Reduced-rank STAP performance analysis

The space-time radar problem is well suited to the application of techniques that take advantage of the low-rank property of the space-time covariance matrix. It is shown that reduced-rank (RR) methods outperform full-rank space-time adaptive processing (STAP) when the space-time covariance matrix is estimated from a data set with limited support. The utility of RR methods is demonstrated by theoretical analysis, simulations and analysis of real data. It is shown that RR processing has two opposite effects on the performance: increased statistical stability which tends to improve performance, and introduction of a bias which lowers the signal-to-noise ratio (SNR). A method for evaluating the theoretical conditioned SNR for fixed RR transforms is also presented. It Is shown that while best performance is obtained using data-dependent transforms, the loss incurred by the application of fixed transforms (such as the discrete cosine transform) may be relatively small. The main advantage of fixed transforms is the availability of efficient computational procedures for their implementation. These findings suggest that RR methods could facilitate the development of practical, real-time STAP technology     

Eigenanalysis-based space-time adaptive radar: performance analysis

The space-time radar problem is well suited to the application of techniques that take advantage of the low rank property of the space-time covariance matrix. The performance of an eigenanalysis-based detector with respect to convergence rate and robustness to calibration errors is analyzed. Analytical expressions are developed for receiver operating curves when the clutter signal environment is assumed to be Gaussian. The curves are derived from the asymptotic expansion of the distribution of the principal components of the covariance matrix. Simulation results are provided to corroborate the theoretical analysis. Examples from the Mountain-Top dataset are used to illustrate the higher convergence rate and increased robustness of the eigenanalysis method.     

Angle of arrival estimation of coherent signals using an arraydoublet in motion

Shown here is how the estimation of signal parameters via relational invariance techniques (ESPRIT) algorithm may be used with a single pair of antennas in motion to estimate angles of arrival (AOA) for coherent signals. The approach exploits the Doppler frequency shifts caused by the doubler in motion. With this estimator, the number of signals that can be handled is not limited by the size of the array, as in the usual ESPRIT application, but by an adjustable parameter. A theoretical performance analysis of the estimator and typical examples showing the use of this estimator are given     

Exploiting GNSS signal structure to enhance observability

There are a number of different error sources, such as multipath and thermal noise, which corrupt satellite navigation waveforms from their theoretical structure. However, even under ideal conditions the broadcast signals have some degree of deformation as a result of the practical individual hardware implementation. For the most demanding users of satellite navigation, such as aircraft navigation and landing systems, it is important to characterize the nominal signal structure in order to detect minimal variations resulting from hardware-based errors. Thus far such precorrelation Global Navigation Satellite System (GNSS) signal quality monitoring has been performed through high gain antennas, which allow for raising the GNSS spectrum above the thermal noise floor and observing the structure of the signal directly at the front end output. This paper describes a new approach to achieve such observability based on signal processing techniques, such as dithering and averaging, which leverage the repetitive nature of the GNSS signal. The paper presents how these techniques can drastically improve the signal-to-noise ratio (SNR) in postprocessing, allowing for the direct analysis of GNSS signals using traditional front end designs and conventional antennas. Results are predicted using the appropriate theory and validated using data collected from the Global Positioning System (GPS).     

基于纠删码的遥测链路丢帧恢复技术

针对遥测系统在恶劣天气条件、航天器姿态调整、天线指向偏离、脉冲干扰等各种因素影响下出现的遥测信号闪断以至接收机丢帧问题,提出一种适用于闪断信道的纠删码技术,用于恢复丢失的数据帧.首先介绍LDPC（低密度奇偶校验）纠删码的编译码算法,然后构造一种以短码长的随机码为外码、IRA（不规则重复累积）LDPC码为内码的级联纠删码,最后给出纠删码的应用策略和实现纠删码所需的CCSDS（空间数据系统咨询委员会）协议扩展.通过仿真实验和分析,得到了LDPC纠删码和级联纠删码的纠删性能,得出级联纠删码与LDPC纠删码均可获得接近理想的纠删性能的结论,且级联纠删码的构造更为灵活,可适应不同的应用环境,对CCSDS协议进行适当扩展后即可支持纠删码在CCSDS遥测链路中的应用.     

