDR criterion has a good effect on the suppression of multi-path signals,but the MVDR criterion has a self-zeroing problem.In this paper,it is suggested to use the beam-former based on the Linear Constrained Minimum Variance Criterion(LCMV),since it is better to solve the self-zeroing problem on MVDR algorithm.