Doppler Spectral Moments of Radar Scattering from Turbulent Ionized Wakes

The Doppler frequency shift, and spectral spread, of the radar scattering return from turbulent underdense ionized wakes are explicitly related here to the wake mean velocity, and to the wake turbulent velocity fluctuations and mean velocity gradients, respectively, via spectral moments of the matched-filter receiver response.     

Resonant Cavity Measurements of Ionized Wakes

A transmission resonant cavity technique, which is suitable for making measurements of electron line densities and collision frequencies in the ionized wakes of hypervelocity projectiles, is described. With this method electron density measurements can be made over six orders of magnitude. Resonant cavity design requirements and limitations of the method are discussed. Typical data from measurements behind projectiles traveling at speeds up to 6.5 km/s are given.     

Angular Spectra in Radar Applications

Recent advances in the art of estimating spectral densities have led to speculation that these same techniques could be used to improve angular resolution in radar applications. An improvement in angular resulution would be particularly helpful in sepingrating low angle returns from their surface reflected images. Despite the duality between time and space, however, it turns out that the low angle radar problem is different from the usual spectral estimation problem in a rather fundamental way. The unfortunate result is that the improved spectral estimation techniques are of little, if any, value in solving the low angle tracking problems.     

The Shape of Doppler Spectra from Precipitation

From October 1982 through May 1983 an extensive weather clutter registration program was executed near the Dutch coast. Coherent echo series of 2 s were obtained from a cluster of adjacent antenna pencil beams every 10 or 15 min., mainly between 16:00 and 08:30 h and on the weekends. The beam cluster was pointed toward the intensity maximum of the clutter volume. The radar operated at 5650 MHz. Spectra with 10 Hz Doppler resolution have been computed by averaging over 19 discrete Fourier transforms of overlapping and tapered subseries of 200 echo vectors. To quantify the deviation from a Gaussian shape a spectral variability is defined which is computed for every estimated spectrum. It is found that the deviation from Gaussian is considerable in about one-fourth of the spectra. A selection of "typical worst case" spectra is presented.     

高超声速尾迹雷达散射湍流场研究

为给高超声速再入尾迹亚密湍流雷达散射分析提供所需的脉动背景场参数 ,本文提出计算非平衡再入湍流尾迹脉动等离子体场的理论方法。在研究高超声速尾迹流动特征的基础上 ,推导、使用包括化学组份浓度脉动强度的k ε g湍流模型 ,用以封闭高超声速尾迹雷诺平均控制方程 ,并用全隐式有限差分求解。以M∞ =2 1 .2 6、Re∞D =1 .33× 1 0 6的小钝锥体流动为例 ,得到的结果说明 :流场的流向和径向参数分布合理 ;在转捩点后较近距离内湍流脉动影响较大 ,随着向下游流动脉动影响迅速减弱 ;本文计算尾迹湍流脉动等离子体场的方法是可行的     

Doppler Radar Tracking of Hostile Fire

Two fixed parameters define a two-dimensional projectile track relative to a radar: an arrival angle, and the expected distance of closest approach (miss distance). Both parameters can be obtained explicitly, the angle from the history of the phase difference between two elements of an interferometer antenna, and the miss distance from the history of the Doppler frequency.     

Clutter Spectra of Low PRF AMTI Pulse-Doppler Radar

Approximate expressions are derived for the video clutter spectra in the receiver of a low pulse repetition frequency (PRF), airborne moving target indicator (AMTI), pulse-Doppler radar for both step-scanning and continuous-scanning antennas. The receiver is assumed to process the received waveform with a clutter-tracking oscillator and a window function is employed to obtain short-term spectra. Except for the broadening effects of the window function, it is shown that the clutter spectrum can be simply related to the antenna voltage-gain pattern. It is further shown, in the scanning antenna case, that the combined spectral broadening due to platform motion and antenna scanning cannot be assumed to be the result of the convolution of the separate effects unless the antenna gain pattern has a Gaussian shape. The approximate clutter expressions are illustrated by examples and are shown to agree well with the results of computer calculations.     