Navigation system design using time-varying complementary filters

A new methodology for the design of navigation systems for autonomous vehicles is introduced. Using simple kinematic relationships, the problem of estimating the velocity and position of an autonomous vehicle is solved by resorting to special bilinear time-varying filters. These are the natural generalization of linear time-invariant complementary filters that are commonly used to properly merge sensor information available at low frequency with that available in the complementary region. Complementary filters lend themselves to frequency domain interpretations that provide valuable insight into the filtering design process. This work extends these properties to the time-varying setting by resorting to the theory of linear differential inclusions and by converting the problem of weighted filter performance analysis into that of determining the feasibility of a related set of linear matrix inequalities (LMIs). Using this set-up, the stability of the resulting filters as well as their "frequency-like" performance can be assessed using efficient numerical analysis tools that borrow from convex optimization techniques. The mathematical background that is required for complementary time-varying filter analysis and design is introduced. Its application to the design of a navigation system that estimates position and velocity of an autonomous vehicle by complementing position information available from GPS with the velocity information provided by a Doppler sonar system is described.     

Design Principles of MIMO Radar Detectors

This paper considers the problem of multiple-input multiple-output (MIMO) radars employing space-time coding (STC) to achieve diversity. To this end, after briefly outlining the model of the received echo, a suitable detection structure is derived, and its performance is expressed in closed form as a function of the clutter statistical properties and of the space-time code matrix. Interestingly, this receiver requires prior knowledge of the clutter covariance, but the detection threshold is functionally independent thereof. At the transmitter design stage, we give two criteria for code construction: the first is based on the classical Chernoff bound, the second is an information-theoretic criterion. Interestingly, the two criteria lead to the same condition for code optimality, which in turn specializes, under the assumption of uncorrelated clutter and square code matrix, in some well-known full-rate space-time codes. A thorough performance assessment is also given, so as to establish the optimum achievable performance for MIMO radar systems.     

Design, implementation and evaluation of parallel pipelined STAP onparallel computers

Performance results are presented for the design and implementation of parallel pipelined space-time adaptive processing (STAP) algorithms on parallel computers. In particular, the issues involved in parallelization, our approach to parallelization, and performance results on an Intel Paragon are described. The process of developing software for such an application on parallel computers when latency and throughput are both considered together is discussed and tradeoffs considered with respect to inter and intratask communication and data redistribution are presented. The results show that not only scalable performance was achieved for individual component tasks of STAP but linear speedups were obtained for the integrated task performance, both for latency as well as throughput. Results are presented for up to 236 compute nodes (limited by the machine size available to us). Another interesting observation made from the implementation results is that performance improvement due to the assignment of additional processors to one task can improve the performance of other tasks without any increase in the number of processors assigned to them. Normally, this cannot be predicted by theoretical analysis     

Performance analysis of radar antenna systems

If modern airborne radar systems are to function properly, the radar antenna radiation patterns must meet certain specifications. Until recently, most radar antennas were designed and tested in a clean antenna environment, i.e., there is no near field scattering from host structures or radome effects. However, these higher order effects are the matter of increasing concern with added performance demands in the ever-increasing jammer and clutter interference environments. We investigated the capabilities and limitations of currently available analysis techniques and computer codes for installed performance of airborne radar antenna systems. Then we developed an extended ray-optical technique that could predict total installed performance of airborne radar antenna systems, i.e., the near field scattering from aircraft structures and radome effects. The new analysis technique utilized a ray-tracing method in both airframe and radome simulation. Thus, it can efficiently predict the total installed performance of large radar antenna systems on an aircraft structure     

Optimizing computational efficiency and user convenience in plasma simulation codes

It is usually important to write plasma simulation codes in such a way that they execute efficiently and are convenient to use. I discuss here practical techniques to achieve this goal. Numerical algorithms must be well formulated and advantage taken of machine architecture in casting the algorithm into a high level language such as Fortran. The advantages of writing critical routines in Assembler are discussed. For large simulation codes, disks must often be used as a temporary store for working data. Efficient methods for doing this are presented. Codes must not only be well organized for ease of implementation and maintenance, but also for ease of use. Ways are suggested for packaging codes such that setup, batch production, restarting and diagnostic postprocessing is facilitated. Particular emphasis is placed on graphics postprocessors, since they must be used in real time with graphics terminals as well as with hardcopy devices.     

