我国VLBI观测网的射电源表

本文给出了上海、乌鲁木齐和昆明三个台站组成的VLBI网进行大地测量的射电源表，画出了这些射电源的天球分布图，计算了每颗射电源同时可被三个台站观测的时间图，利用它们可以编制观测计划表。     

天球参考架的稳定源选取及其最新实现

天球参考架是对理想的天球参考系的实现,其稳定性直接决定了所描述天体运动规律及某些地球物理现象的精度。基于1979-2007年期间全球IVS(international VLBI service)网近30年的观测资料,利用上海天文台的Calc_Solve系统对天测与测地VLBI资料进行解算和分析,得到一系列由射电源坐标(赤经和赤纬)构成的时间序列。通过对该时间序列的稳定性的检验和分析,制定选择稳定源的标准,得到一套稳定的河外射电源表列,最终实现由173颗稳定的定义源与135颗补充源构建的稳定天球参考架。     

一种新型的大地测量手段——卫星源射电干涉测地系统简介

一、一次有意义的试验1980年12月17日，美国麻省理工学院的C.C.Councelman与他的同事们进行了一次有意义的测量试验。他们利用便携式MITES系统（微型射电干涉仪终端）观测了Haysfack天文台附近三个测量标志间的边长。这次试验是利用全球定位系统（GPS）卫星讯号作为射电源进行干涉测量的。测量工作重覆进行了12天。每次观测5颗仰角在20°以上的卫星。没有丢掉一个数据，将全部数据进行了处理。每一天     

昆明站流动VLBI首次长距离实验观测

我国流动VLBI首次在昆明与上海佘山站组成长基线观测。本文对实验观测、射电源的选取、数据文件的生成以及数据处理等情况作了介绍，并对流动VLBI今后的观测提出了一些建议。     

FAST天线用于VLBI观测系统性能分析

讨论了射电天线用于天文观测和VLBI大地测量时的几项性能参数;列出了我国现有射电天线组成系统时的工作性能和观测精度;计算了利用我国现有射电天线与FAST天线组成基线进行观测时的系统性能以及所要求射电源的最低流量密度.由此可以看出FAST天线用于VLBI观测具有巨大潜力.     

FAST天线用于VLBI观测系统性能分析

讨论了射电天线用于天文观测和VLBI大地测量时的几项性能参数；列出了我国现有射电天线组成系统时的工作性能和观测精度；计算了利用我国现有射电天线与FAST天线组成基线进行观测时的系统性能以及所要求射电源的最低流量密度。由此可以看出FAST天线用于VLBI观测具有巨大潜力。     

流动VLBI选址探讨

本文结合流动VLBI参加的"地壳运动观测网络工程"和"探月工程",从建立和维护我国地心参考框架和深空探测两个角度讨论了流动VLBI站址的选择、以及所选站址对我国VLBI观测网的改善。提出了将流动VLBI站设在海南,这样既增加了南北基线长度,也扩大了可观测射电源的范围,同时在建立和维持我国地心参考框架方面也是较好的选择。     

最小二乘配置中粗差的可发现性和可区分性问题

研究最小二来配置中粗差与随机信号的区分性问题。从观测值残差和信号估值及其统计性质出发，根据高斯马尔可夫模型两个备选假设理论，给出了最小二来配置模型中观测粗差的可区分且可发现的表达式。     

河外射电源地面VLBI观测的相对论时延模型

重新研究了河外射电源地面VLBI观测的相时论时间延迟模型，给出了一个严格解析的表达式。根据公式可以得到数据处理中常用的Zhu-Groten模型、Shapiro模型和IERS推荐模型，详细地给出了时间延迟理论模型的计算步骤，介绍了各种时间系统的转换和使用方法。     

ASTRON—G卫星定轨精度和地面跟踪效率分析

结合最新数据,探讨了GNSS系统对最新的空间VLBI计划(VSOP2)--ASTRON-G卫星定轨情况、地面跟踪网对卫星的跟踪效率,以及地面观测站与卫星实现联合观测射电源的情况等,并给出了分析结果.     

