Extending the Field of View With Phased Array Techniques: Results of European SKA Research

The radio frequency window of the square kilometre array is planned to cover the wavelength regime from centimeters up to a few meters. For this range to be optimally covered, different antenna concepts are considered. At the lowest frequency range, up to a few gigahertz, it is expected that multibeam techniques will be used, increasing the effective field-of-view to a level that allows very efficient detailed and sensitive exploration of the complete sky. Although sparse narrow-band phased arrays are as old as radio astronomy, multioctave sparse and dense arrays now considered for the SKA require new low-noise design, signal processing, and calibration techniques. The successful implementation of these new array techniques has already been introduced for the use of phased array feeds upgrading existing telescopes: enhancing aperture efficiency as well as effective field-of-view. Especially the development of low-cost array antenna design will allow a cost-effective large-scale implementation for the SKA. This paper addresses these new capabilities, emphasizing the R&D work done in Europe and aims to provide insight into the status of enabling technologies and technical research on polarization, calibration, and side-lobe control that will unleash the potential of phased arrays for future growth of radio astronomy synthesis arrays.     

On refocusing a radio telescope to image sources in the near fieldof the antenna array

A theoretical model is developed for describing the operation of a radio telescope using techniques from optical coherence theory. This model is then used to develop a more systematic approach to the problem of refocusing a radio telescope to image sources which are within the near-field of the antenna array when the antenna array is in the far field of the source distribution. This is done by refocusing the telescope onto the sources in analogy to a photographic camera. Two methods for refocusing the telescope are described which would require no modification to most existing telescopes. In the first method, refocusing is accomplished either electronically by curving the antenna array into a portion of a hemisphere with a center at the source, or, alternatively by processing the correlation function data obtained from the antennas in a somewhat modified manner     

基于T 型稀疏发射阵列的傅里叶望远镜

傅里叶望远镜通过改变任意两个发射望远镜的位置来获取目标不同的空间频率分量,采集足够多的傅里叶分量值后进行信号处理即可重构目标图像。为了降低系统成本且尽量获得目标的细节信息,提出了一种T 型稀疏发射阵列的激光束发射配置方案,对4 种目标与均匀间隔发射阵列重构的图像进行了对比研究。T 型单臂放置11 个发射望远镜,连续抽取目标的8 个低频信息,再抽取3个高频分量。通过与单臂11 和16 个的均匀间隔发射阵列的重构图像进行对比发现:文中采用的发射阵列重构图像远优于同等数量发射器个数的单臂均匀放置方式,但整体成像能力略差于均匀16,这也是稀疏发射阵列不可避免的。     

A subspace-tracking approach to interference nulling for phasedarray-based radio telescopes

Several next-generation radio telescopes, now in the planning stages, are based on phased-array technology. One reason for this is to make use of adaptive nulling techniques to combat radio frequency interference, which is a growing problem for radio astronomy. This paper presents a low-complexity approach to interference nulling which is suitable for use in such systems. The approach uses subspace-tracking to identify interference, followed by spatial projections to place deep nulls in the directions of interferers. This technique overcomes two limitations of power-minimization algorithms (e.g., "minimum variance"), namely power inversion and pattern rumble, which create serious problems for radio astronomy. Furthermore, this technique imposes a lower computational burden and provides side information which is useful in later stages of data processing. Performance results from a phased array demonstrator system and a simulation are presented     

Australian SKA Pathfinder: A High-Dynamic Range Wide-Field of View Survey Telescope

The Australia SKA Pathfinder (ASKAP) is a new telescope under development as a world-class high-dynamic-range wide-field-of-view survey instrument. It will utilize focal plane phased array feeds on the 36 12-m antennas that will compose the array. The large amounts of data present a huge computing challenge, and ASKAP will store data products in an archive after near real-time pipeline processing. This powerful instrument will be deployed at a new radio-quiet observatory, the Murchison Radio-astronomy Observatory in the midwest region of Western Australia, to enable sensitive surveys of the entire sky to address some of the big questions in contemporary physics. As a pathfinder for the SKA, ASKAP will demonstrate field of view enhancement and computing/processing technology as well as the operation of a large-scale radio array in a remote and radio-quiet region of Australia.     

Earth-rotation aperture synthesis

Resolution for radio astronomy in the order of 1" is necessary for the study of distant radio galaxies and quasars, for detecting faint sources, and for the mapping of clouds of hydrogen and other molecules. To obtain these resolutions many new or planned radio instruments use arrays of moderate size radio telescopes to synthesize large physical apertures. These instruments are generally composed of one or several linear arrays and utilize the rotation of the earth to change the relative orientation of the array and the radio source. The techniques used for earth-rotation aperture synthesis are discussed. The response of a two-element interferometer and the geometry associated with earth-rotation synthesis are reviewed; the current and proposed designs for these instruments and their performance are described; and, finally, the inversion methods for determining the angular power distribution of a radio source from the array response (visibility function) are outlined.     

