Clutter Suppression Properties of Weighted Pulse Trains

A common but troublesome requirement on radar sensors is the detection of a target in the interference from undesired scatterers, or clutter. Systems with coherent processing of pulse trains are uniquely suited for the purpose because, with pulse trains, it is possible to concentrate the receiver output for particular values of Doppler and thus suppress the clutter by Doppler filtering. This paper discusses to what degree the effectiveness of the method can be enhanced by tapering, or weighting, of the pulse amplitudes. The general results are illustrated by computer-plotted response functions for weighted pulse trains. The clutter suppression efficiency of weighting is calculated both for unilateral weighting in the receiver and for bilateral weighting in both receiver and transmitter. The significance of additional phase weighting is discussed and the results for pure amplitude weighting are compared with publishedwork on phase and amplitude weighting.     

宽带扩频阵列信号处理技术研究

提出了一种基于时域分析的复合加权宽带阵列信号处理新方法,该方法通过群时延、相位和幅度复合加权处理,可以避免复杂的频域聚焦算法,最后给出了处理方法的详细算法和实现框图。     

MODIFIED WEIGHTED GRADIENT PROJECTION METHOD AND RESEARCH TO THE KINEMATIC CONTROL OF REDUNDANT MANIPULATORS

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Noise Prediction for Rate Multiplier and Binary Adder Frequency Synthesizers

The theory for design of a rate multiplier or a binary adder frequency synthesizer for a specified level of noise attenuation is developed. Examples are presented to demonstrate the design procedure and the pattern of the pulse train output. The phase jitter associated with each pulse is characterized and illustrated in the time domain. The maximum absolute phase deviation is used as a measure of the phase modulation amplitude of the pulse train. Computer simulation resulted in simple equations approximating the amplitude of worst case spectral components.     

Another Source of VOR Omni Error

In the vicinity of reflecting objects, a VOR receiver picks up both the direct and reflected signals. This summation adds both amplitude and phase modulation to the signal that were not present at the transmitter. The error due to this phase modulation is fairly evident since the desired directional information is transmitted as a relative phase. What is not so evident is the fact that the amplitude modulation can also produce a phase error. This happens when asymmetrical filters are used to process the amplitude-modulated signal. The asymmetrical filter converts some of the amplitude-modulated sidebands into phase modulation which is recorded as a direct phase error.     

A Technique for Improving the Clutter Performance of Coherent Pulse Train Signals

The clutter performance of a radar using coherent pulse train signls depends upon the phase and amplitude weighting used on transmit and receive. This paper describes an iterative technique to determine better weightings. Each iteration improves the signal-to-interference ratio, where the interference is the sum of the mean square noise voltage and the mean square clutter voltage at the time of peak signal. Two examples are presented to illustrate the method and to demonstrate that in many cases the procedure will yield weightings that may be used in a system where the transmitter permits no amplitude weighting.     

Optimum Signals for Radar

The research reported herein deals with the general problem of the selection of radar waveforms. The investigation is specifically concerned with the synthesis of radar signals which are optimum in the sense that they are characterized by ambiguity surfaces minimized over certain predetermined regions of the ambiguity plane. The weighted ambiguity surface is utilized as the weighted error criterion. This error criterion is mathematically tractable and pertinent to radar system performance but is not unduly restrictive as some orientation parameters are left unspecified for subsequent cost or penalty function analysis. The signal optimization is approached by variational techniques augmented by equality and inequality constraints, for example, limiting the amount of bandwidth or frequency modulation to be less than some system requirement. Several examples are presented demonstrating the optimization techniques and providing a minimum error for the stated problem. It is shown that for any given type of amplitude modulation of the radar signal, the variance or dispersion of the ambiguity surface is not decreased for any type of phase modulation added. The optimum signal for an elliptical weighting function is derived for several cases. The minimum error is shown to depend upon the constraints and the unspecified orientation parameters and, for one case, on the second moment of the signal.     

