共查询到20条相似文献，搜索用时 15 毫秒
1.
Raj Thilak Rajan Albert-Jan Boonstra Mark Bentum Marc Klein-Wolt Frederik Belien Michel Arts Noah Saks Alle-Jan van der Veen 《Experimental Astronomy》2016,41(1-2):271-306
The past decade has seen the advent of various radio astronomy arrays, particularly for low-frequency observations below 100 MHz. These developments have been primarily driven by interesting and fundamental scientific questions, such as studying the dark ages and epoch of re-ionization, by detecting the highly red-shifted 21 cm line emission. However, Earth-based radio astronomy observations at frequencies below 30 MHz are severely restricted due to man-made interference, ionospheric distortion and almost complete non-transparency of the ionosphere below 10 MHz. Therefore, this narrow spectral band remains possibly the last unexplored frequency range in radio astronomy. A straightforward solution to study the universe at these frequencies is to deploy a space-based antenna array far away from Earths’ ionosphere. In the past, such space-based radio astronomy studies were principally limited by technology and computing resources, however current processing and communication trends indicate otherwise. Furthermore, successful space-based missions which mapped the sky in this frequency regime, such as the lunar orbiter RAE-2, were restricted by very poor spatial resolution. Recently concluded studies, such as DARIS (Disturbuted Aperture Array for Radio Astronomy In Space) have shown the ready feasibility of a 9 satellite constellation using off the shelf components. The aim of this article is to discuss the current trends and technologies towards the feasibility of a space-based aperture array for astronomical observations in the Ultra-Long Wavelength (ULW) regime of greater than 10 m i.e., below 30 MHz. We briefly present the achievable science cases, and discuss the system design for selected scenarios such as extra-galactic surveys. An extensive discussion is presented on various sub-systems of the potential satellite array, such as radio astronomical antenna design, the on-board signal processing, communication architectures and joint space-time estimation of the satellite network. In light of a scalable array and to avert single point of failure, we propose both centralized and distributed solutions for the ULW space-based array. We highlight the benefits of various deployment locations and summarize the technological challenges for future space-based radio arrays. 相似文献
2.
《天文和天体物理学研究(英文版)》2021,(4)
The very low frequency(VLF) regime below 30 MHz in the electromagnetic spectrum has presently been drawing global attention in radio astronomical research due to its potentially significant science outcomes exploring many unknown extragalactic sources,transients,and so on.However,the nontransparency of the Earth's ionosphere,ionospheric distortion and artificial radio frequency interference(RFI) have made it difficult to detect the VLF celestial radio emission with ground-based instruments.A straightforward solution to overcome these problems is a space-based VLF radio telescope,just like the VLF radio instruments onboard the Chang'E-4 spacecraft.But building such a space telescope would be inevitably costly and technically challenging.The alternative approach would be then a ground-based VLF radio telescope.Particularly,in the period of post 2020 when the solar and terrestrial ionospheric activities are expected to be in a 'calm' state,it will provide us a good chance to perform VLF ground-based radio observations.Anticipating such an opportunity,we built an agile VLF radio spectrum explorer co-located with the currently operational Mingantu Spectra Radio Heliograph(MUSER).The instrument includes four antennas operating in the VLF frequency range 1-70 MHz.Along with them,we employ an eight-channel analog and digital receivers to amplify,digitize and process the radio signals received by the antennas.We present in the paper this VLF radio spectrum explorer and the instrument will be useful for celestial studies of VLF radio emissions. 相似文献
3.
K.P. Stewart B.C. Hicks P.S. Ray P.C. Crane N.E. Kassim R.F. Bradley W.C. Erickson 《Planetary and Space Science》2004,52(15):1351-1355
We are developing and testing active baluns and electrically short dipoles for possible use as the primary wide band receiving elements in the low-frequency array (LOFAR) for long wavelength radio astronomy. Several dipoles of various designs and dimensions have been built and tested. Their useful range occurs when the dipole arms are approximately to one wavelength long and the feedpoint is less than wavelength above ground. An eight-element NRL LOFAR test array (NLTA) interferometer has been built and fringes have been observed from the brightest celestial sources in the frequency range from 10 to 50 MHz. The antenna temperatures vary from about 10% to 100% of the average brightness temperature of the galactic background. With these parameters it is easy to make the amplifier noise levels low enough that final system temperature is dominated by the galactic background. 相似文献
4.