Self-Calibration for the LOFAR Radio Astronomical Array

LOFAR is a low-frequency radio astronomical array currently under development in The Netherlands. It is designed to produce synthesis images of the most distant celestial objects yet observed. Due to high redshift levels, observations must be at unusually low frequencies (30-240 MHz), over large apertures (100 km), using thousands of antennas. At these frequencies, Earth's ionosphere acts as a random refractive sheet which over the large aperture induces source direction dependent gain and phase errors that must be estimated and calibrated out. Current radio astronomy "self-calibration" algorithms do not address direction dependence and will not work in the LOFAR environment. This paper presents a formal study of the parameter estimation problem for LOFAR calibration. A data model is proposed, and a Cramer-Rao lower bound (CRB) analysis is developed with a new general formulation to easily incorporate a variety of constraining signal models. It is shown that although the unconstrained direction dependent calibration problem is ambiguous, physically justifiable constraints can be applied in LOFAR to yield viable solutions. Use of a "compact core" of closely spaced array elements as part of the larger array is shown to significantly improve full array direction dependent calibration performance. Candidate algorithms are proposed and compared to the CRB.     

可展开空间光学望远镜技术发展及展望

为了获得更高的角分辨率,空间光学望远镜的口径越来越大,口径超过4 m的空间望远镜将难以突破现有运载火箭整流罩有效包络的限制。另一方面,在研制周期及成本等方面拥有较大优势的微纳光学遥感卫星也对提高空间分辨率和集光面积有广泛的需求,需要在较小的发射体积里容纳下较大的光机系统,以降低发射成本。可展开空间光学望远镜将成为解决发射尺寸受限的可行方式。从大口径空间天文望远镜、分块式可展开对地观测望远镜和光轴方向可展开微纳卫星光学望远镜等方面对国内外可展开空间光学望远镜的研究现状进行了综述。对可展开空间光学望远镜涉及到的一些关键技术和发展趋势进行了阐述和归纳。     

红外焦平面探测器数字读出电路研究

读出电路是红外焦平面探测器组件的重要组成部分,其性能对探测器乃至整个红外成像系统的性能有重大影响.随着硅CMOS工艺的发展,数字化读出电路以及读出电路片上数字信号处理等功能得以实现,能够大幅度提高红外焦平面探测器的性能.以红外焦平面探测器对读出电路的要求入手,分析了读出电路各性能参数对红外焦平面探测器性能的影响,介绍了读出电路的数字化技术及各种实现方式以及数字积分技术.CMOS技术的发展使得数字积分技术在红外焦平面探测器读出电路中得以实现,有效解决了读出电路的电荷存储容量不足的问题,极大地提高了探测器性能.     

大口径射电望远镜相位阵馈源关键技术研究

馈源作为射电天文望远镜的关键部件，其性能至关重要。面向新疆奇台QTT 110 m射电望远镜和上海天马65 m射电望远镜相位阵馈源设计需求，本研究团队开展了抗射频干扰的高温超导滤波器、宽带Vivaldi天线阵列和低噪声GaAs MMIC芯片电路的研究工作。这些新型馈源技术、方案可为我国大口径射电望远镜的设计提供有效的理论与实验基础。     

基于软件无线电的相控阵雷达系统研究

主要论述了相控阵雷达系统中软件无线电的实现结构，首先从软件无线电的概念出发，论述了在相控阵雷达系统中采用软件无线电技术的必要性和可能性；其次详细地论述了相控阵雷达系统中软件无线电的实现形式：即天线子系统、收/发子系统、信号处理子系统。最后对软件无线电技术在现代雷达中的发展进行了展望。     

智能天线技术及应用研究评述

智能天线是提高无线传输性能的一项关键技术,是无线通信领域持续的研究热点之一,其中部分研究成果已经在无线通信、雷达、电子对抗等广泛的领域获得了成功的应用,并且未来无线系统设计中会越来越多的采用智能天线技术.为此,文章首先简要回顾了有关智能天线技术的发展历史,介绍了其中主要的阵列处理算法原理,给出了基于软件无线电和FPGA/DSP等可重配置软硬件系统设计参考方案,通过一些设计实例分析了应用中的实际问题和解决途径,最后提出了有关这一技术领域未来发展的展望.     

