On Conopulse Radar Theoretical Angle Tracking Accuracy

The Cramer-Rao analysis method is applied to a conopulse radar to determine a lower bound on the variance of angle tracking error. The analysis allows the calculation of the variance independent of the form of the signal processor attached to the antenna. Under reasonable assumptions it is found that angle error variance is larger than that of a similar monopulse system by a factor of 2.     

Mismatched Filtering of Sonar Signals

A replica correlator (matched filter) is an optimum processor for a receiver employing a pulse of continuous wave (CW) signal in a white Gaussian noise background. In an active sonar, however, when the target of interest has low Doppler shift and is embedded in a high reverberation background, this is not so. High sidelobes of the correlator frequency response pass a significant portion of the signal contained in the mainlobe of the reverberation spectrum. In order to reduce the sidelobes of the correlator output spectrum and at the same time keep the increase in its 3 dB bandwidth to a small amount, we propose lengthening of the replica of the transmitted signal and weighting it by a Kaiser window. It is demonstrated that by extending the weighted replica by 50 percent compared with the transmitted signal, it is possible to reduce the sidelobe levels to at least 40 dB below the mainlobe peak, with the concomitant increase of the 3 dB band-width by less than 5 percent. The degradation in signal-to-noise ratio (SNR) performance for such a ?mismatched? filter receiver with respect to the matched filter is less than 1.5 dB.     

Imperfect Carrier Recovery Effect on Filtered PSK Signals

Coherent demodulation of a PSK signal requires the generation of a local carrier phase reference. Methods are given to determine the detection loss caused by noisy phase recovery and its use in the coherent detection of filtered BPSK and QPSK signals. It is assumed that the phase noise can have a static part and a random component with a Tikhonov-type distribution. The static part is mostly due to offset frequency tracking of the PLL used to recover the carrier, while the random component is due to thermal noise present in the carrier recovery loop and is also due to the random nature of the phase modulation. It is shown that the probability of error of BPSK and QPSK can be expressed as a finite sum of a set of strictly alternating converging series when the number of ISI terms is finite. Upper and lower bounds on the probability of error have been derived when this number becomes infinite and we show how this error rate can be computed with any desired accuracy. Numerical results are presented for various values of static error and phase noise variance when the transmit and receive filters are 4-pole Butterworth filters. For filtered PSK signals and for a bit error rate of 10-6, our results show that the additional degradation in presentday receiver systems due to imperfect carrier recovery can be less than 0.1 dB for BPSK and less than 1 dB for QPSK.     

Multipath Angle Error Reduction Using Multiple-Target Methods

The usual methods of reducing multipath angle errors in monopulse tracking radar achieve only limited success because they do not attack the root of the problem. A more correct approach is to accept the multipath signal as a second target and utilize a two-target signal processor which angle tracks both wavefronts. The processor will decouple the return signals so that relatively interference-free data on both waves are obtained. In this paper a signal processor for separating signal from (N - 1) multipath components is developed. The processor is then specialized to the case of only one multipath signal and evaluated by a computer simulation. Data show that large improvements are possible as compared to the usual monopulse tracking system. In particular, the usual large bias errors at low elevation angles are eliminated. Tracking precision compares favorably with the theoretically best possible for two-target tracking systems.     

一种多径条件下的MLS接收机测角误差分析新方法

为了准确分析多径环境条件下微波着陆系统接收机测角误差,在深入分析微波着陆系统(MLS)测角原理的基础上,提出了一种基于扫描波束主瓣等效替代的误差分析方法。该方法通过把往返波束脉冲主瓣用高斯钟形替代,准确计算了接收的扫描脉冲-3dB门限前后沿误差,建立了多路径条件下统一实用的接收脉冲包络波峰位置偏移误差模型和锁住闸门测量误差模型。为验证模型准确性,以特定的多径环境条件为例,计算MLS接收机的测角误差,将计算结果与利用Mathias经典模型的计算结果进行对比,并通过计算机仿真进行验证。结果表明,采用主瓣替代方法能够较好地克服经典模型精确分析误差方面的使用不足和缺陷,且对多径条件下接收机测角精度的准确评定具有一定的实用价值。     