Doppler Radar for Impact Drag Measurements

A Doppler radar tracking system has been used successfully to measure impact drag coefficients for several water-entry configurations. Hemisphere-cylinder and cone-cylinder models were launched vertically into a tank of water at velocities between 100 and 200 feet per second. These launchings were evaluation tests for a system to be used in a new facility at the Naval Ordnance Laboratory?the Hydroballistics Tank. Planned launchings in that facility will be at velocities up to 3000 feet per second. Knowledge of the drag coefficient profile (CD versus depth of penetration) is important in the design of high-velocity water-entry weapons.     

Comments on "Angular Spectra in Radar Applications"

In [1], we proposed using "data adaptive" spectrum estimation techniques (e.g., maximum likelihood and maximum entropy) for radar problems such as separating low angle returns from ground reflections. W. White suggests [2] that these techniques will be of little value due to the correlation between the various received signals. In this rebuttal, we present additional conceptual arguments, simulation results and field data which support the utility of the techniques set forth in [1].     

Performance of Doppler Radar in the Presence of Random Fading

This paper deals with the performance of pulse Doppler radar in the presence of random fading. The behavior of this radar is studied as a statistical problem to bring out the limiting bounds of the ambiguity diagram and the nature of variance with respect to the Doppler frequency. The performance of the radar insofar as the first sidelobe is concerned, is shown to be better in the presence of fading than in the normal case. In a particular case where 75 pulses out of an aggregate of 250 pulses are missing, the first sidelobe level is 20.0 dB down from the main lobe with a probability of 23 percent.     

FM Laser Noise Effects on Optical Doppler Radar Systems

A detailed analytical study is made of the effect of FM noise on a laser carrier frequency which is used in a Doppler radar. Both long-term drift and short-term FM noise are considered. The case of high modulation index of the noise is permitted by the theory. Forward as well as slanted beams are examined. Curves have been calculated for each case to allow rapid estimates of the bandwidth requirements to accomodate the laser noise. This, in turn, will give the resolution limit of the radar caused by that noise. A summary of the results is given.     

Radar/Sonar Signal Design for Bounded Doppler Shifts

In many detection and estimation problems, Doppler frequency shifts are bounded. For clutter or multipath that is uniformly distributed in range and symmetrically distributed in Doppler shift relative to the signal, detectability of a point target or a communication signal is improved by minimizing the weighted volume of the magnitude-squared autoambiguity function. When clutter Doppler shifts are bounded, this volume is in a strip containing the range axis on the range-Doppler plane. For scattering function estimation, e.g., for weather radar, Doppler flow meters, and distributed target classifiers, it is again relevant to minimize ambiguity volume in a strip. Strip volume is minimized by using a pulse train, but such a signal has unacceptably large range sidelobes for most applications. Other waveforms that have relatively small sidelobe level within a strip on the range-Doppler plane, as well as small ambiguity volume in the strip, are obtained. The waveforms are composed of pulse pairs that are phase modulated with Golay complementary codes.     

Adaptive Radar Detection in Doubly Nonstationary Autoregressive Doppler Spread Clutter

