VSAR: a high resolution radar system for ocean imaging

The velocity synthetic aperture radar (VSAR) is a conceptual synthetic aperture radar (SAR)-based sensor system for high resolution ocean imaging. The VSAR utilizes data collected by a multielement SAR system, to extract information not only about the radar reflectivity of the observed area, but also about the radial velocity of the scatterers in each pixel. This is accomplished by making use of the phase information contained in multiple SAR images, and not just the magnitude information as in conventional SAR. Using this velocity information, the VSAR attempts to compensate for the velocity distortion inherent in conventional SAR and to reconstruct the ocean reflectivity. We present the basic theory of the VSAR system and its performance. We also provide an analysis of the VSAR imaging mechanism for a statistical model of the radar returns, designed to capture the effects of speckle and of resolution degradation due to the decorrelation of the radar returns     

OS-CFAR thresholding in decentralized radar systems

In a decentralized detection scheme, several sensors perform a binary (hard) decision and send the resulting data to a fusion center for the final decision. If each local decision has a constant false alarm rate (CFAR), the final decision is ensured to be CFAR. We consider the case that each local decision is a threshold decision, and the threshold is proportional, through a suitable multiplier, to a linear combination of order statistics (OS) from a reference set (a generalization of the concept of OS thresholding). We address the following problem: given the fusion rule and the relevant system parameters, select each threshold multiplier and the coefficients of each linear combination so as to maximize the overall probability of detection for constrained probability of false alarm. By a Lagrangian maximization approach, we obtain a general solution to this problem and closed-form solutions for the AND and OR fusion logics. A performance assessment is carried on, showing a global superiority of the OR fusion rule in terms of detection probability (for operating conditions matching the design assumptions) and of robustness (when these do not match). We also investigate the effect of the hard quantization performed at the local sensors, by comparing the said performance to those achievable by the same fusion rule in the limiting case of no quantization     

Multiparametric importance sampling for simulation of radar systems

The performances of the importance sampling (IS) techniques are improved by using multiparametric distortions of the input random processes. The analysis of different constant false-alarm rate (CFAR) algorithms confirms the usefulness of this method. The potential of this new approach is fully exploited if optimization techniques are used to obtain the optimum distortions and to avoid bias in the estimates     

A technique for enhanced slant range resolution for SAR systems inknowledge-based environment

Synthetic Aperture Radar (SAR) is an airborne (or spaceborne) radar mapping technique for generating high resolution maps of surface target areas including terrain. High resolution is achieved by coherently combining the returns from a number of radar transmissions. The resolution of the images is determined by the parameters of the emissions, with more data giving greater resolution. A requirement of the Microwave Radar Division's SAR radar is to provide classification of targets. This paper presents a technique for enhancing slant range resolution in SAR images by dithering the carrier centre frequency of the transmitted signal. The procedure controls the radar waveforms so they will optimally perform the classification function, rather than provide an image of best quality. It is shown that a Knowledge-Based engineering approach to determining the waveform of the radar gives considerably improved performance as a classifier of targets (of large radar cross-section), even though the corresponding image is degraded     

基于规则的机位附近滑行行为建模研究

航班量快速增长提高了机场的机位利用密度,机位附近的滑行过程更加复杂,容易引发航班延误,必须优化航班滑行过程,减少机坪区滑行冲突.通过详细分析机坪区及其附近区域的滑行规则,归纳出实用的知识库,建立起知识库模型,再利用Java+MySql平台进行实现,最后以厦门高崎机场厦门航空公司的专有机坪和厦航的实际航班数据为仿真实例,进行系统仿真.通过比较仿真结果和机坪调度人员的实际操作,可知仿真相似度高.该研究结果是可行、有效的,能够提供机坪调度的实时方案,有利于减轻机坪调度人员的工作负荷,同时该结果也可以作为调度问题的预测等相关问题研究的参考.     

