【作者】 李博；

【导师】 袁业术；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2010， 博士

【摘要】 分布式多载舰地波超视距雷达,简记为DMS-SWOTHR (Distributed Multi-Ship based Surface Wave Over-The-Horizen Radar),是一个在岸基和单载舰地波超视距雷达基础上发展起来的、以海上舰队为装载平台的全新体制的地波超视距对海探测雷达系统。相对于前两种雷达,它功能更强大、性能更先进,结构也更趋复杂。这种雷达除具有舰载OTHR (Over-The-Horizen Radar)机动性强的特点之外,最突出的优点是整体雷达系统资源可根据探测需求、信号环境和目标类型进行灵活配置和重构,因此能够实现雷达系统资源对作战使命的最佳匹配和对探测任务的使用效率最大化。同时,这种体制的雷达又是一个高度复杂而又精密的多普勒分析与分辨系统,它是靠在多普勒域里鉴别雷达目标和背景杂波在运动特性上的微小差别来区分和检测目标的。因此,它既是一个功能强大的对海探测系统,同时也是一个极富挑战性的研究课题。目前在国际上,对这种新体制超视距雷达的研究还是一个空白。本文就是以这种新体制超视距对海雷达为研究背景,从建立分布式移动多基系统的雷达系统模型和推导分布式移动多基系统的目标特性和杂波特性等基础研究开始,对分布式移动多基系统中的目标与杂波特性、分布式移动多基系统的阵列重构与信号重构技术以及基于重构虚拟大孔径阵列的信号相参处理等重要技术进行了比较深入的研究。在详细推导了分布式移动多基系统的复杂动态几何关系、回波信号模型以及一阶Bragg海浪展宽谱表达式基础上,重点开展了一阶海杂波展宽谱中的舰船目标提取、基于分布式系统的虚拟大孔径阵列重构以及在重构大阵列上应用空间超分辨技术来改善系统角分辨能力等技术的研究。首先,针对一发多收DMS-SWOTHR的几何结构特点,分析了岸基窄波束条件下双基地SWOTHR的海杂波基本理论,阐述了一、二阶海杂波的产生机理及数学模型。海面对高频段电磁波能产生Bragg谐振散射,海浪回波中的一阶分量是由满足Bragg谐振散射条件的重力波导致的,谐振散射条件不仅与雷达工作频率有关而且是双基地角的函数;二阶海杂波的回波能量比一阶海杂波小很多,但其产生机理却复杂得多;有向波高谱对一、二阶海杂波谱的大小起着重要作用。继而,推导了DMS-SWOTHR中任意收、发站平台与运动点目标三者之间的动态几何模型;并以此为基础分析了一阶海杂波谱的展宽机理,给出了DMS-SWOTHR中一阶Bragg海浪回波谱展宽的数学模型。雷达平台运动时,雷达距离分辨单元内不同方向的一阶海杂波谱受到不同程度的调制,导致一阶Bragg峰被展宽;而收、发站平台之间不可避免的相对运动又引入了时变的多普勒频移,这使得在多普勒域里一阶海杂波展宽谱的结构变得异常复杂,进而导致在DMS-SWOTHR系统中检测舰船目标变得非常困难。影响展宽谱中舰船目标检测的主要干扰是由于雷达平台运动而扩散到目标所在多普勒单元的一阶海杂波,通过对雷达平台运动合成孔径效应的分析可知:由于雷达平台运动而映射到任一雷达分辨单元里的一阶海浪的空间方位是已知的;且从概率意义上讲,落入目标所在分辨单元的一阶海杂波的空间方位与目标方位在绝大多数情况下是不同的。这种特定的动态空间几何关系,为在展宽的一阶海浪谱中检测舰船目标提供了技术上的可能性,这也构成了本文的基础和出发点。而对于舰船目标与一阶展宽海浪处于同一方位的特殊情况,可通过改变雷达系统工作频率和雷达系统的运动状态(或雷达平台的相对位置或运动速度或运动方向)来解决。接着,推导了DMS-SWOTHR系统中雷达回波信号的数学表达式。从其相位表达式来看,无论是径向匀加速船目标、向心加速度较大的船目标,还是具有时变多普勒频移的一阶海杂波,其相位关系都是相同的;但其物理意义却不尽相同。若不考虑统计特性的影响,同一距离单元中不同方向的一阶海杂波相对幅度可由双基地雷达方程求得。虽然二阶海杂波也同样会受到平台运动的调制,但由于它本身就是方位向上的全谱,在DMS-SWOTHR系统中与在岸基OTHR中的谱线结构变化不大,仍为一连续谱。因此,依据对高频段海浪回波统计特性分析结果,每个距离-多普勒单元中的二阶海杂波谱和大气噪声,在统计意义上可统一看作是加性高斯噪声。综合以上信号模型分别给出了海杂波展宽谱中有无船目标情况下的仿真结果,这些数据将用于目标提取。然后,针对回波信号多普勒谱中时变与非时变信号分量并存的情况,本文提出了基于施密特正交化理论的空域零点自适应方法来对消一阶海杂波,从而实现了在展宽的一阶海杂波谱中的船目标检测。这样不但能首先检测出非时变目标信号分量,而且可以大大减少多分量干扰源在后续处理中产生的交叉项。基于乘积型高阶模糊函数(PHAF)的迭代算法被用来提取时变目标信号对应的径向运动参数。在应用迭代算法过程中,当估计出一个目标的所有相位参数后即将该目标从待分析信号中剔除,这样做的优点是能有效降低由交叉项引起的确定性噪声水平。仿真结果表明基于PHAF的相位参数估计方法存在信噪比门限,但高信噪比时的均方误差(MSE)曲线非常接近克拉美-罗下限(CRLB);另外,在检测方案的循环处理过程中,参数估计精度因误差传播而有所下降。最后,基于DMS-SWOTHR的特定系统结构,提出了面向雷达系统分辨单元的阵列重构方法。针对入射到不同载舰上均匀线阵(ULA)的同一信号源所在的距离单元和多普勒单元,分别给出了实现大阵列相参信号处理所要满足的充分条件。在条件满足情况下,每个接收阵可视为某一分布式阵列的均匀线性子阵(ULSA)。为实现基于子阵列的跨平台相参信号处理,本文引入了虚拟插值阵列(VIA)变换方法。针对DMS-SWOTHR系统中各子阵间的高稀疏性使现有VIA变换技术在整个观测域上产生较大插值误差的情况,提出了一种基于预估计的VIA变换方法。其基本思想是:假设至少一个ULSA的接收数据可用来定位距离-多普勒单元内的强相关源,并指定若干较窄且不相重合的角度区间的并集作为插值区,插值区仅覆盖方位预估值而不考虑整个非插值区。该方法不仅保证了插值区的变换精度,而且避免了同一分辨单元内的强干扰源入射到非插值区的情形。此外,该方法舍弃了噪声预白化处理,代之以适当增加用于前后向空间平滑(FBSS)处理的虚拟ULA的子阵个数。这样不但可以达到解相关的目的,而且压低了噪声基底。通过蒙特卡洛仿真,本文提出的方法应用在DMS-SWOTHR中在低信噪比条件下比单载舰阵列情形获得更高的目标方位分辨性能和估值精度。更多还原

【Abstract】 On the basis of both shore-based and shipborne surface wave over-the-horizon radar (SWOTHR), Distributed Multi-Ship based SWOTHR (DMS-SWOTHR) system is just beginning to develop, which employs more transmitters and receivers mounted on various ships in a battle group. Compared with the first two radar systems, DMS-SWOTHR is more advanced, but it has a much more complex configuration. Besides outstanding agility and maneuverability, DMS-SWOTHR can flexibly configure and reconstruct the entire system resource according to the detection requirement, the signal environment and the target type, and hence it can provide the optimal matching for the operational mission and the maximum using efficiency for the detection task. Also, DMS-SWOTHR is a highly complex and delicate Doppler resolution system that implements target detection and extraction by distinguishing the weak motion difference between the target and the background clutter in the Doppler domain. By far, the referable data of the