Interferences in Phase-Locked Loops

An analysis of the behavior of a second-order phase-locked loop is presented when an unwanted signal is added to the useful signal. Both signals are sinusoidal and unmodulated, and the analysis is made in the absence of additive noise. When the loop remains locked on the useful signal, a parasitic signal exists at the phase detector output. This signal produces a parasitic phase modulation of the VCO and a static phase error in the loop. The parasitic signal amplitude, the parasitic phase modulation index, and the static phase error are calculated. A necessary condition for the loop to remain in lock is derived. When the loop is initially unlocked, locking can occur either on the useful signal or on the unwanted signal, depending on the amplitude ratio and the frequency difference of the two signals. A formula allowing one to compute the pull-in time is obtained. When the loop locks on the useful signal, acquisition can be slower or faster than in the absence of an unwanted signal. The same phenomenon is observed when the loop locks on the unwanted signal.     

Ambiguity Functions for Quadratic Phase-Coded Pulse Trains

The matched filter ambiguity function is presented for a burst waveform composed of repeated subbursts, each one of which consists of N pulses in which the phase is varied quadratically from pulse to pulse. The resulting ambiguity function exhibits small residual ambiguities along the delay axis separated by the reciprocal of the pulse repetition frequency (PRF). A cross-ambiguity function is derived which reduces these ambiguities to zero amplitude. A third cross-ambiguity function is presented for a receiver matched to a generalized Hamming weighted repeated quadratic burst. The location in the delay/Doppler plane of the waveform ambiguities for these waveforms is compared with that of an uncoded pulse burst.     

Spectral Analysis of ELT Signals for SARSAT

Search and rescue satellite-aided tracking (SARSAT) relays the emergency locator transmitter (ELT) signals of distressed aircraft to an earth station for spectral analysis. Of considerable importance are the characteristics of the spectrum of the ELT signal since the probability of locating the downed aircraft is closely related to the quality of the ELT signal spectrum itself. In this paper, it is shown that the spectrum can be adversely affected by a number of factors including the phase and frequency characteristics of the carrier and their interaction with the amplitude modulation. Two new models are proposed which greatly reduce the self-generated interference produced by ELT units presently being used.     

Some Results on Digital Chirp

Generating chirp waveforms by means of phase coding yields a simple, cost-effective mechanization. The coding process, however, introduces phase errors whose effect must be included in the design. An approximate analysis is presented, valid for moderate to high compression ratios, which allows error effects on compressed pulse amplitude and sidelobes to be calculated in a simple manner. The anaylsis provides criteria for selecting the coding bit width (sample rate), weighting network bandwidth, and phase-coder quantization interval and transition times. Weighting functions for maximizing the signal-to-noise ratio (SNR) or for producing desired close-in sidelobe performance are derived, as is an exact expression for the transmitted spectrum. Numerical results are presented for Gaussian and the maximum-SNR weighting. The results indicate that performance will be satisfactory for many applications.     

暂态信号形成的参量阵声场特性

暂态参量阵技术容易获得低频宽带信号,是诸多声呐系统广泛使用的信号模式。在众多描述非线性声波 传播的理论模型中,KZK方程能够准确地描述有限振幅声波传播的衍射、吸收及非线性效应,是描述参量阵非线性 声场最为精确的方程之一。文章从KZK方程的时域求解出发,对常见暂态信号自解调形成的参量阵进行分析。 理论研究表明,暂态信号参量阵的声场特性主要与信号包络的载波、上升沿有直接关系。高斯包络脉冲的上升沿 越陡峭,相同轴向位置的低频分量幅值越大,相同峰值功率前提下的能量转换效率越高；当被调制载波的中心频率 一致时,正线性调频信号调制得到的参量阵声压幅值要略高于负线性调频信号。文章可为暂态信号参量阵的辐射 波形选择提供初步的理论指导。     

A Semi-Empirical Approach to the PLL Threshold

This correspondence considers the response of the PLL near threshold to an input consisting of a modulated carrier and white, Gaussian noise. For high input signal-to-noise power ratios ?, the output noise power is Gaussian with a parabolic spectrum. As ? is decreased, the PLL tends to lose lock which gives rise to impulses or ?spikes? in the output with a resulting white power spectrum. The additional output noise due to these ?spikes? causes a threshold in the output signal-to-noise ratio. Unfortunately the loss of lock rate in the PLL depends on the modulation as well as the noise power. A semiempirical approximate expression for the loss of lock rate as a function of the noise and sinusoidal frequency modulation is presented and is used to determine the optimum design procedure for PLL's to demodulate FM signals of varying modulation indexes, ?.     