Large area solar cells for future space power systems

Space silicon solar cell technology has matured to the extent that large-area planar silicon cells can be fabricated in sizes up to 8 cm×8 cm with efficiencies up to approximately 15%. In order to achieve substantially higher efficiencies, cells based on GaAs are required. It is shown that, subject to certain boundary conditions, the efficiency of GaAs/Ge cells can reach 24% when used in the dual-junction configuration or approximately 19.5% if the Ge substrate is passive. The electrooptical properties of these cells are reviewed, and prospects for achieving these efficiency goals are presented. Experimental performance data are given     

An interpolation algorithm to improve range estimation for thelinear frequency modulated radar

Radar with the linear frequency-modulated signal may range the target. Its range resolution is decided by the signal frequency bandwidth. This paper proposes an interpolation algorithm based on fast Fourier transform to improve the accuracy of range estimation without increasing the frequency bandwidth. The algorithm application in practice is described. The simulation result of estimation accuracy is also given     

An approach for parameter estimation of combined CPPM and LFM radar signal

In this paper, the problem of parameter estimation of the combined radar signal adopting chaotic pulse position modulation (CPPM) and linear frequency modulation (LFM), which can be widely used in electronic countermeasures, is addressed. An approach is proposed to estimate the initial frequency and chirp rate of the combined signal by exploiting the second-order cyclostationarity of the intra-pulse signal. In addition, under the condition of the equal pulse width, the pulse repetition interval (PRI) of the combined signal is predicted using the low-order Volterra adaptive filter. Simulations demonstrate that the proposed cyclic autocorrelation Hough transform (CHT) algorithm is theoretically tolerant to additive white Gaussian noise. When the value of signal noise to ratio (SNR) is less than 4 dB, it can still estimate the intra-pulse parameters well. When SNR = 3 dB, a good prediction of the PRI sequence can be achieved by the Volterra adaptive filter algorithm, even only 100 training samples.     

An extended chirp scaling algorithm for spaceborne sliding spotlight synthetic aperture radar imaging

A system impulse response with low sidelobes is critical in synthetic aperture radar（SAR） images because sidelobes contribute to noise and interfere with nearby scatterers. However,the conventional tricks of sidelobe suppression are unable to be exactly applied to the case of spaceborne sliding spotlight SAR due to great azimuth shifts in both time and frequency domains. In this paper, an extended chirp scaling algorithm is presented for spaceborne sliding spotlight SAR data imaging. The proposed algorithm firstly uses the spectral analysis（SPECAN） technique to avoid the azimuth spectrum folding effect and then employs the chirp scaling（CS） algorithm to achieve data focusing, i.e., the so-called two-step approach. To suppress the sidelobe level, an efficient strategy for the azimuth spectral weighting which only involves matrix multiplications and short fast Fourier transformations（FFTs） is proposed, which is a post-process executed on the focused SAR image and particularly simple to be implemented. The SAR image processed by the proposed extended CS algorithm is very precise and perfectly phase-preserving. In the end, computer simulation results verify the analysis and confirm the validity of the proposed algorithm.     

An experiment with an S-band radar using pulse compression andrange sidelobe suppression for meteorological measurements

A major technology barrier to the application of pulse compression for the meteorological functions required by a next generation ATC radar is range/time sidelobes which mask and corrupt observations of weak phenomena occurring near areas of strong extended meteorological scatterers. Techniques for suppressing range sidelobes are well known but without prior knowledge of the scattering medium's velocity distribution their performance degrades rapidly in the presence of Doppler. Recent investigations have presented a “doppler tolerant” range sidelobe suppression technique. The thrust of the work described herein is the extension of previous simulations to actual transmitted dispersed/coded waveforms using the S-band surveillance radar located at Rome Laboratory Surveillance Facility. The objectives of the experiment are: 1) to extend the verification of the simulation of the Doppler tolerant technique; and 2) to demonstrate that the radar transmitter, waveform generator, and receiver imperfections do not significantly degrade resolution, performance or reliability of meteorological spectral moment estimates     