researches on DMS-SWOTHR system are still absent, and this radar system has to be confronted with great challenges.Based on the above background, this dissertation begins with the fundamental researches on the distributed mobile radar system model and the characteristics of target and clutter, and then the further researches on the array and signal reconstruction techniques and the coherent signal processing techniques based on a reconstructed large array are made. After obtaining the dynamic geometry model, the received signal model and the Doppler-broadening expression of the first-order Bragg sea waves, this dissertation concentrates on the following problems both in theory and in simulation: how to extract the ships whose Doppler frequencies appear in the spreading domain of the first-order Bragg lines, how to perform the array reconstruction, and how to make the existing spatial super-resolution algorithms be applicable to the reconstructed large array.Firstly, in the DMS-SWOTHR system with a single transmitter and multiple receivers, since the receivers are mounted on different ships and separated from each other physically, this dissertation analyzes the basic theory of sea clutter for the shore-based narrowbeam bistatic SWOTHR, including the mechanisms of first- and second-order sea clutter and their mathematical models. According to the analysis, the sea surface is a type of special scatterer in high frequency bands. The first-order Bragg scattering is caused by gravity-waves with specific wavelength that is related to the radar operating frequency and the bistatic angle; the second-order sea clutter power is much weaker than the first-order power but its mechanism is much more complex; and the directional sea wave height spectrum has a remarkable effect on the first- and second-order sea clutter.Next, the dynamic geometry model of the transmitter platform, an arbitrary receiver platform and a moving target is derived. Based on the model, this dissertation analyzes the spreading mechanism of the first-order Bragg line in DMS-SWOTHR and presents a mathematical spreading model. The Doppler frequencies of sea echoes are simultaneously modulated by different radial velocity components projected from the radar platform motion, which results in the spreading of the first-order Bragg line; moreover, the time-varying Doppler frequencies are imparted due to the unavoidable motion difference between the transmitter and receiver platforms. Hence, the Doppler-broadening spectrum of the first-order sea clutter becomes very complex, which further weakens the ship target detection. The main interference for the detection of ship targets in the spreading domain is the first-order sea clutter from the direction different from the targets’azimuth angles but with the same Doppler frequency considering the platform motion. According to the synthetic aperture effect of radar platform motion, the azimuth of the first-order sea clutter falling into an arbitrary Doppler cell is known, and in a statistical sense, it is different from that of the target in the same cell. These conditions offer the technical possibilities for the target extraction. When the azimuth angles of the target and the first-order sea clutter are equal, changing the radar operating frequency or the motion of the radar system is effective.The above-mentioned geometrical relation is further employed to obtain the received signal model. As viewed from the phase expression after demodulation process and conventional range transform, the mathematical forms are the same for the ship target with a constant radial acceleration, the ship target with a relative centripetal acceleration, and the first-order sea clutter interference with a time-varying Doppler shift. Based on the