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.     

Effects of finite weight resolution and calibration errors on theperformance of adaptive array antennas

Adaptive antennas are now used to increase the spectral efficiency in mobile telecommunication systems. A model of the received carrier-to-interference plus noise ratio (CINR) in the adaptive antenna beamformer output is derived, assuming that the weighting units are implemented in hardware, The finite resolution of weights and calibration is shown to reduce the CINR. When hardware weights are used, the phase or amplitude step size in the weights can be so large that it affects the maximum achievable CINR. It is shown how these errors makes the interfering signals “leak” through the beamformer and we show how the output CINR is dependent on power of the input signals. The derived model is extended to include the limited dynamic range of the receivers, by using a simulation model. The theoretical and simulated results are compared with measurements on an adaptive array antenna testbed receiver, designed for the GSM-1800 system. The theoretical model was used to find the performance limiting part in the testbed as the 1 dB resolution in the weight magnitude. Furthermore, the derived models are used in illustrative examples and can be used for system designers to balance the phase and magnitude resolution and the calibration requirements of future adaptive array antennas     

High Power Radar Implementation of Coherent Waveforms

The radar use of coherent burst waveforms to obtain clutter suppression is summarized and problems arising from the high power implementation of such waveforms are discussed. These problems arise from the nonlinear nature of the typical high power radar transmitter and result in loss of subpulse-to-subpulse amplitude and phase accuracies, causing clutter suppression degradation. adaptive control loop used to measure transmission errors and provide continuous updating to minimize such errors is proposed. Residual transmission errors resulting via use of the control loop are calculated and shown to have an insignificant effect upon the clutter suppression properties of the coherent waveform. Experimental verification of control loop performance is presented.     

Thinned spectrum ultrawideband waveforms using stepped-frequencypolyphase codes

An ultrawideband (UWB) radar can interfere with external RF sources because of the mutual occupancy of the same frequency band. A stepped-frequency polyphase code (SFPC) waveform is proposed as a generic UWB waveform whose interference with the external RF sources is significantly reduced. The subpulses of the individual stepped-frequency (SF) pulses are phase coded using small phase perturbations. This results in a waveform which places nulls at the frequency locations of the external RF sources. Because the phase perturbations are small, a mismatched filter which uses the unperturbed pulses (no phase modulation) as a reference signal results in a simple receiver design and a small mismatch loss on receive. Furthermore, the proposed methodology also has application to narrowband or wideband radars     

噪声调制连续波雷达信号波形射频隐身特性

雷达信号波形的射频(RF)隐身性能是雷达系统能否适应现代战场环境的重要因素,雷达射频隐身信号波形设计是现代雷达系统设计中的重要课题.首先,在介绍随机噪声信号雷达原理的基础上,基于Schleher截获因子阐述了噪声调制连续波雷达信号波形的射频隐身特性.然后,分析了高斯噪声相位和频率调制连续波雷达输出自相关函数和高斯噪声相...     

用于雷达波形的无损压缩编码

讨论了用算术编码实现波形数据的无损压缩。对波形数据进行无损压缩时 ,如果把每个可能的样本值看作一个符号 ,符号表会非常庞大 ,导致算法不可行。基于这种原因 ,本文用一个符号表示一定范围内波形数据 ,而不是单个波形数据 ,然后在此基础上设计了一种编码方案。先是无损线性预测 ,去除了波形数据之间的相关性。并假定产生的差值序列具有白功率谱 ,幅度服从高斯分布 ,第二步是算术编码。最后给出了对雷达回波数据进行压缩的实验结果。