The University of Chile transit radiotelescope is a 528-dipole array operating at 45 MHz. We present a comparison of an experimental study of the antenna radiation pattern with the basic theoretical pattern in three dimensions. We concentrate in the meridian plane diagram since this is particularly difficult to measure for an array like ours. The comparison shows excellent agreement. We have measured several important antenna parameters like the effective area as a function of zenith distance, the orientation of the plane of the array and the pointing accuracy. We include a detailed treatment of these subjects since not much information related to low frequency arrays for radio astronomy can be found in the literature. We discuss the importance of knowing these parameters in the preparation of the 45-MHz Sky Survey under way at the University of Chile Radio Observatory. 相似文献
5.
An effective wide-band (10 to 60 MHz) active antenna element has been developed. The cost of one short (3 m), thin dipole with built-in amplifier and metal construction is less than 45 euro. It was shown both theoretically and experimentally that the upper limiting frequency is at least 60 MHz, the dynamic range is 90 dB/V and the share of the amplifier noise to the background antenna temperature is about 10%. The developed active dipole was tested by building a 30-element antenna array and comparing its parameters with one of the subpart of the UTR-2 radio telescope having passive dipoles of 8.6 m in length and 1.8 m in diameter. The 3C461 ionospheric scintillation spectra observed in the experiments show that the sensitivities and noise-immunities of both antennas are close. This proves the availability using of a short cheap active dipole in new generation giant radio telescopes. 相似文献
6.
A. Kontogeorgos P. Tsitsipis C. Caroubalos X. Moussas P. Preka-Papadema A. Hilaris V. Petoussis C. Bouratzis J.-L. Bougeret C. E. Alissandrakis G. Dumas 《Experimental Astronomy》2006,21(1):41-55
We present the improved solar radio spectrograph of the University of Athens operating at the Thermopylae Satellite Telecommunication
Station. Observations now cover the frequency range from 20 to 650 MHz. The spectrograph has a 7-meter moving parabola fed
by a log-periodic antenna for 100–650 MHz and a stationary inverted V fat dipole antenna for the 20–100 MHz range. Two receivers
are operating in parallel, one swept frequency for the whole range (10 spectrums/sec, 630 channels/spectrum) and one acousto-optical
receiver for the range 270 to 450 MHz (100 spectrums/sec, 128 channels/spectrum). The data acquisition system consists of
two PCs (equipped with 12 bit, 225 ksamples/sec ADC, one for each receiver). Sensitivity is about 3 SFU and 30 SFU in the
20–100 MHz and 100–650 MHz range respectively. The daily operation is fully automated: receiving universal time from a GPS,
pointing the antenna to the sun, system calibration, starting and stopping the observations at preset times, data acquisition,
and archiving on DVD. We can also control the whole system through modem or Internet. The instrument can be used either by
itself or in conjunction with other instruments to study the onset and evolution of solar radio bursts and associated interplanetary
phenomena. 相似文献
7.
8.
The LOw Frequency ARray (LOFAR) is a next-generation radio telescope which uses thousands of stationary dipoles to observe celestial phenomena. These dipoles are grouped in various ‘stations’ which are centred on the Netherlands with additional ‘stations’ across Europe. The telescope is designed to operate at frequencies from 10 to 240 MHz with very large fractional bandwidths (25?–?100 %). Several ‘beam-formed’ observing modes are now operational and the system is designed to output data with high time and frequency resolution, which are highly configurable. This makes LOFAR eminently suited for dynamic spectrum measurements with applications in solar and planetary physics. In this paper we describe progress in developing automated data analysis routines to compute dynamic spectra from LOFAR time–frequency data, including correction for the antenna response across the radio frequency pass-band and mitigation of terrestrial radio-frequency interference (RFI). We apply these data routines to observations of interplanetary scintillation (IPS), commonly used to infer solar wind velocity and density information, and present initial science results. 相似文献
9.