The unifying role of the coarray in aperture synthesis for coherentand incoherent imaging

Systems of two-dimensional (2-D) imaging arrays and apertures are considered from the point of view of their performance in the imaging of spatially incoherent as well as coherent source distributions. Such systems find applications in radar, sonar, and ultrasound imaging, as well as in applications such as seismology and radio astronomy. For linear imaging techniques related to beamforming and based on the Fourier transform relationship between the source distribution and the aperture plane measurements, the point spread function of the system completely characterizes its performance. This function is determined by the geometry of the physical aperture or array as well as the weighting that can be applied to measurements. It is shown that the introduction of the concept of coarray, both for receive apertures in incoherent imaging and for transmit/receive systems in reflection-mode coherent imaging, provides a convenient and elegant framework within which many apparently isolated techniques for point-spread function or aperture synthesis can be understood. In addition to this unifying role, coarray concept gives new insight into the aperture synthesis process, which allows interesting new imaging techniques to be developed, especially in coherent imaging     

RFI mitigation of GNSS signals for radio astronomy: problems and current techniques

Mitigation of man-made radio frequency interference (RFI) is a problem of increasing importance for radio astronomy. Indeed, numerous terrestrial and satellite services of all kinds create lots of electromagnetic signals in a large part of the radio spectrum that are likely to disturb radio astronomical observations. Moreover, the advent of large, highly sensitive radio telescopes such as SKA, LOFAR, etc., will permit the observation of extremely faint and distant radio sources which exhibit large amounts of red shift, hence overlapping with unprotected bands in the radio spectrum. Among various possible causes of RFI, GNSSs have specific characteristics which can be handled appropriately to design efficient mitigation techniques. GNSS signals are generally well documented and the ephemerides of satellites are well known. Therefore, various partially or fully informed methods can be used, which exploit knowledge of the characteristics of the modulations. Adaptive cancellation techniques can also be used that use auxiliary observations coming from additional antennas. Besides, knowing the spatial direction of the disturbing source, spatial filtering and related techniques that can be implemented in antenna arrays are likely to mitigate RFI with minimum knowledge of the GNSS. After recalling some radio astronomy basics, the paper presents a few examples of RFI caused by satellite systems on radio-astronomical observations. Then, we give a brief overview of state-of-the-art RFI mitigation techniques for radio astronomy and we present in some details the principles and results pertaining to some methods that seem particularly appropriate to mitigate RFI arising from GNSS signals. The importance of cooperation between GNSS designers and astronomers is finally pointed out.     

平方公里阵天线新技术综述

天线是射电望远镜系统关键组成部分, 其性能对于望远镜的观测能力有着决定性的影响.数十年来, 射电望远镜天线技术不断发展, 在有效接收面积、观测带宽和视场等各个方面取得了巨大进步,并推动了新的科学发现.文章对射电天文领域出现的多项先进的天线技术, 包括整体成型面板、相控阵馈源、超宽带单波束馈源和孔径阵列等进行了详细的介绍.作为国际上正在积极推进的平方公里阵(Square Kilometre Array, SKA)的关键技术, 这些技术已经取得了重要突破, 其他领域的天线也可有所借鉴.     

Radio Astronomy in the Early Twenty-First Century

This paper serves as an introduction to the contributions in this Special Issue on ldquoAdvances in Radio Telescopes.rdquo After a very short historical view of the emergence of Radio Astronomy, we refer to earlier IEEE special issues on this subject and mention recent instruments in the domain of millimeter wavelength radio telescopes, developments in very long baseline interferometry and the planned Square Kilometre Array (SKA). After a short discussion of site selection aspects for the new telescopes we conclude with a summary of the major astronomical and astrophysical problems which will be studied by the new instruments described in the following papers.     

Performance evaluation of non‐Gaussian channel estimation techniques on ultrawideband impulse radio channels

We study the problem of adaptive channel estimation for a multipath channel on an ultrawideband (UWB) impulse radio (IR) system. The purpose of the work is to demonstrate that the statistical characteristics of the UWB‐IR environment are strongly non‐Gaussian and that as a result, non‐Gaussian signal processing techniques are both efficient and powerful in a UWB‐IR environment. The multipath channel estimation problem has been chosen as an illustrative example because UWB‐IR systems often operate in complex multipath environments. We study both blind and training‐based estimation techniques and demonstrate that even simple non‐Gaussian strategies can achieve significant performance improvement when compared to more common second‐order estimation techniques. Copyright © 2006 John Wiley & Sons, Ltd.     

基于软件无线电的相控阵雷达系统研究

从软件无线电的概念出法,论述了在相控阵雷达系统中采用软件无线电技术的必要性和可能性;其次详细地论述了相控阵雷达系统中软件无线电的实现形式:即天线子系统,收、发子系统,信号处理子系统。最后对软件无线电技术在现代雷达中的发展进行了展望。     

某型号雷达天线阵精度设计

精度是雷达、卫星通讯地面站和射电望远镜的一个重要性能指标。影响整个设备精度的因素很多，本文主要讨论与结构设计有关的天线阵两侧俯仰轴对天线指向精度的影响。