基于LAS码的星群多目标测控技术研究

针对星群多目标同时测控问题,基于星群轨道根数的时延特性和多普勒频移特性分析可得,星群多目标测控的上行链路遥控与测距信号可实现S CDMA（Synchronous Code Division Multiple Access,同步码分多址）,由于星间距离较小,下行链路遥测与测距信号满足QS CDMA（Quasi synchronous code division multiple access,准同步码分多址）.上行链路遥控和测距信号形式设计为PCM BPSK CDMA（Pulse Code Modulation Binary Phase Shift Keying Code Division Multiple Access,脉冲编码调制二进制相移键控码分多址）,上行链路信号采用Gold序列扩频;下行链路遥测和测距信号形式设计为PCM BPSK CDMA,根据总的时延差,提出下行链路采用基于等长脉冲间隔法构造的LAS（Large Area Synchronous,大区域同步）码扩频.结果表明：比特信噪比Eb/N0为10.5 dB时,遥控误码率为1＆#215;10 6;Eb/N0为9.6 dB时,遥测误码率为1×10^5——与达到相同误码性能的Gold序列相比有1 dB改善,因此,LAS码相比于Gold码能够获得更好的误码性能.     

一种在紧缩场进行捕获跟踪试验的方法

为满足不断增长的天基中继业务需求,航天器星间链路常使用大口径高增益天线,且配置自动捕获和跟踪设备,以完成对目标航天器的自动跟踪.随着天线口径的增大,对场地空间要求不断增大,地面验证试验变得愈加困难.针对越来越难以实现的捕获跟踪外场试验,提出一种在紧缩场进行捕获跟踪试验的方案,方案详细分析了在紧缩场进行试验的可行性.通过设置跟踪通道校相因子,很好地分离了角误差信号,获得良好的S曲线以及非常小的交叉耦合,从而验证了在紧缩场进行捕获跟踪试验是可行的.相对于传统的远场测试方法,该方法具有不受气候影响、费用低等优点.     

Leading Edge Estimation Errors

The leading edge estimator (LEE) of a pulse signal is defined as the instant at which a filtered version of the received noisy signal passes a preset threshold. A rigorous analysis for a rectangular pulse model of the signal results in an exact probability density function for the LEE, valid within the time interval of the leading edge of the filtered pulse. Possible occurrence of the threshold crossing outside of this interval is considered to be an anomalous estimate, since it leads to a gross error in comparison with the regular cases. It is found that the density function of the LEE error is asymmetrical and therefore biased, that the probability PA of anomalous estimation increases with the filter bandwidth, thus setting a well definable limit to the latter and that, for prespecified PA, the minimum bias and variance are proportional, respectively, to R-1 and R-2, minima being obtained by allowing for the largest bandwidth compatible with PA. On the other hand, for given bandwidth the variance decreases only as R-1. Here R is the signal-to-noise energy ratio. Results are presented in form of parameterized graphs.     

Simulation of the Canadian GOES Signal Processor

An international experiment is being conducted to assess the use of geostationary spacecraft to relay 406 MHz distress beacon signals. The development is described of a signal processor that will search a nominal 10 kHz portion of the 406 MHz band, detect anddemodulate a small number of beacons, and produce an alertmessage with low probability of message error. It is shown that thenominal composite link C/No is about 30 dB Hz. The proposedprocessor is described and simulation results are presented. Theresults indicate that the processor has a threshold of about26 dB · Hz, a message error rate of 2 percent or less, and a messagetransfer time of less than 10 min.     

