Coherent aperture synthesis by an incoherent signal

The procedure of aperture synthesis by an incoherent source during displacement of the receiving system in space is considered. The advantages of the method are the possibility of aperture synthesis with the help of antenna arrays consisting of two receivers and its applicability to both the source of a quasimonochromatic signal and that possessing a continuous spectrum. The method is applicable with the same limitations as the known methods using the procedures of signal phasing. The results of mathematical simulation of the synthesis upon location of two sources with close frequencies at different distances and in the presence of noise, as well as the results of the in situ experiment with a reverberation interference, are given. The algorithm for decreasing the time of aperture synthesis without resolution deterioration is proposed and tested in situ.     

Experimental study of the directional characteristics of a vector-scalar array

We analyze the spatial spectra of a vector-scalar array when the signals are processed using methods with different resolution. The method of presenting the signals used in the research allowed us to apply the previously developed method of detecting and estimating the parameters of signal sources obtained in the calculation of the theoretical characteristics of detecting the signal sources and realization of the algorithms of signal processing. We compared the resolution of vector-scalar and scalar arrays with the same aperture. The experiments were carried out in stationary conditions and in the towing regime. During the towing regime, the accuracy of determination of the source location was controlled using the GPS receivers. It was shown experimentally that the signal to noise ratio at the output of the receiving array is three times greater if the vector-scalar array is used rather than the scalar one. The level of the lateral background appeared to be three times smaller during operation with the vector-scalar array than with the scalar one even in the towing regime. The results of measurements are confirmed by theoretical calculations.     

Modeling of Processing Algorithms for Processes at the Output of a Combined Receiver and Combined Array Channels

Using a computer-based simulation method, we consider the noise immunity of an individual combined receiver consisting of sound pressure and vibration velocity receivers, as well as the noise immunity of a plane acoustically transparent array consisting of such receivers, are considered. The computer simulation results verify the earlier developed analytic method of calculating the noise immunity coefficient of hydroacoustic arrays for the multiplicative processing algorithm for processes in their channels. It is shown that if the anisotropy of the noise field acting on the combined receiver and array varies in a very wide range, the maximum noise immunity is ensured not by multiplicative but by additive processing.     

Incoherent signal source resolution based on coherent aperture synthesis

A technique is proposed for resolving two incoherent signal sources of the same frequency and significantly different intensities with similar angular coordinates. The technique is based on aperture synthesis of a receiving array, first, by the signal of higher-power source and the estimate of its angular coordinate with subsequent subtraction of the signal spectrum from the angular spectrum of the received field. This makes it possible to achieve aperture synthesis and estimate the angle of arrival of a higher-power signal. Thus, the technique is of interest not only for synthesized apertures, but also for arrays with a filled aperture, since it eliminates the restrictions imposed by the presence of lateral lobes of the array response. Our mathematical simulation data demonstrate the efficiency of this technique in the detection and location of weak signals against the background of high-power noise sources even at their close angular positions.     

Experimental investigation of potentialities of seismoacoustic sea-bottom sounding using coherent pulse signals

We describe the results of experimental investigations of the seismoacoustical sounding of the bottom structure of the Caspian Sea. They were obtained using a ship towed hydroacoustic emitter of LFM pulse signals in several frequency ranges of frequency band from 100 to 1000 Hz. Based on the high coherence and relatively high frequencies of emitted signals, the results point to feasibility of substantial improvement in noise immunity and resolution of sounding the bottom rocks?? structure at depths of up to 1000 m thanks to combined application of a series of procedures of coherent processing of incoming signals. The processing involves matched filtering of individual pulses, coherent accumulation of pulse trains within the horizontally uniform bottom area, and adaptive path accumulation of pulses accounted for inclination of individual reflecting layers. The resulting gain in noise immunity came to about 30 dB, which points to possibility of efficient use of relatively low-power (up to 100 W) coherent sources for seismoacoustic sounding of sea bottom at minimal damage to local ecology.     

拖曳阵被动合成孔径目标深度稳健估计

使用水下无人平台作为载体的拖曳阵进行被动目标深度估计具有灵活性高和隐蔽性好的优点,针对实际应用中存在的平台自噪声和阵列瞬时随机加速度扰动问题,提出了一种稳健的目标深度估计方法。该方法分为三个步骤,首先对阵元接收信号进行自适应噪声抵消和相位抖动滤波,然后对声压进行距离积分实现简正波模态估计,最后计算模态匹配度,最大值对应的深度为目标深度估计结果。仿真表明在干扰背景下该方法的目标深度估计稳健性优于传统方法,声源频率、合成孔径距离和信干比决定了目标深度估计误差。利用实验数据验证了该方法对水下低频线谱声源的深度估计能力。     

水下线谱噪声源识别的波束域时频分析方法研究

从联合空时频三维信息从发, 提出了波束域时频分析识别水下运动航行器低频线谱噪声源位置的方法. 首先, 利用小孔径圆环阵的超指向性波束形成, 将各线谱噪声源匀速通过正横位置附近时产生的多普勒信号在时域上分离. 其次, 分别使用伪Wigner-Ville分布和调频小波变换两种时频分析方法对波束输出的信号进行处理, 得到各噪声源信号的时频图像. 最后, 转换时间坐标到空间并参考配置信标, 即可识别低频线谱噪声源在水下航行器上的位置. 该方法解决了阵列识别水下低频噪声源的孔径受限问题, 同时对处理同频相干噪声源也适用. 仿真试验结果表明: 两种波束域时频分析方法都能较精确地识别低频线谱噪声源的位置; 在测量系统信息的配合下, 波束域调频小波变换的识别效果更优.     