The problem of adaptive radar detection in clutter which is nonstationary both in slow and fast time is addressed. Nonstationarity within a coherent processing interval (CPI) often precludes target detection because of the masking induced by Doppler spreading of the clutter. Across range bins (i.e., fast time), nonstationarity severely limits the amount of training data available to estimate the noise covariance matrix required for adaptive detection. Such difficult clutter conditions are not uncommon in complex multipath propagation conditions where path lengths can change abruptly in dynamic scenarios. To mitigate nonstationary Doppler spread clutter, an approximation to the generalized likelihood ratio test (GLRT) detector is presented wherein the CPI from the hypothesized target range is used for both clutter estimation and target detection. To overcome the lack of training data, a modified time-varying autoregressive (TVAR) model is assumed for the clutter return. In particular, maximum likelihood (ML) estimates of the TVAR parameters, computed from a single snapshot of data, are used in a GLRT for detecting stationary targets in possibly abruptly nonstationary clutter. The GLRT is compared with three alternative methods including a conceptually simpler ad hoc approach based on extrapolation of quasi-stationary data segments. Detection performance is assessed using simulated targets in both synthetically-generated and real radar clutter. Results suggest the proposed GLRT with TVAR clutter modeling can provide between 5–8 dB improvement in signal-to-clutter plus noise ratio (SCNR) when compared with the conventional methods.     

Rotating Blades Radio Interference in a Helicopter-Borne CW Doppler Radar

Radio interference generated in a helicopter-borne continuous wave (CW) Doppler radar system due to the rotating blades is analyzed. This problem has been previously treated for the case of pulse Doppler radar systems with very narrow (near zero) beamwidth. In this case the strong interference component returning directly from the blades (with no ground reflection) need not be considered as it reaches the receiver when it is still blinded. In the case of a CW Doppler radar, however, this interference component must be included. Numerical calculations show that the total blade interference power level, dominated by the direct component, is higher than that of the direct ground clutter in the radar clutter region. It decreases approximately as (f - fo)-4 in the radar clear region. It stays, however, well above the thermal noise level which might cause false alarm and degrade the radar performance.     

Doppler Radar Error Equations for Damped Inertial Navigation System Analysis

The use of Doppler radar measurements to provide velocity damping for an aircraft inertial navigation system is considered. Three different Doppler antenna configurations are examined: two-axis stabilized, azimuth stabilized, and data stabilized antennas. A general reference velocity error equation is presented and appropriately evaluated for each of the Doppler configurations. Specific elements of the error equations are examined and physically interpreted for both local-level and space-stable inertial systems. Detailed examination of the interaction of Doppler radar and inertial navigation velocity error mechanisms is provided.     

FM/AM Noise Test Set for a Pulsed Doppler Radar

A new test method to measure the amplitude noise and phase noise in both CW and pulsed CW signals of a Ku-band pulsed Doppler radar is described. These noises are measured in a simulated environment of radar operation; thus the test results may give direct information to determine radar subclutter visibility. In comparison with the conventional noise test method, this new method not only gives more meaningful results but also can obtain results much faster in testing. Actual test system design is described by block diagrams and theoretical analysis. A method to determine approximate frequency jitter in a transmitter signal is also described.     

Stability Measurement Problems and Techniques for Operational Airborne Pulse Doppler Radar

In the deployment of pulse Doppler (PD) radar, determination of phase and amplitude stability is the most difficult measurement problem. Unique requirements are placed on pulse and carrier stability so that the radar can perform in strong clutter. Because of subclutter visibility and sensitivity specifications, coherent noise, which is insignificant for noncoherent pulse radars, becomes extremely important. In solving the measurement problem, special support equipment was developed which is considered to have reached such a degree of refinement that it is probably one of the most technically advanced pieces of field test equipment supporting any operational radar. This paper discusses stability requirements, sources of instability, and the combination of techniques selected for verification of compliance of the PD radar with the stability requirements. The results of a program to develop special field support equipment to satisfy the measurement requirements are emphasized. Results of field experience and the special training required of military field personnel to enable them to effectively use this relatively complex support equipment are discussed.     

基于遗传算法的多部测速雷达布站优化研究

通过部署于不同地点的多部无源多普勒测速雷达,可以对有辐射信号源的机动飞行目标进行跟踪测量,并且可以对目标位置和速度信息进行最佳估计。本文探讨了遗传算法在测速雷达布站优化中的使用方法,分析了误差传播矩阵,建立了简易目标函数,利用遗传算法对信标体制下的多普勒测速单站的布站几何进行了优化。