Angle measurement in the presence of mainbeam interference

The problem of reducing interference impinging on an antenna array when the sources lie in the main beam is addressed. Adaptive antenna arrays are incorporated to form adapted sum and difference beams in which the interference signals are suppressed. Monopulse error curves are then obtained, providing the necessary distortion correction curves across the entire mainbeam tracking angle region. New Cramer-Rao (C-R) bounds on the angle estimation error are derived with generalized assumptions on the signal amplitude and phase. The bounds previously derived by others are valid under different conditions. With these generalized assumptions on the signal characteristics, a Monte Carlo simulation is performed, based on the estimation procedure presented, to determine the angle estimation error. These errors are compared with the C-R bounds. Good performance is shown for sufficient S/N₀ and angular separation between the target and the interference sources     

拉普拉斯白噪声下的分组Turbo码

目前,传统航空测控通信所采用的纠错码大多是建立在高斯信道基础上的。然而,航空测控环境中不可避免存在着多种尖锐的噪声,测控通信纠错码的可靠性能在非高斯信道中尚未得到充分的研究。分析了一类国际航空遥测的分组Turbo码（BTC）在拉普拉斯白噪声信道下的译码和性能。将传统Chase迭代译码算法引入到拉普拉斯白噪声信道中,建立相应的数学模型,同时,基于该数学模型设计了3种不同的译码接收器下的BTC译码方案。仿真结果验证了该数学模型的正确性与可行性,在误码率为10^-4时最佳译码方案相比于硬限幅接收机有3.7 dB的增益,相比原有的高斯信道下的接收机仅有0.6 dB的性能损失。     

比相单脉冲雷达测角与角闪烁研究

比相单脉冲雷达采用和差通道接收信号比相的方法测角。一般认为,雷达事实上并不是在测量目标的方向,而是在测量目标的角闪烁方向。从特殊的两点目标出发,分析了单点目标和复杂目标下比相单脉冲雷达和差通道方式测角的原理与内在异同,探讨了比相单脉冲雷达测角与角闪烁的关系,提出有关雷达测角的新的观点。     

Multiframe detector/tracker: optimal performance

We develop the optimal Bayes multiframe detector/tracker for rigid extended targets that move randomly in clutter. The performance of this optimal algorithm provides a bound on the performance of any other suboptimal detector/tracker. We determine by Monte Carlo simulations the optimal performance under a variety of scenarios including spatially correlated Gaussian clutter and non-Gaussian (K and Weibull) clutter. We show that, for similar tracking performance, the optimal Bayes tracker can achieve peak signal-to-noise ratio gains possibly larger than 10 dB over the commonly used combination of a spatial matched filter (spatial correlator) and a linearized Kalman-Bucy tracker. Simulations using real clutter data with a simulated target suggest similar performance gains when the clutter model parameters are unknown and estimated from the measurements     

基于数字包络检波的RNSS通道时延标定方法

提出一种采用数字处理的时延测试方法,用于对导航卫星导航信号发射通道分数码片时延的精确测量。该方法是通过高速A/D（模/数）转换器,对导航卫星下行的BPSK（二进制相移键控）信号和卫星导航秒脉冲进行双通道采样,读取采样数据并进行数据处理。根据秒脉冲信号触发门限上升沿确定时延测量起点,对BPSK采样数据进行平方律检波,获取码片换相点,计算换相点和秒脉冲之间的分数码片时延,并进行滤波器时延校准,从而得到导航卫星发射链路的分数码片时延,该方法不需要进行伪随机信号的捕获和跟踪,测量精度主要取决于采样器采样率。通过在测试中使用一根校准电缆对该方法进行验证,验证结果表明,采用本文提出测试方法的测量误差优于0.3ns。     

Modeling and analysis for the GPS pseudo-range observable

In this paper, a digital system for the Global Positioning System (GPS) pseudo-range observable is modeled and analyzed theoretically. The observable is measured in a GPS receiver by accurately tracking the pseudorandom noise (PRN) code phase of the input GPS signal using a digital energy detector and a digital delay lock loop (DDLL). The following issues are presented: (1) mathematical modeling of the digital PRN code acquisition and tracking system, (2) the closed-form expression derivation for the detection and false-alarm probabilities of the acquisition process and for the variance of code phase tracking error, and (3) the linear and nonlinear performance analysis of the DDLL for optimizing the receiver structures and parameters with tradeoff between the tracking errors due to receiver dynamics and due to input noise     