基于时延估计的分裂阵时域波束形成技术

大孔径拖曳线列阵受舰艇横向机动、洋流影响和水动力影响时会产生一定的阵形畸变,阵形畸变使得波束形成时阵列流型失配,进而降低了目标方位分辨率和阵处理的增益。在无法进行阵形估计时,基于时延估计的分裂阵时域波束形成技术将大孔径拖曳线阵分为左右两个子阵分别做波束形成,通过加权广义相关时延估计算法估算对应波束的时延差,再依据估算时延差对左右两个波束进行延时求和得到最终的波束信号。仿真和海试数据证明,相对于全阵直接做波束形成的方法,基于时延估计的分裂阵的时域波束形成技术有效提高了目标方位分辨率和阵处理的增益。     

风成噪声背景下垂直阵阵列信噪比随声源深度的变化规律

风成噪声是海洋中最广泛存在的环境噪声、是被动声纳处理性能的重要影响因素.它具有一个典型的物理性质是很大程度上只对应中高阶模态.阵列信噪比计入了阵列采样辐射声强、背景噪声功率和阵处理增益,是决定声纳阵列处理性能的关键物理量.本文研究了受风成噪声模态结构影响,典型夏季浅海环境中垂直阵阵列信噪比随声源深度的变化关系.在简正波模深函数采样完整的假设条件下,理论证明了阵列信噪比随声源深度的变化可近似为低阶模态幅度强度(模深函数模值的平方)随深度变化的线性叠加,且模态阶数越低,贡献越大;并且,在强风成噪声背景、显著负梯度环境下该变化规律可由1阶模态幅度强度随深度的变化近似独立表征.以上结果表明,在同一声源距离条件下,声源置于水体下半部分时的阵列信噪比比置于海面附近更大,并且在位于1阶模态峰值点所在深度附近时达到最大.典型负声速梯度浅海环境中的仿真实验结果对理论分析进行了验证,并表明在一定条件下阵列信噪比随声源深度的变化与声源距离近似无关.     

Three-dimensional imaging spectrometry by fully passive interferometry

We have developed a method to obtain a set of spectral components of three-dimensional (3D) images for a spatially incoherent, polychromatic light source distribution by a fully passive interferometric technique. The principle of the method is based on the measurement of a five-dimensional (5D) spatial coherence function and signal processing including the synthetic aperture technique and spectral decomposition. This paper gives a mathematical formulation illustrating the principle of the method, and we report an experimental demonstration in which the measured object is composed of two statistically uncorrelated point sources. The experimental results verify the applicability of our method to conventional spatially incoherent, polychromatic objects.     

The possibility of using the equivalent plane wave model to increase the efficiency of taking bearings of low-frequency signals in shallow water

The possibility of approximating the sound field in the region of interference maxima using the equivalent plane wave model with the actual amplitude and the average ??effective?? phase velocity calculated or measured by the phase gradient at the array aperture is discussed. The method is substantiated by studying the mode, interference, and phase structures of the low-frequency sound field along with the spatial responses of an extended linear array. For bottom-moored or towed geophysical arrays whose sizes are large compared to the wavelength, both the necessity and the possibility of reducing the error in taking the bearing of a sound source in a waveguide are justified. The use of the proposed model is recommended for approximate matching of the array to the transfer function of the waveguide to reduce the bearing error.     

Interferometric Direction Finding by a Vector-Scalar Receiver

A method for interferometric direction finding of a broadband sound source in an oceanic waveguide by a single vector-scalar receiver is presented. The method is based on the double Fourier transform of the interference pattern formed during motion. The efficiencies of the proposed direction finding method and the method based on measuring the delay times of signals arriving at spaced scalar receivers are compared based on the natural experiment results. The noise immunity of the interferometric direction finding method is considered.     