Taufiq Hidayat Achmad Munir Budi Dermawan Anton Timur Jaelani Stéphane Léon Dading Hadi Nugroho Andriyan Bayu Suksmono Putra Mahasena Premana Wardayanti Premadi Dhani Herdiwijaya Chatief Kunjaya Zadrach Ledoufij Dupe Budi Brahmantyo Denny Mandey Muhammad Yusuf Hesti Retno Tri Wulandari Falahuddin Arief Muhammad Irfan Agus Triono Puri Jatmiko Evan Irawan Akbar Hery Leo Sianturi Jehunias Leonidas Tanesib Ali Warsito Judhistira Aria Utama 《Experimental Astronomy》2014,37(1):85-108
We report the first measurements of radio frequency spectrum occupancy performed at sites aimed to host the future radio astronomy observatory in Indonesia. The survey is intended to obtain the radio frequency interference (RFI) environment in a spectral range from low frequency 10 MHz up to 8 GHz. The measurements permit the identification of the spectral occupancy over those selected sites in reference to the allocated radio spectrum in Indonesia. The sites are in close proximity to Australia, the future host of Square Kilometre Array (SKA) at low frequency. Therefore, the survey was deliberately made to approximately adhere the SKA protocol for RFI measurements, but with lower sensitivity. The RFI environment at Bosscha Observatory in Lembang was also measured for comparison. Within the sensitivity limit of the measurement equipment, it is found that a location called Fatumonas in the surrounding of Mount Timau in West Timor has very low level of RFI, with a total spectrum occupancy in this measured frequency range being about 1 %, mostly found at low frequency below 20 MHz. More detailed measurements as well as a strategy for a radio quiet zone must be implemented in the near future. 相似文献
10.
N. Razavi-Ghods E. de Lera Acedo A. El-Makadema P. Alexander A. Brown 《Experimental Astronomy》2012,33(1):141-155
The new generation of radio telescopes, such as the proposed Square Kilometer Array (SKA) and the Low-Frequency Array (LOFAR) rely heavily on the use of very large phased aperture arrays operating over wide band-widths at frequency ranges up to approximately 1.4?GHz. The SKA in particular will include aperture arrays consisting of many thousands of elements per station providing un-paralleled survey speeds. Currently two different arrays (from nominally 70?MHz to 450?MHz and from 400?MHz to 1.4?GHz) are being studied for inclusion within the overall SKA configuration. In this paper we aim to analyze the array contribution to system temperature for a number of regular and irregular planar antenna array configurations which are possible geometries for the low-frequency SKA (sparse disconnected arrays). We focus on the sub-500?MHz band where the real sky contribution to system temperature (T sys ) is highly significant and dominants the overall system noise temperature. We compute the sky noise contribution to T sys by simulating the far field response of a number of SKA stations and then convolve that with the sky brightness temperature distribution from the Haslam 408?MHz survey which is then scaled to observations at 100?MHz. Our analysis of array temperature is carried out by assuming observations of three cold regions above and below the Galactic plane. The results show the advantages of regular arrays when sampled at the Nyquist rate as well as their disadvantages in the form of grating lobes when under-sampled in comparison to non-regular arrays. 相似文献
11.
12.
A low frequency (40–150 MHz) radioheliograph for observations of the solar corona is in operation for the last few years at
the Gauribidanur radio observatory, about 100 km north of Bangalore. The array has 32 antenna groups and a 1-bit digital correlator
system consisting of 1024 channels is used as the back-end receiver. This paper describes the latter and results of the associated
system tests. 相似文献
13.
Fine structure observations of the frequency spectrum of the S-component in the solar radio emission are described. Measurements were carried out in August 1976 and August 1977 using a 22 m parabolic antenna and a radiospectrograph operating over the frequency range 5.0 to 7.0 GHz, with the resolution 60 MHz. Measurement techniques are described. Fine structures (150–800 MHz) as great as 20% of the local source radiation level were observed in radio emission spectra of a number of these sources. The spectrum structures observed were changed in the process of active region development. 相似文献
14.
E. P. Abranin Yu. M. Bruck V. V. Zakharenko A. A. Konovalenko 《Experimental Astronomy》2001,11(2):85-112
The paper describes the preamplification antenna system (PAS) ofthe decameter radio telescope UTR-2. The new PAS have the continuous frequency range 8 to 40 MHz. The dynamic range is: 54 dbm at low frequencies and 30 dbm at high frequencies. To extendthe dynamic range the signal spectrum is divided and summed up with the aid of frequency selective devices, each consisting oftwo identical antimetric bandpass filters of even order and twohybrid adders. Within each of the three bands, the signal is amplified in multistage amplifiers with a deep linear frequencyindependent negative feedback in each stage. Part One presents thegoals, underlying ideas and structure of the PAS. Part Twodescribes details of the design, specific parameters and some observational results. 相似文献
15.
