利用国内VLBI网跟踪大椭圆轨道卫星

2004年7月,昆明VLBI站经过改造,由上海、乌鲁木齐和昆明站组成的中国VLBI网(CVN)采用统一的MARK4格式编制器和CVN硬盘记录系统,对大椭圆轨道卫星“探测1号”的2圈轨道的共同可视弧段进行了跟踪观测．软件相关处理程序已成功地用于检测卫星遥测信号的干涉条纹和数据相关处理．采用基于条纹幅度的加权最小二乘条纹拟合方法,获得了卫星VLBI观测量及其精度估计,完成了卫星VLBI观测量的3基线闭合误差检验．应用河外射电源校准方法和多频点相位校正信号提取方法,进行了台站钟差和仪器延迟等系统误差改正．经系统差改正后的卫星VLBI观测量序列已用于“探测1号”卫星的轨道确定．     

利用VLBI数据确定"探测一号"卫星的轨道

双星计划的“探测一号”是中国首颗真正严格意义上的科学实验卫星,其运行轨道为中国迄今所发射的卫星中距地球最远,远地点地心距达7．8万公里．采用射电天文的VLBI技术可以对“探测一号”以及更远的深空目标,如探月飞行器实现跟踪．为了验证VLBI技术在我国探月计划中的作用,上海天文台组织了国内目前仅有的上海、乌鲁木齐和昆明3个台站对“探测一号”进行试跟踪,利用对“探测一号”约两天的VLBI观测数据,确定“探测一号”卫星的轨道,对VLBI的定轨能力做初步的探讨．按照测控部门提供的初轨 (其精度仅保证跟踪)推算的轨道与VLBI时延的拟合误差平均约2 km,时延率的拟合误差平均约15 cm／s．而利用VLBI数据定轨后的拟合程度相对于初轨有了很大的改善,结果表明,单独利用VLBI时延定轨,时延的拟合精度约5．5 m,作为外部检核的VLBI时延率的拟合精度在2 cm／s左右．单独利用VLBI时延率定轨,时延率的拟合精度约为1．3 cm／s,作为外部检核的VLBI时延的拟合精度约为29 m．而若将时延和时延率数据联合定轨,采用其内符精度加权,VLBI时延和时延率的残差分别为5．5 m和 2 cm／s．为了合理地评估VLBI定轨的真实精度,利用模拟数据进行误差协方差分析,结果表明VLBI定轨精度受动力学模型误差的影响较大,由于"探测一号”卫星的动力学模型难以精确确定,所以利用两天弧段的VLBI数据确定“探测一号”卫星轨道的位置误差为km量级,而速度误差可达cm／s量级．模拟计算还表明, VLBI和USB数据联合定轨可以大大提高定轨精度．     

A Model of Relation between Fluctuation of Double Differential Total Ionospheric Electron Content and Angular Distance of the Two Satellites Observed by Same-beam VLBI

Time delay and phase fluctuation are produced when the signals of a spacecraft are transmitted through the ionosphere of the earth, which give rise to a great influence on the measurement precision of VLBI (Very Long Baseline Interferometry). Using the 1-year same-beam VLBI data of 2 satellites (Rstar and Vstar) in the Japanese lunar exploration project SELENE, we obtained a model of the relation between the fluctuation of double differential total electron content in the ionosphere and the angular distance of the two satellites. For the 6 baselines, the root mean square r of fluctuation (in units of TECU) and the angular distance of the two satellites θ (in units of ^°) has a relation of r = 0.773θ + 0.562, and for the 4 VLBI stations, the relation is r = 0.554θ + 0.399 from the baselines inversion. The results can serve as a reference for the derivation of differential phase delay and for the occultation observation and study of planetary ionospheres.     

类星体3C 286的一个早期VLBI观测结果

利用基于自校正和ＣＬＥＡＮ算法的现代ＶＬＢＩ混合成图技术,对Ｐｅａｒｓｏｎ等人于１９７６年观测得到的类星体 ３Ｃ ２８６在 １８ ｃｍ波段上的 ＶＬＢＩ数据重新作了处理．由此得到的图像要大大好于最初的处理结果,也获得了更多的有用信息．用几个Ｇａｕｓｓ模型去拟合校正后的ＵＶ数据,发现中央亮区可以被分解为两个致密结构,这与后来的高分辨率观测相一致．与不同历元的ＶＬＢＩ观测结果相比较,发现这两个致密结构间的距离不但在同一时间随观测频率变化,而且很有可能在同一观测频率上随时间变化．     

同波束VLBI差分时延率研究

通过分析SELENE的两颗小卫星Rstar和Vstar的同波束VLBI数据,发现两卫星间差分时延率中由中性大气和电离层引起的长周期变化已被去除,且在小角距时,短周期变化也能绝大部分被去除。但角距离越大,去除部分越小。通过将2008年一年中8个台站组成的所有基线的相关相位拟合残差数据分别平滑积分30 s与60 s后,再通过5点直线拟合求得时延率,得出单卫星时延率随机误差分别约为0.32 ps/s RMS与0.20 ps/s RMS,而差分时延率随机误差分别为0.19 ps/s~0.32 ps/s RMS与0.09 ps/s~0.17 ps/s RMS。单卫星的时延率随机误差在秋季最大,春季最小,而差分时延率的随机误差在全年基本稳定。     

