流动VLBI数据采集系统及其控制软件的设计与使用

结合我国流动VLBI观测站的研制与建设，介绍流动VLBI观测站的数据采集系统及其系统控制软件MOFS，在整个VIBI观测期间，MOFS软件控制数据采集系统的自动运转，并具有对系统参数的监测功能，长期的使用表明，流动VLBI观测站的数据采集系统及其控制软件很好地满足了我国流动VLBI观测的要求。     

基于VLBI观测的基线改正方法

针对甚长基线干涉测量(VLBI)技术中,测站坐标精度与高精度的VLBI宽带观测不相匹配的问题,该文提出一种利用VLBI本身观测结果分析测站坐标改正量的方法。利用观测得到的时延值扣除几何时延及各种已知误差项后,残余部分还存在由基线矢量误差引起的时延、两地原子钟同步误差和系统误差引起的时延,给出残余时延与基线矢量误差等的计算模型。采用国家授时中心VLBI2010系统观测数据,计算结果表明,吉林-喀什基线的基线长度改正量约为11.467 7cm,观测值与拟合值的对比表明这种方法的可行性。     

VLBI观测的电离层延迟改正模型研究

电离层是大气层中的一个电离区域，高度范围大约在60－1000km。电磁波信号穿越电离层时其传播速度会发生变化，传播路径也会略微发生弯曲，从而使信号的传播时间乘以在真空中的光速不等于信号源至测站的几何距离。对VLBI观测来讲，电离层引起的差异可达近百米百米。文中从电磁波的传播原理出发，讨论了信号传播速度和传播路径变化引起的VLBI观测延迟；对目前采用的各种电离层延迟模型进行了分析总结；并指出单频率VLBI观测应顾及高阶项和路径弯曲的影响或使用区域性电离层延迟改正模型。     

利用中国VGOS站开展国际联测的EOP精度仿真分析

甚长基线干涉测量(very long baseline interferometry, VLBI)是测量地球定向参数（earth orientation parameters,EOP）的主要空间测地技术之一,中国正在建设名为VLBI全球观测系统（VLBI global observing system, VGOS）的新一代测地VLBI站,通过国际联测优化站网构型是实现高精度EOP测量的必由之路。以3个中国VGOS站为核心站,通过引入2个国外站构建5站联合观测网,分析评估了不同站网构型的EOP测量能力。针对每个站网构型,通过调整4个约束条件的权重因子批量生成相应的观测纲要,采用蒙特卡洛仿真方法选择最优的观测纲要,评价指标为EOP解算值的可重复性。仿真结果表明,由中国站、南非哈特比站以及澳大利亚霍巴特站组成的网型EOP测量能力最强,相对于中国3站组成的网型,dUT1测量精度提高5.7倍,极移的X、Y分量的测量精度分别提高2.8倍和18.3倍。仿真结果可为后续开展高精度EOP组网观测提供参考依据。     

DCW－01型流动VLBI测量仪时频系统性能检测

我国自行研制的第一台DCW 0 1型流动VLBI测量仪已在昆明进行了试观测 ,文中介绍了该仪器的时频系统 ,并在观测实验中对其精度、稳定度作了检测。结果显示 ,所采用的时频系统 ,其精度及稳定度分别优于 1× 1 0 -12 和 1× 1 0 -14 ,较好地满足了流动VLBI测量仪时频系统的既定要求 ,能够精密、可靠地适用于流动VLBI的观测实验。最后对仪器的检测工作提出了一些改进和建议。     

微型激光测距与甚长基线干涉测量的天线旋转中心的直接解法

精确确定SLR (Satellite Laser Rainging)和VLBI (Very Long Baseline Interferometry) 天线的旋转中心是并址站归心基线测量的关键问题。本文利用两类约束条件建立旋转中心与观测标志之间的直接关系,第1类约束是SLR或VLBI上的观测标志绕其旋转轴旋转形成1个由平面和球面相割得到的平面圆,第2类约束是SLR或VLBI的旋转中心与其垂直轴旋转圆心在同一铅垂线上,与其水平轴旋转圆心在同一水平面上。根据这两类约束条件建立相应的条件方程,利用标志点观测值直接解算旋转中心的坐标及其协方差阵。利用我国2个GNSS (Global Navigation Satellite System) 与SLR或VLBI并址站的实测数据,求解了基于本文直接解法的空间归心基线。结果表明,与已有分步解的差值优于1mm。     

VGOS超宽带观测的首次UT1测量数据处理及分析

在国际甚长基线干涉测量（very long baseline interferometry, VLBI）大地测量与天体测量服务组织协调下,首次利用隶属于VLBI全球观测系统（VLBI global observing system, VGOS）的美国Kokee和德国Wettzell观测站及并置的传统VLBI观测站开展了世界时（universal time, UT1）联合测量试验,观测数据在上海VLBI中心进行了干涉处理。结果表明,VGOS超宽带观测系统的UT1测量精度约为7 μs,并置基线的传统S/X双频系统测量精度约为14 μs,VGOS系统的UT1解算结果优于S/X系统。通过试验建立了从相关处理、相关后处理到UT1参数解算的完整数据处理流程,验证了上海VLBI相关处理机的VGOS数据处理能力,为承担国内和国际VGOS观测数据的相关处理任务奠定了基础。     