bistatic radar equation, the relative amplitudes of the first-order sea clutter from different directions within the range cell are not equal when neglecting the statistical factor. Although the second-order sea clutter are also affected by the platform motion, the second-order spectrum, compared with that in the shore-based case, is still a continuum and changes indistinctively. Thus, the second-order sea clutter continuum and the atmospheric noise in each range-Doppler cell are both considered as the additive noise whose amplitude and phase are modeled by Rayleigh and uniform distributions, respectively. This dissertation gives the simulation results under different conditions based on the above signal models.Then, for the radar echoes containing time-varying and non-time-varying signal components in the Doppler domain, this dissertation proposes a scheme that is a recursive procedure for target extraction. The main idea is that the orthogonal weighting technique is performed to cancel the broadened first-order Bragg lines, so that not only the uniformly moving targets could be first detected due to little coherent integration loss (CIL) but also the cross-terms to be produced in the subsequent steps are greatly reduced in advance, and then the product high-order ambiguity function (PHAF) based spectra are obtained to estimate the corresponding radial motion parameters of the nonuniformly moving targets, respectively. In this scheme, an arbitrary target, once extracted fully, has to be removed for the purpose of suppressing the deterministic noise generated by the cross-terms. The simulation results shows that at low peak SNR, the PHAF-based method exhibits a threshold effect, but at high peak SNR, the performance is very close to the corresponding Cramer-Rao lower bound (CRLB). In addition, the estimation accuracy degrades because of the error propagation as the“peeling”algorithm proceeds.Finally, based on the special configuration of DMS-SWOTHR, this dissertation proposes a radar resolution cell oriented array reconstruction method. The coherence conditions for a given signal source are analyzed in both the range and Doppler domains, respectively, when impinging on multiple uniform linear arrays (ULAs) mounted on the DMS-WOTHR platforms, respectively. If the conditions are both satisfied, all the receiving arrays can be regarded as the uniform linear subarrays (ULSAs) of a distributed array. To implement the cross-platform coherent signal processing by using the array reconstruction technique to synthesize a larger receiving array, the virtual interpolated array (VIA) transform is introduced. The existing robust VIA transform techniques can cause unacceptable interpolation errors over the entire field of view because of high sparseness of the distributed array. For that, this dissertation proposes a preestimation-based VIA transform method by specifying a union of nonoverlapping narrow subsectors as the interpolated sector to cover only the source locations preestiamted roughly on an assumption that at least a single ULSA is available for localizing highly correlated sources within the range-Doppler cell. This method not only guarantees the interpolation precision in the interpolated sector but also avoids the situation of strong interference sources in the same cell impinging on the array outside the sector in multisource scenarios. In addition, it skips noise prewhitening and employs more subarrays of the virtual ULA for forward-backward spatial smoothing (FBSS) that plays a key role in noise floor reduction as well as correlated source decorrelation. Monte Carlo simulations show that the proposed method used in DMS-SWOTHR performs much better in both the azimuth resolution and estimation precision at low SNRs.更多还原