System aspects and transmission impairments of active phased arrays for satellite communications

The communications link and system aspects of active phased arrays that are used in multiple-beam satellite systems are assessed through measurements and analysis. Three link parameters are investigated and their effects on the overall carrier-to-interference ratio (CIR) are quantified. The first parameter is the intermodulation components that are generated at the nonlinear amplifier outputs and contribute to well-formed interference in the far-field radiation of the array. The second is the bit-error ratio (BER) degradation due to the multi-carrier operation of the active array. Measurement results are shown to demonstrate this effect. The third link parameter is the cochannel interference caused by frequency reuse in multiple-beam systems. The paper starts by reviewing early developments of phased arrays for multiple-beam satellite communications applications. A key component in these developments is the modular monolithic microwave integrated circuit (MMIC) beam-forming matrices that generate a number of simultaneous and independently digitally controlled beams     

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.     

Radar: The Cassini Titan Radar Mapper

The Cassini RADAR instrument is a multimode 13.8 GHz multiple-beam sensor that can operate as a synthetic-aperture radar (SAR) imager, altimeter, scatterometer, and radiometer. The principal objective of the RADAR is to map the surface of Titan. This will be done in the imaging, scatterometer, and radiometer modes. The RADAR altimeter data will provide information on relative elevations in selected areas. Surfaces of the Saturn’s icy satellites will be explored utilizing the RADAR radiometer and scatterometer modes. Saturn’s atmosphere and rings will be probed in the radiometer mode only. The instrument is a joint development by JPL/NASA and ASI. The RADAR design features significant autonomy and data compression capabilities. It is expected that the instrument will detect surfaces with backscatter coefficient as low as −40 dB.RADAR Team LeaderThis revised version was published online in July 2005 with a corrected cover date.     

Some Effects of Hard Limiting in Adaptive Antenna Systems

Adaptive antennas are often implemented with the Applebaum-Howells-type adaptive processor usually include a hard limiter between each antenna port and its correlation mixer, primarily for dynamic range compression. Brennan and Reed [3] analyzed the effects of hard limiting, and their conclusions suggest that it does not degrade the steady-state performance of the adaptive processor. Standard and hard-limited processors are compared and it is shown that when the two types of processor have the same sensitivity threshold, the hard-limited one can fail to provide sufficient interference cancellation when the correlation matrix of input signals has two or more eigenvalues of differing magnitudes. The consequence of hard limiting is that (depending on the processor design parameters) the larger of two or more signals can capture the hard limiter, allowing the smaller signals to pass through the processor essentially unattenuated. It is also shown that when a hard-limited processor is designed to provide the same cancellation as a standard one, it must have essentially as large a dynamic range as the standard, processor; therefore, it offers no advantage of dynamic range compression. Moreover, the hard-limited processor lacks a constant sensitivity threshold, which can be a desirable feature of a standard processor. Specific examples are presented for identical-element array antennas and for multiple-beam antennas.     

Improved DC/AC interface inverter for high-frequency spaceapplications

A DC/AC interface inverter for a high-frequency space power distribution system that uses dual asymmetrical resonant bridges is examined. Compared to the conventional approach in which two symmetrical bridges are used, the asymmetrical bridges arrangement offers better use of the installed kVA rating and efficiency of the inverter system. A simplified approach for modeling the inverter system in steady-state is presented, and a criteria to select the resonant components is given. A detailed simulation is performed for the following: a symmetrical inverter system with a resistive load, an asymmetrical inverter system with a resistive load, and an asymmetrical inverter system with the type-1A AC/DC converter load     

Instrumentation for balloon and rocket experiments

A number of plasma, particle and field detectors used on rocket investigations in and above the Earth's atmosphere are described. Emphasis is on magnetospheric and solar-interplanetary studies. A balloon-borne X-ray telescope system with 20 pointing accuracy is discussed. A PCM telemetry system used on both balloons and rockets to handle scientific data is described including a simple Doppler ranging system that gives location to 1.5 km. A system to reduce and analyze PCM data on the ground is discussed.     

基于FPGA的雷达信号模拟、采集与处理系统

介绍了基于FPGA设计并实现了一种雷达信号模拟、采集与处理的系统。系统主要由计算机和带有双路高速A/D和双路高速D/A的信号处理卡组成。信号处理卡以FPGA为核心控制与处理芯片,主要完成雷达信号模拟、雷达信号处理和雷达信号采集等功能,与计算机通讯使用USB2.0接口,采用数据抽取、坐标查表映射和DirectDraw等技术在计算机显示器上以P显和A显方式进行实时显示。该系统提供在线配置功能,用户可通过USB接口对FPGA程序进行配置或升级,无须专用配置芯片,简化了电路板设计,提高了系统的灵活性。     

直升机旋翼/发动机一体化仿真模型

 建立铰接式、叶片为刚性的非线性旋翼模型和发动机的非线性气动热力学模型以及简化的供油系统和传动机构模型，并将各子系统模型组合成旋翼／发动机一体化模型。在此基础上研究了一体化系统模型对操纵量改变的动态响应以及供油系统改变对系统的影响。     

一种高精度数据采集及控制系统设计

数据采集与控制系统是现代检测与控制技术的核心,工业控制领域对数据采集与控制系统的性能要求越来越高。设计了一种可拓展通道数目的高精度数据采集和控制系统,系统采用一主多从的模式,整个控制系统由CAN总线构成通讯网络,中央控制器负责整体调度各节点控制器的工作,节点控制器有3路高精度A／D和6路高精度D／A资源,负责各节点信号采集与控制。     

Pulse-Position Modulation Based on Energy Detection

A pulse-position modulation (PPM) system based on energy detection is described. The system is shown to be applicable to a communication system in which the arrival times of signals are unknown. The decision statistics are chisquare distributed (either central or noncentral), and recursive methods of computing probabilities of error are derived. A simple digital processor for implementing the system is also described.     

