Medium PRF Performance Analysis

A discussion of various types of x-band airborne radars is presented together with their systematic development through the years to the present time. Starting with simple, low pulse-repetition frequency (PRF) radars for measuring radar-target range, airborne radar development proceeded with more sophisticated high PRF Doppler radars where radar-target range and range rate were measured simultaneously. The use of Doppler (frequency) in signal processing allowed the separation of moving from nonmoving targets (ground), enabling the detection of moving targets in the presence of ground clutter. More recent developments in waveform generation and selection has resulted in the development of medium PRF radars, whereby a greater degree of tactical flexibility in target detection is achieved by combining the desirable features of both low and high PRF radars. Part of the available literature gives an overview, together with a specific example of the design and performance of an airborne medium PRF radar. Here, however, the systematic evolution of these radars is emphasized and the necessary theoretical background is developed for their performance calculations. Modern day airborne radars may be equipped with all three modes of operation, low, medium, and high PRF, allowing the operator to utilize the mode best suited for the tactical encounter. Low PRF and high PRF radars have been described elsewhere and are given here primarily for the sake of completeness and for the necessary background for developing medium PRF radar equations. They are also needed for developing the reasons why medium PRF radars came into being.     

Signal-to-Noise Ratio Calculations in Pulse and Pulse Doppler Radars

In low pulse-repetition frequency (PRF) pulse radars, signal-to-noise ratio (SNR) is usually calculated on a per pulse basis and this value is then multiplied by the number of pulses integrated to obtain the SNR for a given duration of target illumination. In high PRF pulse Doppler radars, SNR is usually calculated by using the centerline power of the transmitted signal spectrum as the target return power because the centerline is kept in the receiver and returns of the PRF lines are notched out [1]. We show here that both methods of SNR calculations are entirely equivalent for matched transmit-receive radar systems.     

A comprehensive survey on the methods of angle of attack measurement and estimation in UAVs

Angle of Attack (AOA) is a crucial parameter which directly affects the aerodynamic forces of an aircraft. The measurement of AOA is required to ensure a safe flight within its designed flight envelop. This paper intends to summarise a comprehensive survey on the measurement techniques and estimation methods for AOA, specifically in Unmanned Aerial Vehicle (UAV) applications. In the case of UAVs, weight constraint plays a major role as far as sensor suites are concerned. This results in selecting a suitable estimation method to extract AOA using the available data from the autopilot. The most feasible and widely employed AOA measurement technique is by using the Multi-Hole Probes (MHPs). The MHP measures the AOA regarding the pressure variations between the ports. Due to the importance of MHP in AOA measurement, the calibration methods for the MHP are also included in this paper. This paper discusses the AOA measurement using virtual AOA sensors, their importance and the operation.     

Performance comparison of PRF schedules for medium PRF radar

Previous work has shown how evolutionary algorithms (EAs) are an effective tool in optimising the selection of pulse repetition frequency (PRF) values of medium PRF schedules in an airborne fire control radar (FCR) application requiring target data in three PRFs. The optimisation is driven by the requirement to minimise range/Doppler blindness whilst maintaining full decodability. In this paper we detail work in which the optimisation process is applied to design novel short medium PRF schedules requiring target data in just two PRFs. The paper reports on the testing of a variety of near-optimum schedules to compare their blindness, decoding, and ghosting performances. The results show that in many situations, the 2 of N schedules are a practical alternative to conventional 3 of N processing.     

Medium PRF set selection using evolutionary algorithms

This paper presents a new and novel method of selecting multiple pulse repetition frequency (PRF) sets for use in medium PRF pulsed-Doppler radars. Evolutionary algorithms are used to minimise the blind areas in the range/Doppler space. The evolutionary algorithm allows optimal solutions to be generated quickly, far faster than with exhaustive searches, and is fully automatic, unlike existing techniques. The evolved solutions compare very favorably against the results of both an exhaustive search and existing published PRF set selection methods. This evolutionary approach to generation of PRF sets is a major advance in medium PRF radar design.     

Medium PRF radar PRF selection using evolutionary algorithms

Evolutionary algorithms are applied to the optimization of pulse repetition frequency (PRF), for both eight-and nine PRFs, in medium PRF radar while considering the detailed effects of sidelobe clutter and many other technical factors. The algorithm presented also ensures that all the solutions produced are fully decodable and have no blind velocities. The evolutionary algorithm was able to identify near-optimum PRF sets for a realistic radar system with only a modest computational effort.     