地球卫星VLBI观测研究进展

随着地球卫星在资源、导航定位、授时、跟踪以及数据中继等经济和军事领域的作用不断增强,传统的测距测速技术已无法满足对卫星定位精度的实际需求。由于VLBI技术具有高精度角分辨率的特点,人们尝试将其应用于人造地球卫星的观测。针对卫星VLBI观测方面,首先介绍了卫星VLBI观测技术的基本原理及其与传统河外射电源VLBI观测的差异,然后介绍了中外学者应用VLBI技术进行地球卫星观测的研究进展及相关成果,最后对以往近地卫星VLBI观测研究存在的问题进行总结,并讨论了目前以及将来可能的解决方案。     

The most compact bright radio-loud AGN—I. A new target sample selected for the space VLBI

We investigated the archival ground-based VLBI images of the extragalactic radio sources included in both the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck catalogues, and selected 49 bright and compact sources as potential targets for space Very Long Baseline Interferometry (VLBI) observations at mm wavelengths. These sources have a flat radio continuum spectrum between 33 and 94 GHz. They are identified as core-dominated active galactic nuclei (AGN), located at declinations above ?40^°, and have never been observed with ground-based VLBI at 86 GHz. The radio properties of the 49 new sources are presented. We compare this new sample with similar samples of compact AGN available from earlier studies. The new candidates, together with the existing bright compact AGN sample identified from 86-GHz ground-based VLBI imaging surveys, form a catalogue of more than 160 AGN. These could be primary targets for mm-VLBI observations on the ground, as well as for future mm-wavelength space VLBI missions such as the project with two satellites currently under study in China.     

A model of geometric delay in Space VLBI

A model which includes the relativistic effect is derived that can be applied to space very long baseline interferometry(SVLBI) while taking observations of sources at infinite distance. In SVLBI,where one station is on a spacecraft,the length of the baseline and the orbiting station's maximum speed in an elliptical orbit around the Earth is much larger than the ground-based VLBI,which leads to a larger delay and higher delay rate. The delay models inside VLBI correlators are usually expressed as fifth-order polynomials during a limited time interval,which are evaluated by firmware in the correlator and track delays in the interferometer over the limited time interval. The higher SVLBI delay rate requires more accurate polynomial fitting and evaluation,as well as more frequent model updates.     

Model of VLBI observation within the relativistic framework

In this paper we derive the post-newtonian expressions for the VLBI time delay and gravitational delay in the barycentric coordinate system of the solar system. We discuss the effect of the various bodies and their range of action. From the transformation between the barycentric and the geocentric systems we then give the VLBI observational model in the geocentric system. Our final results are given by formula (16) for the gravitational delay and by formula (25) for the VLBI time delay.     

月球交会对接VLBI差分时延研究

中国探月3期任务中,月球交会对接技术是任务成功的重要保障.利用嫦娥3号(CE03)绕月飞行的VLBI (Very Long Baseline Interferometry)时延数据,模拟仿真绕月交会对接过程中,同波束VLBI观测模式下,差分群时延的变化情况.仿真结果显示,在远程导引段,轨道器和上升组合体轨道距离保持100 km,持续半小时,差分群时延很好地反映了两者的轨道信息,可以用于定轨定位;自主控制段,上升组合体靠近轨道器,在轨道距离从5 km减小到20 m过程中,上升组合体加速追赶轨道器时,差分群时延快速趋近于0,上升组合体减速远离轨道器时,差分群时延绝对值快速变大.最后,利用嫦娥3号奔月段同时发射两个DOR (Differential One-Ranging)信号的VLBI时延数据,计算差分相时延,初步展示了月球交会对接过程中同波束VLBI差分相时延的误差情况.     

OQ208, a non-ideal VLBI calibrator of flux density

OQ208 is used ore and more frequently as a VLBI calibrator of flux density. A VLBI image of OQ208 at 5 GHz is firstly provided in this paper. Its structure consists of a compact core and a component. The angular distance between the core and the component is about 7 milli-arcsecond; the position angle is −125 degrees. In principle, OQ208 is not an ideal VLBI calibrator of flux density. This paper provides the structural information for correcting the structure effect when OQ208 is to be used as a calibrator.     

