RFI measurements and mitigation for FAST

Radio Frequency Interference(RFI)mitigation is essential for supporting the science output of Five-hundred-meter Aperture Spherical radio Telescope(FAST)due to its high sensitivity.In order to protect FAST from RFI,an Electromagnetic Compatibility(EMC)study has been carried out and the operation of a Radio Quiet Zone(RQZ)is ongoing.RFI measurements of the telescope instruments and monitoring of the active radio services outside the site have revealed the radiation properties of the RFI sources.Based on the measurement results and theoretical analysis,various EMC methods have been implemented for the telescope to decrease the RFIs.Meanwhile,the main RFI sources in the FAST RQZ,such as mobile stations,broadcast stations and navigation instruments,have been identified,and the technical measures have been adopted to protect the quiet radio environment around the site.The early science outputs of FAST have demonstrated the efficiency of RFI mitigation methods.     

高灵敏度VLBI和FAST计划

概述了VLBI天体物理的近期发展，主要成就及其受到的灵敏度制约，简要地回顾的目前还处在不太成熟阶段的低频VLBI的探索及近十余牛对提高VLBI灵敏度的硬件和软件两方面的尝试，并在此基础上，着重讲座了我国坟划中的FAST射电望远镜对提高VLBI灵敏度的可能贡献，展望了因为FAST的参与而获得的中低频高灵敏度VLBI的天体物理课及其意义。     

East Asian VLBI Network observations of active galactic nuclei jets:imaging with KaVA+Tianma+Nanshan

The East Asian Very Long Baseline Interferometry(VLBI) Network(EAVN) is a rapidly evolving international VLBI array that is currently promoted under joint efforts among China, Japan and Korea. EAVN aims at forming a joint VLBI Network by combining a large number of radio telescopes distributed over East Asian regions. After the combination of the Korean VLBI Network(KVN) and the VLBI Exploration of Radio Astrometry(VERA) into Ka VA, further expansion with the joint array in East Asia is actively promoted. Here we report the first imaging results(at 22 and 43 GHz) of bright radio sources obtained with Ka VA connected to Tianma 65-m and Nanshan 26-m Radio Telescopes in China. To test the EAVN imaging performance for different sources, we observed four active galactic nuclei(AGN) having different brightness and morphology. As a result, we confirmed that the Tianma 65-m Radio Telescope(TMRT) significantly enhances the overall array sensitivity, a factor of 4 improvement in baseline sensitivity and 2 in image dynamic range compared to the case of Ka VA only. The addition of the Nanshan 26-m Radio Telescope(NSRT) further doubled the east-west angular resolution. With the resulting high-dynamic-range,high-resolution images with EAVN(Ka VA+TMRT+NSRT), various fine-scale structures in our targets,such as the counter-jet in M87, a kink-like morphology of the 3 C 273 jet and the weak emission in other sources are successfully detected. This demonstrates the powerful capability of EAVN to study AGN jets and to achieve other science goals in general. Ongoing expansion of EAVN will further enhance the angular resolution, detection sensitivity and frequency coverage of the network.     

The potential of FAST in detecting celestial hydroxyl masers and related science topics

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) will make contributions to studies of Galactic and extragalactic masers. This telescope, with construction finished and now undergoing commissioning, has an innovative design that leads to the highest sensitivity of any single dish radio telescope in the world. FAST's potential for OH megamaser research is discussed, including the sky density of masers detectable in surveys. The scientific impact expected from FAST maser studies is also discussed.     

1-Gbps VLBI, The First Detection of Fringes

We report on the detection of VLBI fringes from quasars by a new VLBI system operating at 1 Gbps (1024 Mbits-per-second). Newly developed 1024 Msps (mega sample-per-second) AD samplers and 1024 Mbps recorders were used for the observations. A correlator with external buffers was used for the 1024 Mbps correlation processing of the tapes data.Our new VLBI system enabled 1024 Mbps VLBI, and this allowed the sampling of a 512 MHz bandwidth from a radio telescope receiver. This is the highest sampling speed ever used for VLBI, and the widest bandwidth used for VLBI observation. Initial sensitivity as evaluated by SNR comparison with earlier VLBI systems produced results to matched the expanded bandwidth. In our first observations, simultaneous optical fibre linked real-time VLBI observations were made to check the validity of data and precisely detemine the clock offsets among the radio telescopes.     