Radio source instability in VLBI analysis

The source position time-series for many of the frequently observed radio sources in the NASA geodetic very long baseline interferometry (VLBI) program show systematic linear and non-linear variations of as much as 0.5 mas (milli-arc-seconds) to 1.0 mas, due mainly to source structure changes. In standard terrestrial reference frame (TRF) geodetic solutions, it is a common practice to only estimate a global source position for each source over the entire history of VLBI observing sessions. If apparent source position variations are not modeled, they produce corresponding systematic variations in estimated Earth orientation parameters (EOPs) at the level of 0.02–0.04 mas in nutation and 0.01–0.02 mas in polar motion. We examine the stability of position time-series of the 107 radio sources in the current NASA geodetic source catalog since these sources have relatively dense observing histories from which it is possible to detect systematic variations. We consider different strategies for handling source instabilities where we (1) estimate the positions of unstable sources for each session they are observed, or (2) estimate spline parameters or rate parameters for sources chosen to fit the specific variation seen in the position-time series. We found that some strategies improve VLBI EOP accuracy by reducing the biases and weighted root mean square differences between measurements from independent VLBI networks operating simultaneously. We discuss the problem of identifying frequently observed unstable sources and how to identify new sources to replace these unstable sources in the NASA VLBI geodetic source catalog.     

昆明站流动ⅥBI站坐标的首次确定

我国第一台流动甚长基线干涉测量（VLBI）设备经近十年的研制目前在昆明进行了试观测，本文介绍了实验观测及数据的获取情况。并对数据文件进行了处理。结果显示，在上海佘山25米天线至昆明流动站近2千公里的基线上，昆明流动站坐标精度优于5cm。基线相对精度优于10^-8。     

Effect of the selection of reference radio sources on geodetic estimates from VLBI observations

This paper evaluates the effect of the accuracy of reference radio sources on the daily estimates of station positions, nutation angle offsets, and the estimated site coordinates determined by very long baseline interferometry (VLBI), which are used for the realization of the international terrestrial reference frame (ITRF). Five global VLBI solutions, based on VLBI data collected between 1979 and 2006, are compared. The reference solution comprises all observed radio sources, which are treated as global parameters. Four other solutions, comprising different sub-sets of radio sources, were computed. The daily station positions for all VLBI sites and the corrections to the nutation offset angles were estimated for these five solutions. The solution statistics are mainly affected by the positional instabilities of reference radio sources, whereas the instabilities of geodetic and astrometric time-series are caused by an insufficient number of observed reference radio sources. A mean offset of the three positional components (Up, North, East) between any two solutions was calculated for each VLBI site. From a comparison of the geodetic results, no significant discrepancies between the respective geodetic solutions for all VLBI sites in the Northern Hemisphere were found. In contrast, the Southern Hemisphere sites were more sensitive to the selected set of reference radio sources. The largest estimated mean offset of the vertical component between two solutions for the Australian VLBI site at Hobart was 4 mm. In the worst case (if a weak VLBI network observed a limited number of reference radio sources) the daily offsets of the estimated height component at Hobart exceeded 100 mm. The exclusion of the extended radio sources from the list of reference sources improved the solution statistics and made the geodetic and astrometric time-series more consistent. The problem with the large Hobart height component offset is magnified by a comparatively small number of observations due to the low slewing rate of the VLBI dish (1°/ s). Unless a minimum of 200 scans are performed per 24-h VLBI experiment, the daily vertical positions at Hobart do not achieve 10 mm accuracy. Improving the slew rate at Hobart and/or having an increased number of new sites in the Southern Hemisphere is essential for further improvement of geodetic VLBI results for Southern Hemisphere sites.     

The impact of radio source structure on European geodetic VLBI measurements

European geodetic very long baseline interferometry (VLBI) sessions (also known as EUROPE sessions) have been carried out on a regular basis for the past 15 years to study relative crustal motions within Europe. These sessions are based on observations of extragalactic radio sources, which serve as distant fiducial marks to establish an accurate and stable celestial reference frame for long-term geodetic measurements. The radio sources, however, are not always point-like on milliarcsecond scales, as VLBI imaging has revealed. In this work, we quantify the magnitude of the expected effect of intrinsic source structure on geodetic bandwidth synthesis delay VLBI measurements for a subset of 14 sources regularly observed during the EUROPE sessions. These sources have been imaged at both X-band (8.4 GHz) and S-band (2.3 GHz) based on dedicated observations acquired with the European VLBI Network (EVN) in November 1996. The results of this calculation indicate that the reference source 0457+024 causes significant structural effects in measurements obtained on European VLBI baselines (about 10 picoseconds on average), whereas most of the other sources produce effects that are only occasionally larger than a few picoseconds. Applying the derived source structure models to the data of the EUROPE5-96 session carried out at the same epoch as the EVN experiment shows no noticeable changes in the estimated VLBI station locations.     