Structures for radar detection in compound Gaussian clutter

The problem of coherent radar target detection in a background of non-Gaussian clutter modeled by a compound Gaussian distribution is studied here. We show how the likelihood ratio may be recast into an estimator-correlator form that shows that an essential feature of the optimal detector is to compute an optimum estimate of the reciprocal of the unknown random local power level. We then proceed to show that the optimal detector may be recast into yet another form, namely a matched filter compared with a data-dependent threshold. With these reformulations of the optimal detector, the problem of obtaining suboptimal detectors may be systematically studied by either approximating the likelihood ratio directly, utilizing a suboptimal estimate in the estimator-correlator structure or utilizing a suboptimal function to model the data-dependent threshold in the matched filter interpretation. Each of these approaches is studied to obtain suboptimal detectors. The results indicate that for processing small numbers of pulses, a suboptimal detector that utilizes information about the nature of the non-Gaussian clutter can be implemented to obtain quasi-optimal performance. As the number of pulses to be processed increases, a suboptimal detector that does not require information about the specific nature of the non-Gaussian clutter may be implemented to obtain quasi-optimal performance     

末制导雷达目标选择能力及其计算方法

目标选择能力是末制导雷达的一个效能指标,阐述了在定义该效能指标的基础上,给出了定量计算该指标的计算方法.     

High energy density regenerative fuel cell systems for terrestrialapplications

Regenerative Fuel Cell System (RFCS) technology for energy storage has been a NASA power system concept for many years. Compared to battery-based energy storage systems, RFCS has received relatively little attention or resources for development because the energy density and electrical efficiency were not sufficiently attractive relative to advanced battery systems. Even today, RFCS remains at a very low technology readiness level (TRL of about 2 indicating feasibility has been demonstrated). Commercial development of the Proton Exchange Membrane (PEM) fuel cells for automobiles and other terrestrial applications and improvements in lightweight pressure vessel design to reduce weight and improve performance make possible a high energy density RFCS energy storage system. The results from this study of a lightweight RFCS energy storage system for a remotely piloted, solar-powered, high altitude aircraft indicate an energy density up to 790 wh/kg with electrical efficiency of 53.4% is attainable. Such an energy storage system would allow a solar-powered aircraft to carry hundreds of kilograms of payload and remain in flight indefinitely for use in atmospheric research, Earth observation, resource mapping, and telecommunications. Future developments in the areas of hydrogen and oxygen storage, pressure vessel design, higher temperature and higher pressure fuel cell operation, unitized regenerative fuel cells, and commercial development of fuel cell technology will improve both the energy density and electrical efficiency of the RFCS     

Rank detector preprocessor for glint reduction in a tracking radar

A rank detector is used to detect instantaneous received power fades in a tracking radar. On detection of a fade, censorship of the angular position measurement is implemented in a Kalman tracking filter. It is shown that this technique can typically give a 15% angular tracking improvement on highly dynamic targets     

一种特种阀流量特性计算的经验公式迭代方法

通过分析特种流量调节阀(简称特种阀)的结构特点,建立该特种阀的流量特性数学模型,对其流量特性进行研究,提出一种利用经验公式求解流出系数的快速迭代计算方法。采用该经验公式迭代方法,计算特种阀的流量系数和气体质量流量,并与发动机高空模拟试验数据进行对比分析,验证迭代方法的有效性。对比结果显示,该迭代方法计算的质量流量与实测值具有很好的吻合度,该迭代方法可用于特种阀的工作流量特性计算。     

Integer ambiguity resolution in GPS for spinning spacecrafts

A procedure to compute the integer ambiguity problem when a GPS receiver is used in a multiple antenna configuration attached to a rotating spacecraft is presented. The method is applied to a simulation of an experimental satellite which uses the GPS receiver for attitude determination     