Preface:Planning the scientific applications of the Five-hundred-meter Aperture Spherical radio Telescope

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) is by far the largest telescope of any kind ever built. FAST produced its first light in September 2016 and it is now under commissioning, with normal operation to commence in late 2019. During testing and early science operation, FAST has started making astronomical discoveries, particularly pulsars of various kinds, including millisecond pulsars, binaries, gamma-ray pulsars, etc. The papers in this mini-volume propose ambitious observational projects to advance our knowledge of astronomy, astrophysics and fundamental physics in many ways.Although it may take FAST many years to achieve all the goals explained in these papers, taken together they define a powerful strategic vision for the next decade.     

The role of FAST in pulsar timing arrays

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) will become one of the world-leading telescopes for pulsar timing array(PTA) research. The primary goals for PTAs are to detect(and subsequently study) ultra-low-frequency gravitational waves, to develop a pulsar-based time standard and to improve solar system planetary ephemerides. FAST will have the sensitivity to observe known pulsars with significantly improved signal-to-noise ratios and will discover a large number of currently unknown pulsars. We describe how FAST will contribute to PTA research and show that jitter-and timing-noise will be the limiting noise processes for FAST data sets. Jitter noise will limit the timing precision achievable over data spans of a few years while timing noise will limit the precision achievable over many years.     

Wide-field VLBI imaging

We discuss the technique of Wide-field imaging as it applies to Very Long Baseline Interferometry (VLBI). In the past VLBI data sets were usually averaged so severely that the field-of-view was typically restricted to regions extending a few hundred milliarcseconds from the phase centre of the field. Recent advances in data analysis techniques, together with increasing data storage capabilities, and enhanced computer processing power, now permit VLBI images to be made whose angular size represents a significant fraction of an individual antenna's primary beam. This technique has recently been successfully applied to several large separation gravitational lens systems, compact Supernova Remnants in the starburst galaxy M82, and two faint radio sources located within the same VLA FIRST field. It seems likely that other VLBI observing programmes might benefit from this wide-field approach to VLBI data analysis. With the raw sensitivity of global VLBI set to improve by a factor 4–5 over the coming few years, the number of sources that can be detected in a given field will rise considerably. In addition, a continued progression in VLBI's ability to image relatively faint and extended low brightness temperature features (such as hot-spots in large-scale astrophysical jets) is also to be expected. As VLBI sensitivity approaches the μJy level, a wide-field approach to data analysis becomes inevitable.     

Space VLBI Geodesy: background of an experiment proposal

Space VLBI is a new observing technique which will be available in the second half of this decade by radio telescopes as interferometer elements in orbit. Besides the main astronomical interests, this new development has some very interesting potential applications in satellite dynamics, geodesy and geodynamical research which qualify space VLBI a potential new technique in space geodesy. A geodesy demostration experiment (GEDEX) is being proposed for the Japanese space VLBI satellite-VSOP, with the main objectives of reference frame interconnections and orbit accuracy improvement of the space radio telescope. The paper gives a review of the special characteristics of space VLBI and a general background of the GEDEX proposal.     

VLBI at the Hartebeesthoek radio astronomy Observatory

The Hartebeesthoek Radio Astronomy Observatory has played a key role in the development of very long baseline interferometry (VLBI) in the southern hemisphere since 1971. This paper describes how the VLBI programme evolved and the instrumentation used. Contributions to high resolution mapping of compact radio sources are described, for both the Southern Hemisphere VLBI Experiment, SHEVE, and for Global networks, where HartRAO has made significant improvements in the N-S resolution. The unique geographical location of the telescope has been used to establish the terrestrial reference frame in the southern hemisphere and to measure tectonic motions over the past nine years. The Observatory has also been a fundamental station in extending the celestial reference frame defined by extragalactic radio sources to the southern hemisphere, and results of these programmes are given.     