Partially Adaptive STAP using the FRACTA Algorithm

A partially adaptive space-time adaptive processor (STAP) utilizing the recently developed FRACTA algorithm is presented which significantly reduces the high computational complexity and large sample support requirements of fully adaptive STAP. Multi-window post-Doppler dimensionality reduction techniques are employed to transform the data prior to application of the FRACTA algorithm. The FRACTA algorithm is a reiterative censoring (RC) and detection algorithm which has been shown to provide excellent detection performance in nonhomogeneous interference environments. Two multi-window post-Doppler dimensionality reduction techniques are considered: PRI-staggered and adjacent-bin. The partially adaptive FRACTA algorithm is applied to the KASSPER I (Knowledge-Aided Sensor Signal Processing & Expert Reasoning) challenge datacube. The pulse repetition interval (PRI)-staggered approach with D=6 filters per Doppler bin is found to provide the best detection performance, outperforming the fully adaptive case while simultaneously reducing the runtime by a factor of ten. Using this implementation, partially adaptive FRACTA detects 197 out of 268 targets with one false alarm. The clairvoyant processor (the covariance matrix for each range cell is known) detects 198 targets with one false alarm. In addition, the partially adaptive FRACTA algorithm is shown to be resilient to jamming, and performs well for reduced sample support situations. When compared with partially adaptive STAP using traditional sliding window processing (SWP), the runtime of partially adaptive FRACTA is 14 times faster, and the detection performance is significantly increased (SWP detects 46 out of 268 targets with one false alarm).     

GPS多径信号的自适应滤波估计方法

 研究全球定位系统(GPS)多径信号估计的问题。通过分析自适应滤波器的原理,建立了数字中频信号处理的数学模型,提出一种用自适应滤波实现GPS多径幅度、码相位和载波相位估计的方法。该方法采用不同延迟的伪随机序列对信号进行解扩、解调和累加,得到了作为期望信号的系列自适应滤波相关值。对该方法与其他3种方法进行了理论上的分析比较,得出本方法具有信噪比高、自适应滤波性能好、带有码相位信息和不存在载波模糊度问题等优点。根据各种滤波器算法的特点和本应用的需求,给出了选用递归最小二乘算法实现的方法。通过计算机仿真,验证了提出的方法能够在14 dB的信噪比下,以1个采样间隔的时间延迟分辨率和0.005周的载波相位估计精度估计出GPS L1的多径信号。     

Performance bounds for hot and cold clutter mitigation

A new method is presented for describing the theoretical interference space-time covariance matrix that will be observed in an adaptive airborne radar system under specific topographical conditions. Both hot clutter that is induced by interfering sources and cold clutter that results from the radar transmitter are considered. This method incorporates phenomenology observed under site specific conditions as well as system effects such as array geometry, receiver filtering, and system bandwidth. Use of this formulation rather than sample data analyses that are generally employed enables one to infer performance bounds for site-specific, and thus generally, heterogeneous terrain that are tighter and therefore more meaningful than the thermal noise floor limit     

A system for obstacle detection during rotorcraft low altitudeflight

An airborne vehicle such as a rotorcraft must avoid obstacles like antennas, towers, poles, fences, tree branches, and wires strung across the flight path. Automatic detection of the obstacles and generation of appropriate guidance and control actions for the vehicle to avoid these obstacles would facilitate autonomous navigation. The requirements of an obstacle detection system for rotorcraft in low-altitude Nap-of-the-Earth (NOE) flight based on various rotorcraft motion constraints is analyzed here in detail. It is argued that an automated obstacle detection system for the rotorcraft scenario should include both passive and active sensors to be effective. Consequently, it introduces a maximally passive system which involves the use of passive sensors (TV, FLIR) as well as the selective use of an active (laser) sensor. The passive component is concerned with estimating range using optical flow-based motion analysis and binocular stereo. The optical flow-based motion analysis that is combined with on-board inertial navigation system (INS) to compute ranges to visible scene points is described. Experimental results obtained using land vehicle data illustrate the particular approach to motion analysis     