Azam Zavvari Mohammad Tariqul Islam Radial Anwar Zamri Zainal Abidin Mhd Fairos Asillam Christian Monstein 《Experimental Astronomy》2016,41(1-2):185-195
The e-CALLISTO system is a worldwide network that aims to observe solar radio emission for astronomical science. CALLISTO instruments have been deployed worldwide in various locations that together can provide continuous observation of the solar radio spectrum for 24 h per day year-round. Malaysia-UKM is a strategic equatorial location and can observe the Sun 12 h per day. This paper gives an overview of the spectrum allocation for radio astronomy, which falls in the specified operating frequency band of the CALLISTO spectrometer. The radio astronomy bands are analyzed at the Malaysia-UKM station according to the International Telecommunication Union recommendations. Some observational results are also presented in this paper. 相似文献
16.
相控阵馈源(Phased array feeds, PAFs)接收机作为下一代微波接收机, 为大口径射电天文望远镜的射电干扰(Radio Frequency Interference, RFI)缓解工作带来了新的解决方法. PAFs接收机对射电望远镜焦平面的电磁波进行空域采样, 返回时域阵列信号, 使用最小方差无失真响应(Minimum Variance Distortionless Response, MVDR)波束合成器可以自适应地识别RFI的方向, 同时抑制RFI在输出信号中的功率, 从而达到提升射电望远镜灵敏度的效果. 仿真结果表明MVDR波束合成器对有源高能量的射电干扰有很强的识别能力和一定程度的缓解能力, 同时, 该波束合成器对各阵元信道中加性噪声累积引起的无源干扰有很强的抑制能力, 因此, PAFs接收机的MVDR波束合成器可以增强日益复杂电磁波环境下射电望远镜的抗干扰能力. 相似文献
17.
Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band. 相似文献
18.
Maroulis D. Dumas G. Caroubalos C. Bougeret J. L. Moussas X. Alissandrakis C. Patavalis N. 《Solar physics》1997,172(1-2):353-360
We present the new digital solar radio spectrograph located at the Thermopyles station, Greece, operated by the University of Athens. Observations cover the range from 110 to 600 MHz, using a 7-m parabolic antenna. The reception system uses two techniques in parallel: sweep frequency and multi-channel, the latter being based on the Acousto-Optical technique. The data acquisition system is based on two subsystems, a Sun Sparc-5 workstation and a front end based on a VME Motorola system. The two subsystems are connected through the Ethernet and are operated using the VxWorks real-time package. The daily operation is completely automated: pointing of the antenna to the sun, starting and stopping the observations at pre-set times, acquiring data, compressing data by silence suppression in real time, and archiving the data on a routine manner on DAT tapes. Apart from its usual function, this instrument will be used in conjunction with other instruments, including the Nançay decameter array and the low frequency radio receivers on the Wind spacecraft. 相似文献
19.
作为射电天文接收机系统的关键器件, 低噪声放大器的噪声和增益性能对接收机系统的灵敏度有重要影响. 采用100nm砷化镓赝配高电子迁移率晶体管(pseudomorphic High Electron Mobility Transistor, pHEMT)\lk工艺, 研制了一款可覆盖C波段(4--8GHz)的低噪声放大器(Low Noise Amplifier, LNA). 所设计的LNA采用3级共源级联放大拓扑结构, 栅极、漏极双电源供电. 常温下测试表明, 该LNA在4--8GHz频段内平均噪声温度为\lk60K, 在5GHz处获得最低噪声温度50K, 通带内增益($31\pm1.5$)dB, 输入输出回波损耗均优于10dB, 芯片面积为$2.1\times1.1$mm2, 可以应用于C波段射电天文接收机以及卫星通信系统等. 相似文献
20.
Jayanta Roy Yashwant Gupta Ue-Li Pen Jeffrey B. Peterson Sanjay Kudale Jitendra Kodilkar 《Experimental Astronomy》2010,28(1):25-60
The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation
of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components,
is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes
with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline
uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility
in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection.
Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the
GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using
this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate
sized array like the GMRT. 相似文献