Tracking considerations in selection of radar waveform for rangeand range-rate measurements

The conventional approach for tracking system design is to treat the detection and tracking subsystems as completely independent units. However, the two subsystems can be designed jointly to improve system (tracking) performance. It is known that different radar signal waveforms result in very different resolution cell shapes (for example, a rectangle versus an eccentric parallelogram) in the range/range-rate space, and that there are corresponding differences in overall tracking performance. We develop a framework for the analysis of this performance. An imperfect detection process, false alarms, target dynamics, and the matched filter sampling grid are all accounted for, using the Markov chain approach of Li and Bar-Shalom. The role of the grid is stressed, and it is seen that the measurement-extraction process from contiguous radar "hits" is very important. A number of conclusions are given, perhaps the most interesting of which is the corroboration in the new measurement space of Fitzgerald's result for delay-only (i.e., range) measurements, that a linear FM upsweep offers very good tracking performance     

空间自旋目标宽带雷达干涉三维成像方法

空间目标三维成像可为目标的特征提取、分类与识别提供重要依据。基于L型三天线干涉成像原理,提出了一种宽带雷达条件下空间自旋目标干涉三维成像方法。首先,分析了雷达发射线性调频(LFM)信号条件下,空间自旋目标在距离-慢时间平面上的成像特点,建立了基于距离-慢时间平面的空间自旋目标干涉三维成像模型;其次,针对建立的干涉三维成像模型中,不同散射点的回波在距离-慢时间平面上会相互交叠的问题,对回波曲线分离、交叉点处理以及一维距离旁瓣的影响等进行了讨论,并给出了解决方法,从而获得目标三维图像。与已有方法相比,该方法可有效克服单基雷达三维成像无法获得目标各散射点真实三维位置以及在双/多基雷达三维成像时多部雷达回波联合处理较困难的问题。最后,仿真实验结果验证了所提方法的有效性。     

Robust space-time adaptive processing for airborne radar in nonhomogeneous clutter environments

Space-time adaptive processing (STAP) holds tremendous potential for the new generation airborne surveillance radar, in which the phased array antennas and pulse Doppler processing mode are adopted. A new STAP approach using the multiple-beam and multiple Doppler channels is presented here for airborne phased array radar. The approach with space-time multiple-beam (STMB) architecture is robust to array errors and has very low system degrees of freedom (DOFs). Hence, it has low sample support requirement and it is very suitable for the practical planar phased array radar under nonhomogeneous clutter environments. Meanwhile, a new nonhomogeneous detector (NHD) based on the correlation dimension (CD) is also proposed here, which is used as an effective method to screen tracing data prior to detection processing. It can further improve the performance of the STAP approach in the severely nonhomogeneous clutter environments. Therefore, a scheme that incorporates the correlation dimension nonhomogeneity detector (CD-NHD) with the STMB is recommended, which we term CD-NHD-STMB. The experimental simulation results indicate that: 1) the STMB processor is robust to array element error and has high performance under nonhomogeneous clutter environments; 2) the CD-NHD is also effective on the nonhomogeneous clutter. As a result, the CD-NHD-STMB scheme is robust to array element error and nonhomogeneous clutter, and therefore available for airborne phased array radar applications.     

Fractal interpolation of radar signatures for detecting stationarytargets in ground clutter

The fundamentals of fractal geometry are reviewed, and its application to the millimeter-wave radar detection of stationary targets in a clutter background is described. First, high-range-resolution (HRR) profiles are used to determine the fractal interpolation functions needed to create fractal signatures. The fractal dimension is then determined for these signatures. On the basis of the value of the fractal dimension, the signature is declared to represent either a target of interest or clutter. The results of a CFAR (constant false alarm rate) simulation are presented to illustrate the performance of the method. They indicate that the fractal dimension feature used seems to be independent of amplitude. Thus, the fractal dimension information combined with traditional amplitude processing techniques will improve probabilities of detection