A VLBI Delay Model for Radio Sources at a Finite Distance

A relativistic delay model for Earth-based very long baseline interferometry (VLBI) observation of sources at finite distances is derived. The model directly provides the VLBI delay in the scale of terrestrial time. The effect of the curved wave front is represented by using a pseudo source vector K = (R ₁ + R ₂)/(R ₁ + R ₂), and the variation of the baseline vector due to the difference of arrival time is taken into account up to the second-order by using Halley’s method. The precision of the new VLBI delay model is 1 ps for all radio sources above 100 km altitude from the Earth’s surface in Earth-based VLBI observation. Simple correction terms (parallax effect) are obtained, which can also adopt the consensus model (e.g. International Earth Rotation and Reference Frames Service conventions) to finite-distance radio source at R > 10 pc with the same precision. The new model may enable estimation of distance to the radio source directly with VLBI delay data.     

New VLBI2010 scheduling strategies and implications on the terrestrial reference frames

In connection with the work for the next generation VLBI2010 Global Observing System (VGOS) of the International VLBI Service for Geodesy and Astrometry, a new scheduling package (Vie_Sched) has been developed at the Vienna University of Technology as a part of the Vienna VLBI Software. In addition to the classical station-based approach it is equipped with a new scheduling strategy based on the radio sources to be observed. We introduce different configurations of source-based scheduling options and investigate the implications on present and future VLBI2010 geodetic schedules. By comparison to existing VLBI schedules of the continuous campaign CONT11, we find that the source-based approach with two sources has a performance similar to the station-based approach in terms of number of observations, sky coverage, and geodetic parameters. For an artificial 16 station VLBI2010 network, the source-based approach with four sources provides an improved distribution of source observations on the celestial sphere. Monte Carlo simulations yield slightly better repeatabilities of station coordinates with the source-based approach with two sources or four sources than the classical strategy. The new VLBI scheduling software with its alternative scheduling strategy offers a promising option with respect to applications of the VGOS.     

天球参考系的基本理论和方法研究进展

天球参考系的研究在天体测量、大地测量等研究领域中占有重要的地位。随着配置有CCD的地面光学望远镜和空间望远镜观测精度的不断提高 ,以及其它空间大地测量技术 (VLBI、LLR、SLR和GPS等 )的发展 ,对天球参考系也提出了越来越高的要求。本文从天球参考系的基本理论和方法出发 ,详细地介绍了天球参考系目前的发展状况 ,指出乌克兰里克米亚天文台、上海天文台和IERS在计算河外射电源参考架时采用的小角旋转法公式是错误的 ,并进行了改正     

昆明固置流动VLBI站点归心

参考《全球定位系统（GPS）测量规范》之附录G归心元素测定，采用测GPS的方法，对固置在昆明的流动VLBI站进行了归心计算，在得到流动VLBI用GPS技术所测量的地心坐标同时，指出了参考文献[1]中存在的矛盾与错误，并结合实际应用，给出了一组实用的计算公式。     

The extension of the parametrization of the radio source coordinates in geodetic VLBI and its impact on the time series analysis

The radio sources within the most recent celestial reference frame (CRF) catalog ICRF2 are represented by a single, time-invariant coordinate pair. The datum sources were chosen mainly according to certain statistical properties of their position time series. Yet, such statistics are not applicable unconditionally, and also ambiguous. However, ignoring systematics in the source positions of the datum sources inevitably leads to a degradation of the quality of the frame and, therefore, also of the derived quantities such as the Earth orientation parameters. One possible approach to overcome these deficiencies is to extend the parametrization of the source positions, similarly to what is done for the station positions. We decided to use the multivariate adaptive regression splines algorithm to parametrize the source coordinates. It allows a great deal of automation, by combining recursive partitioning and spline fitting in an optimal way. The algorithm finds the ideal knot positions for the splines and, thus, the best number of polynomial pieces to fit the data autonomously. With that we can correct the ICRF2 a priori coordinates for our analysis and eliminate the systematics in the position estimates. This allows us to introduce also special handling sources into the datum definition, leading to on average 30 % more sources in the datum. We find that not only the CPO can be improved by more than 10 % due to the improved geometry, but also the station positions, especially in the early years of VLBI, can benefit greatly.