Doppler ambiguity resolution using multiple PRF

An algorithm for velocity ambiguity resolution in coherent pulsed Doppler radar using multiple pulse repetition frequencies (PRF) is presented. It relies on the choice of particular values for the PRFs. The folded frequency of the target signal is obtained by averaging the folded frequency estimates for each PRF, and a quasi maximum likelihood criterion is maximized for ambiguity order estimation. The fast implementation of this nonambiguous estimation procedure is based on the fast Fourier transform (FFT), The proposed waveform allows full exploitation of any (even) number of PRFs, which appears to be important for estimation improvement. The effects of the waveform parameters and the folded frequency estimation variance on the performance of the ambiguity order estimation procedure are evaluated theoretically and through computer simulations. Mean square error (MSE) curves are given to assess the Doppler frequency estimation accuracy. Finally, the new method is compared with a classical technique and the implementation of the algorithm in a clutter environment is addressed.     

Properties of Staggered PRF Radar Spectral Components

Expressions for moving target indicator (MTI) improvement factor limitation due to pulse repetition frequency (PRF) staggering and loss of target detectability for various values of Doppler frequency in the passband are presented. It is also shown that the product of variance of stagger periods and clutter variance is an important parameter determining the performance of a staggered PRF MTI radar.     

Mitigating Range Ambiguity in High PRF Radar using Inter-Pulse Binary Coding

The paper proposes a way to increase the energy within a coherent processing interval (CPI) using more pulses instead of longer pulses. Long coded pulses result in masking targets at close range and poor Doppler tolerance. Increasing the number of pulses implies high pulse repetition frequency (PRF), which suffers from range ambiguity and target folding. These drawbacks of a high PRF can be mitigated by inter-pulse coding. The approach suggested here should be attractive for close and mid range applications of radar, ground penetrating radar, ultrasound imaging, and more.     

Pulse Repetition Frequency Discriminator with Complete Harmonic Suppression

A method for selective pulse-train discrimination based on pulse repetition frequency (PRF) is presented. The technique permits acceptance of a pulse train with a desired PRF while rejecting all others, including those with pulse rates harmonically related to the desired one.     

Constrained Improvement MTI Radar Processors

A new class of staggered PRF MTI radar processors is developed in this paper. These processors are constrained to achieve a specified value of MTI improvement and, subject to this constraint, minimize variations in processor response as a function of target Doppler frequency. The selection of both filter weights and PRF stagger sequences is discussed and a number of representative designs are presented.     

弹载合成孔径雷达自适应重频分析与设计

弹载合成孔径雷达(Synthetic Aperture Radar,SAR)的目标距离、视线角由于高速逼近目标而快速变化,这导致传统的固定脉冲重复频率(Pulse Repetition Frequency,PRF)(简称重频)波形难以兼顾弹载SAR雷达在成像各方面的约束条件,故需要根据当前弹体运动和弹目关系变化情况实时计算重频。详细分析了影响重频选择的各项因素,包括避免距离模糊、方位模糊、高度杂波、发射遮挡影响及SAR成像分辨率、系统相参性要求等影响因素,并设计了自适应重频计算的工作流程。某SAR雷达系统实验表明,该设计能够在实际飞行弹道条件下根据实际弹目关系自适应调整脉冲重复频率,从而更好地实现SAR雷达系统的工作性能,有效解决了固定重频波形不能适应弹载SAR工作条件的难题。     

Design of Weighting Functions Under Peak Amplitude and Effective Radiated Voltage Constraints

The problem of designing optimum weighting functions subject to peak amplitude and effective radiated voltage constraints occurs in the design of active phased arrays and in spectrum control of high-pulse-repetition frequency (PRF) pulsed Doppler transmitters. Algorithms for generating such weighting functions are developed using a least-squares technique. Numerical results are presented which show that these algorithms yield substantial performance improvement over textbook weighting functions in favorable circumstances.     