Orbit determination of satellite “Tance 1” with VLBI data

The satellite “Tance 1” of the “Double-Star Program” is the first truly scientific experimentation satellite of China. Its orbit is the farthest so far launched in China, with a geocentric apogee reaching 78 thousand kilometers. The tracking of “Tance 1” and of more distant space targets, such as the lunar exploration craft, can be realized with the VLBI technique of radio astronomy. In order to test and verify the role which the VLBI technique plays in the lunar exploration program of China, Shanghai Astronomical Observatory organized the only 3 tracking stations in China (located at Shanghai, Urumqi and Kunming), to carry out test tracking of “Tance 1,” and used the time delay data obtained to determine the orbit of “Tance 1” over a two-day period, so providing a preliminary assessment of the possibility of VLBI orbit determination. The fitting error of the orbit so obtained is about 5.5 m in the time delay and about 2 cm/s in the delay rate (this for checking only), much better than is provided by the preliminary orbit (used merely for ensuring tracking) in which the corresponding figures are around 2 km and 15 cm/s. Further, if the orbit is determined by using both the time delay and time delay rate data (with weights according to their internal accuracies), then the residuals are 5.5 m in the time delay and 2 cm/s in the delay rate. For an appreciation of the true accuracy of the VLBI orbit determination, we used simulation data (of the observed two-day VLBI data) and found the results depended greatly on the error in the dynamic model of the satellite which, however, is difficult to assess, while the formal residuals are of the order of 1 kin in the delay and of cm/s in the delay rate. The simulation computation also indicates that a joint determination using both VLBI and USB data will have an improved accuracy.     

基于天文开源软件的卫星干涉测量数据处理系统

VLBI (Very Long Baseline Interferometry)技术观测卫星需要对干涉测量数据进行相关和后处理,通过相关、时延校准、条纹搜索,最终得到卫星的基线几何时延.基于天文开源软件建立起一套卫星干涉测量数据处理系统.该系统可工作在实时和事后两种状态,实现相关、中性大气、电离层、钟模型以及仪器硬件的时延校准、条纹搜索、生成基线时延和时延率序列.使用该系统处理北斗GEO (Geosynchronous Earth Orbit)卫星的干涉测量试验数据,得到了精度在1–2 ns量级的卫星基线时延序列.     

VLBI and VLA observations of intraday polarization variability in 0917+624 and 0954+658

Total intensity and polarization λ =6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 0917+624 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 0917+624: both the polarized flux and the polarization position angle varied substantially on time-scales of ∼5–10 h. There is evidence that the VLA polarization variations for 0954+658 occurred in an inner VLBI jet component, where the polarized flux varied by ∼30–40 per cent on time-scales of ∼2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previously identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment suggests that polarization IDV occurs frequently in at least some IDV sources.     

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.     

Prompt determination of universal time corrections in e-VLBI mode

The results of a prompt determination of the Universal Time corrections in the e-VLBI mode on the Quasar VLBI network in 2009–2011 are presented. For this purpose, the hardware-software tools for observational data transfer from the Svetloe, Zelenchukskaya, and Badary Observatories to the Correlation Processing Center (St. Petersburg) over fiber-optic communication lines in a mode close to real time using the specialized Tsunami protocol have been developed. The rms error in the UTC-UT1 corrections with respect to the combined IERS series is 60 μs.     

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.     

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.     

Measurement of Zeeman pattern with VLBI

Magnetic field is believed to play an important role in the evolution of compact HII regions where massive stars are formed. In fact, it exists very few methods to measure it. One of the best method used, is based on the Zeeman effect: under the effect of the magnetic field, a line can be split into two separated lines of opposite circular polarization. The separation in frequency between the lines is directly proportional to the intensity of the magnetic field. One of the difficulty of the method, is to made a correct identification of the Zeeman pairs. The very high resolution achievable by VLBI allows us to demonstrate whether the pairs are spatially coincident (as expected in true Zeeman pairs) or not. The OH emission traces the densest pockets of the neutral gas environment. In addition, because of its non zero electronic angular momentum, the OH radical is a natural magnetometer allowing us to determine both the intensity and the direction of the magnetic field in the outer layers of the compact HII regions. In this poster I reported on the possibilities of VLBI observations of OH masers in the first excited state (J = 5/2) lying at 6 GHz.     

Tidal deformations of the earth from VLBI observations

Based on an analysis of the VLBI observations performed in 1985?C2010 within the framework of international geodetic programs on global networks of stations, we have obtained statistically significant corrections to the parameters of lunisolar tides??the nominal complex Love/Shida numbers. The new integral (frequency-independent) values of these parameters (in 10^?4) are h ⁽⁰⁾ = (6113 ± 3) ? (33 ± 2)i, l ⁽⁰⁾ = (843 ± 1) ? (5 ± 2)i for a total tide; h ⁽⁰⁾ = (6106 ± 3) ? (10 ± 6)i, l ⁽⁰⁾ = (843 ± 1) ? (8 ± 1)i for diurnal tides; and h ⁽⁰⁾ = (6106 ± 3) ? (24 ± 3)i, l ⁽⁰⁾ = (843 ± 1) + (3 ± 1)i for semi-diurnal tides. We have detected a new effect of asymmetry in the horizontal tidal displacements in the direction of tectonic motions for 50 VLBI stations. We have determined upper limits for the influence of the frequency-dependent resonance effects whose estimation accuracy is limited by an abundance of close frequencies in their harmonic expansion. The influence of the transfer function for tides on the VLBI observations has turned out to be lower than the measurement accuracy. In future, positional GPS/GLONASS measurements are planned to be used to refine the resonance effects and the transfer function.