YH-1星载超稳定晶振的频率稳定性的测试与分析

中国首个火星探测器萤火1号拟定在2011年发射升空。探测器上配备了一组(2个)超稳定晶体振荡器,该组晶体振荡器为X波段发射机提供频率标准,向地球发射下行信号。地面台站采用无线电开环多普勒和甚长基线干涉测量技术,对其进行通信、导航和跟踪。分析处理了萤火1号的超稳定晶体振荡器频率实验测试数据,确认晶体振荡器的频率稳定度约为1×10~(12),此结果可以作为火星探测器精密测定轨的有力支撑。     

The Experiment and Analysis on VLBI Observations of Space Passive Hydrogen Maser

China plans to establish a lunar orbital VLBI (Very Long Baseline Interferometer) station around 2025, which will carry a space passive hydrogen maser as the time and frequency reference. Since it is the first time to use a space passive hydrogen maser for VLBI observation, its feasibility needs to be studied and verified. Therefore, we carried out VLBI observations using the space passive hydrogen maser as the frequency reference. In the experiment, the active hydrogen atomic clock and space passive hydrogen maser were used as the frequency standard, and the alternate VLBI observations of China’s Mars probe TW1 (Tianwen 1) were carried out using the 25 m radio telescope at Sheshan, Shanghai, and other VLBI stations. The results of data processing and analysis show that the standard deviation of VLBI residual group delay based on both active hydrogen atomic clock and space passive hydrogen maser are within 0.5 ns, which indicates that the space passive hydrogen maser can meet the accuracy requirements of VLBI measurement for deep space exploration, and verify its feasibility as the frequency standard of lunar orbital VLBI stations.     

星载氢钟VLBI观测实验及分析

中国计划于2025年左右建立月球轨道VLBI (Very Long Baseline Interferometer)测站,将会搭载被动型星载氢钟作为时间频率标准.由于是首次在VLBI观测中使用星载氢钟,需要研究和验证其可行性.因此,利用星载氢钟作为频率基准开展了VLBI观测.实验时,分别使用主动型地面氢钟和被动型星载氢钟作为频率基准,利用上海天文台佘山25 m射电望远镜和其他测站对我国火星探测器天问一号进行了交替VLBI观测.数据处理分析结果表明,基于地面氢钟与星载氢钟的VLBI残余群时延标准差均在0.5 ns以内,表明星载氢钟可满足深空探测VLBI测定轨的精度要求,验证了其作为月球VLBI测站频率基准的可行性.     

Two-Component Structure of the Radio Source 0014+813 from VLBI Observations within the CONT14 Program

We consider a method of reconstructing the structure delay of extended radio sources without constructing their radio images. The residuals derived after the adjustment of geodetic VLBI observations are used for this purpose. We show that the simplest model of a radio source consisting of two point components can be represented by four parameters (the angular separation of the components, the mutual orientation relative to the poleward direction, the flux-density ratio, and the spectral index difference) that are determined for each baseline of a multi-baseline VLBI network. The efficiency of this approach is demonstrated by estimating the coordinates of the radio source 0014+813 observed during the two-week CONT14 program organized by the International VLBI Service (IVS) in May 2014. Large systematic deviations have been detected in the residuals of the observations for the radio source 0014+813. The averaged characteristics of the radio structure of 0014+813 at a frequency of 8.4 GHz can be calculated from these deviations. Our modeling using four parameters has confirmed that the source consists of two components at an angular separation of ~0.5 mas in the north–south direction. Using the structure delay when adjusting the CONT14 observations leads to a correction of the average declination estimate for the radio source 0014+813 by 0.070 mas.     

百秒差距尺度上的4C39.25

通过主要在短厘米波长的全球VLBI观测,已经发现强射电类星体4C39.25在十秒差距尺度结构中的视超光速运动,其运动学图景表明该源是一个很特殊的视超光速源.近几年已提出了若干模型,试图解释这特殊的运动图景.报道不多的百秒差距尺度的VLBI观测,很可能有助于检验提出的模型.本文介绍用欧洲网及上海25m天线,在18cm波长对4C39.25所作的VLBI 成象观测的初步结果,显示百秒差距尺度复杂结构及可能存在的视超光速运动.     

从KARST到FAST---中国天眼的概念起源与决策过程

在大科学时代,大科学工程的建设日益复杂,设计者需综合考虑设备性能、技术储备、经费、风险、环境等因素才能进行合理决策.在国内外天文学发展的背景中,梳理了500 m口径球面射电望远镜(Five-hundredmeter Aperture Spherical radio Telescope, FAST)概念形成的过程以及FAST团队在其中所做的决策,包括大望远镜中国方案的设想、先导单元的提出、主动反射面技术的采用等,并对决策过程进行了分析探讨. FAST诞生于我国与国际天文学发展的互动与融合进程,实现了从跟进到占据先机的转变,可为在相关基础薄弱的领域建设大科学工程提供参考.     