快速判断线性控制系统能控能观性的一种途径

介绍了判断线性控制系统能控性和能观性的一种方法,并提供了一种用 MATLAB 语言编写的快速准确判断线性控制系统能控能观性的软件。     

Design and implementation of a robust controller for a synchronousreluctance drive

A robust controller design for a synchronous reluctance drive system is presented. Based on a simplified model of the system, a robust position controller has been derived. A digital signal processor (DSP), TNO-320-C30, is used to implement the control algorithm. Furthermore, all the current, velocity, and position control loops are executed by the DSP. The system, as a result, is very flexible. Although the hardware circuit of the system is very simple, the synchronous reluctance drive system can accurately control a one-axis table. In addition, the system also has good transient response, load disturbance response, and tracking ability. Several experimental results validate the theoretical analysis     

Buried small objects detected by UWB GPR

A ground penetrating radar (GPR) using short-pulse is developed to detect small and shallow metal objects buried underground. A bistatic mode in which the GPR system uses separate transmitting and receiving antennas is applied. A modified fat dipole antenna is developed for the transmitting and receiving antennas. The prototype of the system is tested in the real environment and 2D visualization of raw data is achieved. We show that the developed system has a good ability to detect underground metal objects, and even small targets of several centimeters.     

270-VDC/hybrid 115-VAC electric power generating system technologydemonstrator

The approach, system architecture, instrumentation, and performance of a technology demonstrator system for the application of 270-VDC/hybrid 115-VAC electrical power generation and distribution to future military aircraft are examined. The design has focused on assembling a highly flexible test system. The loads are typical of those in a hybrid electrical system. The system consists of a single 3-phase, 115-VAC electrical generator whose output is rectified, filtered, and supplied through DC contactors to both a 270-VDC load bus and to the input of an inverter, which supplies a 400-Hz, 115-VAC load bus. A variable-frequency, 115-VAC bus is also available directly from the generator. A prototype variable-speed constant-frequency 120-kVA generator is used. The test facility is designed to provide loads to all power types capable of being produced by the system. The system has provided particularly good insight into various aspects of 270-VDC/115-VAC hybrid systems and their components     

Adaptive Resource Allocation in a Spatial Spectral Processor

A method for achieving an optimal allocation of fixed computational resources in a spatial/spectral filtering system is developed. Specifically, clock cycles per unit time for a given processor are to be optimally allocated. The system is a cascade of three subsystems: an adaptive beamformer, a bandpass filter, and a spectral whitener. Each subsystem has adjustable parameters which are related to required arithmetic operations. A gradient projection algorithm is used to find a best allocation according to specified criteria. Two simulation examples are presented to show how the algorithm operates and to indicate the kind of improvement that is possible in a system design.     

Performance Analysis of a Target Detection System Using Infrared Imagery

The automatic detection of targets in cluttered infrared imagery is considered. The environment for the problem is that of a "fire-and-forget" weapon, and the mission philosophy for such a weapon dictates that the weapon has to find one and only one target in the automatic detection phase. A detection system that meets this requirement is presented. The system uses techniques of image processing and pattern recognition, with the extension that ranking methods are used instead of thresholds to accommodate the requirement of finding one and only one target. A probability model of the system is developed to determine the system performance as a function of throughput and expressions derived for the probability that the object chosen by the system as the target is actually a target. In order to validate the theoretical results, the actual performance of the detection system on a database of 68 infrared images is determined and compared with the predicted performance of the system. It is shown that there is good correspondence between the empirical results and the theoretical performance.     

Design and implementation for a digital synchronous reluctancedrive

A systematic controller design for a synchronous reluctance drive system is presented. This controller consists of two parts: a forward-loop H^∞ controller to improve the transient response, and a load compensator to reduce the load disturbance. Based on a simplified model of the drive system, a control algorithm has been derived. Detailed analysis of the characteristics of the closed-loop system is presented. The effects of the parameter variations are also studied. A digital signal processor, TMS-320-C30, is used to implement the control algorithm. Both the speed control and the position control of the drive system can be implemented by using the proposed control method. Furthermore, all the control loops are executed by the digital signal processor. The system, as a result, is very flexible. The whole drive system performs well although its hardware is very simple. For speed control, the system can be operated at a speed as low as 1 r/min. For position control, the system can accurately control a one-axis table. In addition, the system also has good position tracking ability. Several experimental waveforms validate the simulated results     

Control of an aircraft electric fuel pump drive

The concept of designing a high-speed, permanent magnet, brushless DC motor aircraft fuel pump drive using a cycloconverter link is examined. A combination of sinusoidal and DC steady-state analysis is used to produce a simple model of the system. A closed-loop control system with an outer loop based on speed and an inner loop based on current is postulated wherein a proportional-plus-integral controller is placed in the forward path to assure minimum speed error. Gains are then set to assure that the eigenvalues of the linearized control system lie within the left half s-plane over the entire full range