Research On Ocean Clutter's Characteristics Of Bistatic High-Frequency Ground Wave Radar

For a High-Frequency (HF) ground wave radar, ocean clutter is the main background for ship to be detected and forms a blind zone on Doppler spectrum. To detect ships in this background, Bragg lines must be detected first or some clutter suppressing methods must be adopted to separate ships from Bragg lines. So multidimensional characteristics of ocean clutter are the basis for target and clutter detection. Bistatic ocean clutter has distinguishing qualities from monostatic because it is affected by many factors such as bistatic angle, ocean current, wind direction landform and bistatic layout and others. This means bistatic ocean clutter is more complicated in shape and brings more difficulties in targets detection than monostatic. In this paper the properties of bistatic ocean clutter are investigated by theoretical models and then the conclusions derived by these models are verified by experimental data from the bistatic HF radar. At the same time, also based on these data, multidimensional characteristics of bistatic ocean clutter are analyzed and the methods used to suppress bistatic ocean clutter are discussed. All these analytical results provide helpful theoretical basis for clutter detection.The spread of bistatic ocean clutter spectrum is quantitively analyzed firstly and the spatial distribution of Bragg lines'width is presented. For bistatic HF radar with low range and angle resolution, the bistatic angle over a clutter cell is a variable other than a constant, this will cause the spread of ocean clutter. Traditionally the sprectrum spread caused by bistatic angle's deviation is always illustrated by range resolution and angle resolution seperately, namely one by one. But the bistatic angle's deviation over a clutter cell is in fact determined by both of them simultaneously. So the conclusions derived from the traditional methods always are not consistent with the measured data of bistatic radar. By calculating the deviation of bistatic angle over a clutter cell, the relationship between the Bragg lines'width and the location of clutter cell—namely the spatial distribution of Bragg lines'width, is presented. All these analysis indicate that the most significant spreading area is located at the vicinity of the transmitter and near the baseline and on the Range-Doppler (RD) spectrum the bistatic Bragg lines are two bands skewing inward toward to 0 Hz and gradually spreading. This kind of spread will worsen the spectrm and make targets detection more difficultly. To evaluate the effects of the worsened spectrum, the energy distributionof ocean clutter spectrum (namely morphological characteristics) must be investigated.Usually the coss sections of ocean surface can reveal the morphological characteristics of ocean clutter spectrum. But all the cross sections models of ocean surface presented in pubulished documents are derived from narrow receiving antenna beamwidth in which the bistatic angle is assumed as a constant. Spatial distribution of Bragg line's with shows that if the bistatic angle's deviations are ignored just like what the models of narrow beam does, the spectrum spread could not be revealed. In this paper, the bistatic angle is processed as a variable (namely broad beam). Based on Gill's narrow beam cross sections models of ocean surface, the broad beam models are presented by fractionizing the broad beam into narrow beams. The simulation results show that the broad beam models can reveal the spectrum spread chracteristics above mentioned.The analysis of the measured bistatic sea-echo data proves that the spatial distribution of Bragg lines'width and the characteristics of broad beam models are consistent with the measured data, it means that the sprectrum spread analysis is reasonable and the broad beam models are valid. It also means that the broad models presented here are proved be superior to narrow beam models in revealing the real bistatic ocean clutter spectrum shape.Some factors can not be taken into account in the theoretical models, such as ocean current, wind shear and landform near the layout of the bistatic radar, so the effects of these factors on the clutter spectrum are analyzed by analyzing the measured data. Based on these the multidimensional characteristics of bistatic ocean clutter are obtained.Theoretical analysis and measured data show that bistatic ocean clutter has distinguishing properties with monostatic. Some of them, such as Bragg line's spread and Bragg peaks split, asymmetry of positive and negative Bragg line's shifts and morphological characteristics and so on, make the clutter detection fail, because usually the detections rely on the Bragg lines'predicted value. This makes Bragg lines frequently detected and tracked as targets. To solve this problem, a method suppressing bistaic ocean clutter is presented based on Clean algorithm and the characteristics of bistatic ocean clutter. Besides, MUSIC subspace method's validity of suppressing bistatic ocean clutter is also discussed. Then a joint processing of Clean and MUSIC clutter suppression technique is presented. The processing results by these methods show that they are valid in suppressing bistatic ocean clutter.All these analysis of the multidimensional characteristics of bistatic ocean clutter provide a helpful basis for clutter detection and suppression. And the methods presented to suppress bistatic ocean clutter are proved to be valid and these techniques show very good potential to be used in practice.