From KARST to FAST: The Conceptual Origin of FAST and Its Decision-making Process

In the era of big science, the construction of large science projects is becoming more complex. Designers have to comprehensively consider factors such as instrument performance, technical reserves, funding, risks, and the environment to make a reasonable decision. On the basis of domestic and international astronomy, this paper sorts out the process of the formation of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) concept and the decisions made by the FAST team, including the Chinese concept of the large telescope, the proposal of the prototype, and the adoption of the active reflector, etc. We also discuss the process of decision-making. FAST was born in the process of interaction and integration between Chinese and international astronomy development, and realized the transformation from following up to taking the lead. It can provide a reference for constructing future large science projects with limited foundations in terms of technology and funding.     

Prognostics and health management of FAST cable-net structure based on digital twin technology

The Five-hundred-meter Aperture Spherical radio Telescope(FAST)will be fully commissioned later in 2019.Once commissioned,operation and maintenance of FAST will be the most prominent task.The unique working mode of active shape-changing of FAST cable-net structure makes the traditional maintenance way,which combines routine inspection with preventive maintenances not only expensive,but also unable to effectively avoid potential risks in operations.Therefore,it is necessary to develop an economical and reliable operation/maintenance technology for FAST cable-net structure.In this paper,a Prognostics and Health Management(PHM)system is proposed based on the advanced Digital Twin(DT)technology.Through the finite element analysis of DT model,the current safety status of FAST cablenet is evaluated,and the fatigue life of components in the cable-net is predicted.Hence Condition-Based Maintenance(CBM)of FAST cable-net structure can be realized.The PHM system described in this paper can effectively guarantee the healthy and safe operation of the FAST cable-net structure,greatly improve the maintenance efficiency and reduce the cost for maintenance works.     

脉冲星数据比对分析和可视化系统设计与实现

脉冲星数据比对分析和可视化系统(PSRDB,URL:http://www.psrdb.net/),由FAST(Five-hundred-meter Aperture Spherical Radio Telescope)早期科学数据中心团队为快速开展脉冲星候选体比对分析和数据管理研发.通过前端数据提交页面,接收和维护来自FAST及其他研究机构的候选体数据.目前,PSRDB已收录自1967年人类发现第1颗脉冲星以来所有公开文献发表的2811颗脉冲星样本,并采集了当前主要巡天项目尚未正式发表的源和候选体,如FAST多科学目标同时扫描巡天(CRAFTS)候选体数据.基于入库基础数据,利用位置、周期、色散等参数进行比对分析,辅助科研工作者在线检索匹配已知星表数据,最后将检索匹配、比对分析结果生成图表供进一步分析.目前,PSRDB已被应用于FAST脉冲星搜寻和候选体数据管理.未来,PSRDB可在新源认证、后随观测、观测计划制定和原始数据处理流程设计等方面提供数据和工具支撑.     

Pulkovo combined catalogue of radio source positions PUL 2013

According to the decision of the International Astronomical Union (IAU), since 1998 the International Celestial Reference System has been realized by the ICRF catalogue of extragalactic radio source positions obtained from VLBI observations. Over the past years, the accuracy of the ICRF catalogue data has been increased only through an increase in the number and quality of observations and an improvement in the methods of their processing. Both the first ICRF version and the new ICRF2 version adopted by the IAU in 2009 are based on the catalogues obtained at the same VLBI data processing center. However, the experience of classical astrometry shows that a significant increase in the accuracy of the International Celestial Reference Frame can be achieved by creating combined catalogues, such as the fundamental catalogues of star positions. The same approach was applied to improve the ICRF catalogue. Even the first experience of such a combined solution has shown its high efficiency. Here, a new combined catalogue of radio source positions PUL(2013)C02 is presented. Mainly classical methods based on the expansion of the systematic differences between the input catalogues into series of orthogonal functions with additional improvements have been applied for its creation. Comparison of the combined catalogue obtained with the ICRF2 catalogue has shown that the latter is most likely not devoid of systematic errors at a level of 15–20 μas.