Reduction of Radar Tracking Errors with Frequency Agility

Frequency agility with random frequency in each pulse gives an improvement in radar angle tracking with a monopulse radar. With a conical-scan tracking radar, the glint error is reduced but fading error can be increased, and the net result must be studied in each case. A theory, usable for calculating angle tracking errors with a frequency agile radar, is given, and two examples showing the error reduction are presented. According to the theory, one part of the glint or fading spectra is ``smeared out' to half the pulse repetition frequency. Another part, the size of which depends on the degree of correlation between pulses, keeps the form of the original spectrum.     

Family of multicarrier bi-phase radar signals represented by ternary arrays

A K times L ternary array, comprised of the elements {0, 1, - 1}, with some unique features, represents a multicarrier radar signal with favorable autocorrelation and ambiguity functions. Constructing such an array using Galois fields is described. As in a Costas binary array, only one frequency is transmitted at any time slot, but in our array the same frequency is repeated in several time slots, yielding a signal with considerably larger pulse compression than a Costas signal that uses the same number of frequencies     

北斗导航接收机频率稳定度传递算法

为了提高北斗导航接收机的灵敏度,提升其弱信号跟踪能力,通常需要利用长时间的相干积分来提高环路信噪比。但是,当相干积分时间加长到一定程度时,环路性能反而有所下降,信噪比提升也不能达到理论值。针对由剩余频率误差和晶振误差引起的相干积分能量损失问题,主要研究了频率偏差对环路跟踪性能的影响,并提出了利用频率稳定度传递策略辅助弱信号跟踪的方法,解决了北斗导航接收机弱信号跟踪性能提升的问题,最大程度地改善了相干积分的效果,实现了对弱信号的跟踪。利用软件接收机平台对提出的频率稳定度传递算法进行验证,仿真结果表明该算法可使环路信噪比提升4dB ~5dB,充分说明了其可行性及有效性。     

A Method for Radar Turbulence Detection

A method is described whereby a conventional non-Doppler radar may be used in the measurement and detection of atmospheric turbulence. The technique involves the measurement of the average echo power and the variance of the fluctuation spectrum in each of two separated pulse volumes, and corresponding measure ments of echo power and variance of the detected signals in a channel representing the predetection sum of the signals from those two volumes. It is shown that the variance of the sum channel is the power-weighted sum of the variances in the independent channels plus a term dependent upon the difference in mean radial velocity of the associated pulse volumes. Accordingly, the difference in mean radial velocity can be measured. When the spacing of the sampled volumes is adjusted to represent distance along an aircraft flight plan, the measurement is a measure of the turbulence to be experienced in traversing the space between those volumes. Specific systems for accomplishing the measure ment are discussed.     

A flexible processor for a digital adaptive array radar

A digital beamforming processor for an adaptive array radar is described. The functionality and the architecture of the processor are strongly driven by a goal of achieving adaptive null depths in the 60-dB to 70-dB range, which necessitates substantial preprocessing of each channel. In particular, conversion to baseband quadrature channels is accomplished digitally using a single A/D converter per channel, and FIR (finite impulse response) equalizing filters are employed in each channel to match channel transfer functions. The processor is highly modular, and this not only distributes the total processing load, but also the I/O (input/output) bandwidth requirement. This is accomplished by distributing the adaptive beamforming algorithm systolically across a linear array of processing nodes. The processor is expandable to a different number of channels and sufficiently flexible to be applied to other problems of an array signal processing nature. Experimental data presented demonstrate that the processor is capable of supporting channel-to-channel cancellation of interfering signals to the level of -65 dB