| Sky-wave OTHR works by using the characteristics of ionospheric reflection electromagnetic wave,which makes it possible for sky-wave OTHR to effectively break through the limitation of the earth's surface arc,thus realizing the detection of ultra-long-range targets.Unlike the coastal defense early warning radar deployed on the coast,sky-wave OTHR can be deployed in relatively safe inland areas,thus avoiding precise attack by the enemy.Sky-wave OTHR has the characteristics of long observation distance.However,because it works with the characteristics of ionospheric reflection,it includes sea clutter,ionospheric interference,active interference,noise and so on.Among them,the frequency of sea clutter is very close to the target of slow ship.If we want to detect the target of slow ship,we need to suppress sea clutter.In this paper,skywave over-the-horizon radar is studied,the characteristics of skywave OTHR are analyzed,and the sea clutter component in echo is discussed and modeled.In addition,the sea clutter suppression algorithms are studied.The conventional sea clutter suppression methods include subspace decomposition method,azimuth spread clutter suppression method and space-time adaptive processing method.The improved space-time adaptive algorithms include local joint space-time adaptive processing,space-time multi-beam processing and space-time adaptive processing based on sparse recovery.The main work of this paper can be divided into three parts:Part ?: Sky wave OTHR echo analysis and sea clutter modeling.In this part,the effects of ionospheric interference,active interference and sea clutter on sky-wave OTHR echoes are discussed.Subsequently,the causes of sea clutter,the factors affecting sea clutter and the mathematical and statistical characteristics of sea clutter are studied.At the end of this chapter,the K-distributed clutter is taken as an example to simulate the sea clutter.Part ?: Sky wave OTHR conventional sea clutter suppression algorithm research.Three commonly used sea clutter suppression algorithms are discussed,including subspace decomposition,azimuth spread clutter suppression and space-time adaptive processing.The eigenvalue of the clutter covariance matrix is projected to the clutter subspace by subspace decomposition method.The signal components in the clutter subspace are all sea clutter components,and there is no useful signal part.The clutter suppression can be achieved by removing this component.The azimuth spread clutter suppression algorithm removes the target signal component by constructing a blocking matrix,estimates the clutter signal in the main lobe by using the clutter signal obtained by the side lobe,and subtracts the estimated clutter component from the main lobe by using the results of digital beamforming of the main beam,thus realizing the cancellation of the clutter in the main lobe.Space-time adaptive processing makes full use of the information in the angle-Doppler domain of clutter,and makes full use of the information in the spatial domain and Doppler domain to realize the optimal STAP algorithm in theory.Part ?: improved space-time adaptive algorithm.It includes local joint space-time adaptive processing,space-time multi-beam processing and space-time adaptive processing based on sparse recovery.In the conventional clutter suppression algorithm,the theoretically optimal STAP algorithm is discussed.But the computational complexity of the optimal STAP algorithm is large,so the dimension reduction of the STAP algorithm is also discussed in the follow-up.The local joint space-time adaptive processing of dimension reduction and the space-time multi-beam method with less computational complexity are discussed respectively.Due to the inherent sparsity of sea clutter space-time power spectrum,STAP algorithm based on sparse recovery is introduced into sky-wave OTHR.Finally,the clutter suppression effect is evaluated by increasing the signal-to-interference-noise ratio(SINR). |
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