大型飞机迎角/过载限制器设计

根据典型大型飞机迎角/过载限制器的特点,提出了两种采用积分控制来减小静态误差的大型飞机迎角/过载保护方法,即最大值切换逻辑和加权过渡逻辑方法,深入研究了如何设计迎角/过载限制器参数,探讨了采用以上两种不同方法来进行迎角与过载限制器切换的优劣,系统仿真结果证明了所设计的限制器的有效性。该项研究为我国未来大型飞机如何设计迎角/过载限制器提供了部分理论依据。     

飞机迎角的校准及测量

本文首先叙述了迎角传感器空中校准的必要性和常用校准方法,简要介绍了各种常用空中校准方法的原理及其优缺点。实践表明利用静姿态角法阳参数辨识法相结合进行迎角空中校准,在一定范围内是切实可行的。但在大迎角及失速飞行中因非线性带来的一些问题大大增加了校准的难度。本文针对这一事实提出了用直接测定姿态角加计算航迹角间接确定飞机迎角的方法,解决了Y—7飞机失速试飞中的迎角测量问题。实践表明,该方法可以推广用于一般民用飞机的迎角测量。     

飞机大攻角空间机动气动力建模研究

 准确的气动力模型是大攻角机动仿真与控制等飞行动力学问题研究的基础。针对纵向运动建立的非线性非定常气动力微分方程模型扩展应用于一般空间运动形式,在分析大攻角非定常流动机理和试验数据的基础上,发展了一种多自由度非线性非定常气动力数学模型,并探讨了模型辨识和参数估计问题。用全机构形的大攻角动态风洞试验数据对模型进行的验证表明所发展的气动力模型能够描述大攻角气动力主要的非线性和非定常特性。     

结合视线方向运动补偿的滑动聚束SAR子孔径成像算法

滑动聚束合成孔径雷达(SAR)是一种新兴的成像模式,既可以提高方位向分辨率又能够扩展成像范围。其数据处理时需要考虑两个关键问题:一是系统脉冲重复频率(PRF)不足,方位向信号发生混叠;二是合成孔径长度的增加使运动误差的影响更为突出,运动补偿(MOCO)精度要求提高。基于子孔径技术,提出了一种改进的高分辨率成像算法。划分子孔径克服了PRF不足的问题;子孔径数据处理采用结合视线(LOS)方向运动补偿的Omega-K算法,实现更高精度的运动补偿,提高了聚焦质量。最终的方位向分辨率达到0.1 m,具有实际工程应用价值。点目标仿真和实测数据处理验证了算法的有效性。     

部分三代战机“迎角悬挂”现象的机理分析与仿真计算

参考有关风洞试验结果,探讨了部分三代战机"迎角悬挂"现象的机理,对比分析了不同机型"迎角悬挂"的特点。并以某型三代战机为例,根据其纵向气动和操稳特性曲线及飞控系统特性,利用时域动态响应的方法,仿真计算了该型机大迎角下"迎角悬挂"的有关现象,计算结果与其技术资料的叙述相符。分析与计算表明,大迎角下"迎角悬挂"现象,是由该类型三代战机气动布局决定的一个共性问题;同时,"迎角悬挂"现象的特点又与具体机型的气动布局特点以及重心位置等因素有关。     

Parameter study on lateral moments of banked wings in ground effect

Ground effect dramatically improves the performance of the Wing-in-Ground(WIG)vehicles near the ground. However, making coordinate turnings in ground effect zone may raise complexity in flight control. The Angle of Attack(AOA) and height are believed to be important factors. To find the impact of these factors, three straight rectangular wings with different aspect ratios are simulated via CFD approach. The results show that in normal situations, the rolling moment tends to level the wing but le...     

Effect of an end plate on surface pressure distributions of two swept wings

A series of wind tunnel tests was conducted to examine how an end plate affects the pressure distributions of two wings with leading edge(LE) sweep angles of 23° and 40°. All the experiments were carried out at a midchord Reynolds number of 8×10~5, covering an angle of attack(AOA) range from -2° to 14°. Static pressure distribution measurements were acquired over the upper surfaces of the wings along three chordwise rows and one spanwise direction at the wing quarter-chord line. The results of the tests confirm that at a particular AOA, increasing the sweep angle causes a noticeable decrease in the upper-surface suction pressure. Furthermore, as the sweep angle increases, the development of a laminar separation bubble near the LEs of the wings takes place at higher AOAs. On the other hand, spanwise pressure measurements show that increasing the wing sweep angle results in forming a stronger vortex on the quarter-chord line which has lower sensitivity to AOA variation and remains substantially attached to the wing surface for higher AOAs than that can be achieved in the case of a lower sweep angle. In addition, data obtained indicate that installing an end plate further reinforces the spanwise flow over the wing surface, thus affecting the pressure distribution.