直线电机地铁车辆牵引系统国产化应用研究

为解决国内直线电机(LIM)轨道交通牵引传动系统关键技术及备品备件完全依赖进口、维修维护困难、车辆运营成本高的现状,研究开发了具有完全自主知识产权的大功率强迫风冷式LIM牵引变流器系统。针对LIM特殊结构引起的边端效应等问题对传统间接磁场定向矢量控制方式进行了优化。考虑到LIM次级时间常数发生变化对磁场定向的影响,导致转矩控制性能下降,提出了一种基于最小电流的参数在线辨识方法。所提方法计算简单、不依赖于电机其他参数,试验台测试结果证明了该算法的准确性和有效性。在地铁实际运营线路的试验结果表明,国产化牵引变流器各项性能满足车辆技术要求,而且效率等部分指标优于原车进口牵引系。     

一种基于角度-多普勒补偿的均匀圆形

采用均匀圆形相控阵天线的机载雷达杂波分布随距离变化而变化,各距离单元的杂波不再满足独立同分布的条件,造成统计型空时自适应处理(STAP)器性能下降。基于此,本文建立了均匀圆形天线机载雷达模型,对其杂波分布进行了分析,得出了空间角随阵元数非线性变化的特性造成其杂波距离维分布非均匀的结论。研究了一种均匀圆形天线机载雷达杂波抑制方法,该方法先通过修正的角度-多普勒补偿(MADC)预处理消除在杂波谱中心处的非均匀,再利用基于导数更新(DBU)技术进一步减小在其他方位杂波的非均匀程度。仿真结果表明了该方法的有效性。     

Aerospace launch vehicle control: an intelligent adaptive approach

A theoretical analysis of on-line autonomous intelligent adaptive tracking controller based on emotional learning model in mammalians brain (BELBIC) for aerospace launch vehicle is presented. The control algorithm is provided with some sensory inputs and reward signal, subsequently it autonomously seeks the proper control signal to be executed by actuators, thus eliminating tracking error without pre-knowledge of the plant dynamics. The algorithm is very robust and fast in adaptation with dynamical change in the plant, due to its on-line learning ability. Development and application of this algorithm for an aerospace launch vehicle during atmospheric flight in an experimental setting is presented to illustrate the performance of the control algorithm.     

Susceptibility of DS/FH,M, Binary DPSK to Partial and Full Band Barrage Jamming

Spread spectrum signaling schemes have been proposed to counter unfriendly, electrical jamming threats. In order to establish their effectiveness, such schemes must be analyzed. This work takes a step in this direction by developing the susceptibility equation, or equivalently, the probability of error, of a direct sequence/frequency hopped (DS/FH), binary differential phase-shift keying (DPSK) system when subjected to a barrage jamming signal. Specific system models are established for the receiving system as well as for the jamming signal and the spread spectrum techniques. Both partial and full band jamming strategies are considered. Graphical results are presented with the conclusions summarizing the spread spectrum effectiveness and the deficiencies of the FH processing gain definition.     

多重信号分类算法测向精度的仿真分析

作为空间谱估计理论体系中的标志性算法,多重信号分类算法从1979年提出后就一直是阵列信号处理领域的研究热点之一。针对复杂电磁环境中阵列测向的实际应用问题,从多信源、含噪宽带信号、入射角度、阵列间距这4个因素对多重信号分类算法的测向精度作了仿真分析,得出了一些结论。     

Smart flight control surfaces with microelectromechanical systems

Pitch, roll, and yaw moments can be developed by deflecting and changing the geometry of control surfaces. In this paper, smart flight control surfaces are designed using multi-node microelectromechanical systems (MEMS) to displace control surfaces and change the surface geometry. These MEMS augment translational motion microstructures (actuators-sensors), controlling/signal processing integrated circuits (ICs), radiating energy devices and antennas. The desired pitch, roll, and yaw moments are produced, drag can be reduced, and unsteady aerodynamic flows are controlled by smart flight control surfaces. That is, we achieve aerodynamic moment and active flow control capabilities. The major objective here is to report fundamental and applied research in design of smart flight control surfaces with MEMS-based actuator-sensor-IC arrays controlled by hierarchical distributed systems. We demonstrate the feasibility and effectiveness of the application of smart flight control surfaces for coordinated longitudinal and lateral vehicle control