Analysis of telescope array receivers for deep-space inter-planetary optical communication link between Earth and Mars

Optical communication technology shows promising prospects to fulfill the large bandwidth communication requirements of future deep-space exploration missions that are launched by NASA and various other international space agencies. At Earth, a telescope with a large aperture diameter is required to capture very weak optical signals that are transmitted from distant planets and to support large bandwidth communication link. A single large telescope has the limitations of cost, single point failure in case of malfunction, difficulty in manufacturing high quality optics, maintenance, and trouble in providing communication operations when transmitting spacecraft is close to the Sun. An array of relatively smaller-sized telescopes electrically connected to form an aggregate aperture area equivalent to a single large telescope is a viable alternative to a monolithic gigantic aperture. In this paper, we present the design concept and analysis of telescope array receivers for an optical communication link between Earth and Mars. Pulse-position modulation (PPM) is used at the transmitter end and photon-counting detectors along with the direct-detection technique are employed at each telescope element in the array. We also present the optimization of various system parameters, such as detector size (i.e., receiver field of view), PPM slot width, and the PPM order M, to mitigate the atmospheric turbulence and background noise effects, and to maximize the communication system performance. The performance of different array architectures is evaluated through analytical techniques and Monte-Carlo simulations for a broad range of operational scenarios, such as, Earth-Mars conjunction, Earth-Mars opposition, and different background and turbulence conditions. It is shown that the performance of the telescope array-based receiver is equivalent to a single large telescope; and as compared to current RF technology, telescope array-based optical receivers can provide several orders of magnitude greater data rates for deep-space communication with Mars.     

Resolution and noise in ghost imaging with classical thermal light

The resolution and classical noise in ghost imaging with a classical thermal light are investigated theoretically. For ghost imaging with a Gaussian Schell model source, the dependences of the resolution and noise on the spatial coherence of the source and the aperture in the imaging system are discussed and demonstrated by using numerical simulations. The results show that an incoherent source and a large aperture will lead to a good image quality and small noise.     

Analysis of noise immunity of OFDMA communication systems operated in the presence of interfering stations

We analyze noise immunity of high-speed data transmission systems operated in a complex communication channel with frequency-selective fadings and interference from the neighboring base stations. This problem is very topical for modern orthogonal frequency-division multiple-access (OFDMA) data transmission standards. Such digital communication systems allow one to measure the noise parameters on individual subcarriers simultaneously with reception useful signals. We perform comparative analysis of different methods for measuring the noise characteristics and using them for demodulating the coded signal. Four processing methods with different complexity and noise immunity degree are proposed for receiver signals. Characteristics of the considered receivers are compared for different channel loadings and algorithms of power control by interfering stations. Practical recommendations for using the considered receivers are given. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 533–543, June 2007.     

Probability density function of noise statistics for optically pre-amplified DPSK receivers with optical Mach-Zehnder interferometer demodulation

We present explicitly semi-analytical probability density functions (pdf’s) of noise statistics in DPSK receivers with Mach-Zehnder interferometer (MZI) demodulation with considering the phase noise for the first time. Error performance of DPSK receivers with MZI demodulation is evaluated by using the calculated pdf’s. It is found that DPSK receivers with MZI demodulation and balanced detection are less sensitive to phase noise impact than those with the single-port detection to some extent. Moreover, it is found that ASE-ASE beat noise induced pdf difference in balanced detection compared to single-port detection may result in ∼3 dB improvement in receiver sensitivity mainly depending on optical filtering, ASE-amplified spontaneous emission. Therefore, the measured receiver sensitivity improvement by using balanced detection consist of the improvements due to signal energy difference and ASE-ASE beat noise induced pdf difference compared to single-port detection.     

Immunity of a Detection System for Optimum Space Filtration and Use of an Interference Compensator

The paper studies the dependence of the potential noise immunity of a detection system and noise immunity of a detection system with an interference compensator on the array parameters, angular signal position and local noise, the degree of correlation of distributed noise and spectral densities of the signal power, noise, and interference. The gain in noise immunity of the detection system when the optimal spatial filter is used with respect to the use of an interference compensator is estimated as a function of the degree of correlation of distributed noise and the power of random amplitude–phase errors of the weight coefficients of the array.     

Reconstruction of Rayleigh-Lamb dispersion spectrum based on noise obtained from an air-jet forcing

The time-domain cross correlation of incoherent and random noise recorded by a series of passive sensors contains the impulse response of the medium between these sensors. By using noise generated by a can of compressed air sprayed on the surface of a plexiglass plate, we are able to reconstruct not only the time of flight but the whole wave forms between the sensors. From the reconstruction of the direct A(0) and S(0) waves, we derive the dispersion curves of the flexural waves, thus estimating the mechanical properties of the material without a conventional electromechanical source. The dense array of receivers employed here allow a precise frequency-wavenumber study of flexural waves, along with a thorough evaluation of the rate of convergence of the correlation with respect to the record length, the frequency, and the distance between the receivers. The reconstruction of the actual amplitude and attenuation of the impulse response is also addressed in this paper.     

Noise immunity of a planar array of combined receivers

The far-field noise immunity of a passive direction-finding planar array consisting of combined receivers is considered for different signal processing algorithms. It is shown that there is no point in using combined receivers in arrays with an acoustic baffle.     

Testing underwater bottom-moored antenna arrays in the sea and in a man-made lake

Measurements of response, gain, and noise immunity are carried out for an underwater compensated additive receiving array with randomly spaced hydrophones that is moored at the bottom of a man-made lake with multimode sound propagation. The in-sea locating ability of a similar array is demonstrated with the sources of noiselike signals at frequencies of 5–100 Hz. A dedicated numerical processor is developed and tested for processing the signals received by a random underwater array.