VLBI技术CONT观测的高频ERP解算

国际VLBI测天测地服务机构(IVS)已组织了多次VLBI连续加密观测(CONT),提供了高精度连续的原始观测数据,在地球自转参数(ERP)的连续高频解算中起到积极的作用,揭示了地球自转高频变化的观测资料和理论模型之间的差异,有助于进一步解析其激发机制改进模型.这里使用VLBI资料处理软件系统OCCAM处理了CONT02,CONT05和CONT08数据,并进行ERP高频解算及频谱分析.从各次CONT观测的残差频谱中发现较强周期信号,反映了地球自转的特性.特别是CONT08残差频谱中存在明显的周日项信患,揭示了北半球夏季月份大气激发对地球自转的作用.     

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。     

昆明固置流动VLBI站的定向与定位

结合昆明固置流动VLBI站的特点，介绍了流动VLBI以动到一个的观测点后的定向与定位工作。其中，定向方法采用天文测量中的“北极星任意时角法”^[1]，定位方法采用单台GPS大偏心观测。在归心计算中我们参考了《全球定位系统（GPS）测量规范》^[2]，纠正了该规范存在一些问题，并结合实际应用，给出了一组实用的计算公式。     

VLBI技术的发展与展望

至今,VLBI技术经过迅猛发展,先后经历了以下三个发展阶段:传统VLBI、实时和准实时VLBI以及空间VLBI。本文分别介绍这三种VLBI技术的定义及其特点,通过对比分析其优缺点,展望VLBI技术的未来发展。     

Geodetic VLBI with an artificial radio source on the Moon: a simulation study

We perform extensive simulations in order to assess the accuracy with which the position of a radio transmitter on the surface of the Moon can be determined by geodetic VLBI. We study how the quality and quantity of geodetic VLBI observations influence these position estimates and investigate how observations of such near-field objects affect classical geodetic parameters like VLBI station coordinates and Earth rotation parameters. Our studies are based on today’s global geodetic VLBI schedules as well as on those designed for the next-generation geodetic VLBI system. We use Monte Carlo simulations including realistic stochastic models of troposphere, station clocks, and observational noise. Our results indicate that it is possible to position a radio transmitter on the Moon using today’s geodetic VLBI with a two-dimensional horizontal accuracy of better than one meter. Moreover, we show that the next-generation geodetic VLBI has the potential to improve the two-dimensional accuracy to better than 5 cm. Thus, our results lay the base for novel observing concepts to improve both lunar research and geodetic VLBI.     

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。     

用甚长基线干涉测量（VLBI）数据导出的板块运动参数

本文采用最近获得的ＶＬＢＩ基线变化率，解算了全球５０个ＶＬＢＩ站的站速度。通过对ＶＬＢＩ台站构造稳定性的分析，用位于板块稳定地区台站的站速度求解了欧亚（ＥＵＲＡ）、北美（ＮＯＡＭ）和太平洋（ＰＣＦＣ）三个主要板块之间的相对运动欧拉矢量，这是完全基于ＶＬＢＩ数据导出的板块运动模型，称为ＶＰ－ＭＭ１。通过ＶＰＭＭ１与地学板块运动模型的比较，分析了板块运动的稳定性，得出了几点初步结论。     

Simulations of VLBI observations of a geodetic satellite providing co-location in space

We performed Monte Carlo simulations of very-long-baseline interferometry (VLBI) observations of Earth-orbiting satellites incorporating co-located space-geodetic instruments in order to study how well the VLBI frame and the spacecraft frame can be tied using such measurements. We simulated observations of spacecraft by VLBI observations, time-of-flight (TOF) measurements using a time-encoded signal in the spacecraft transmission, similar in concept to precise point positioning, and differential VLBI (D-VLBI) observations using angularly nearby quasar calibrators to compare their relative performance. We used the proposed European Geodetic Reference Antenna in Space (E-GRASP) mission as an initial test case for our software. We found that the standard VLBI technique is limited, in part, by the present lack of knowledge of the absolute offset of VLBI time to Coordinated Universal Time at the level of microseconds. TOF measurements are better able to overcome this problem and provide frame ties with uncertainties in translation and scale nearly a factor of three smaller than those yielded from VLBI measurements. If the absolute time offset issue can be resolved by external means, the VLBI results can be significantly improved and can come close to providing 1 mm accuracy in the frame tie parameters. D-VLBI observations with optimum performance assumptions provide roughly a factor of two higher uncertainties for the E-GRASP orbit. We additionally simulated how station and spacecraft position offsets affect the frame tie performance.     

Monte Carlo simulations of the impact of troposphere,clock and measurement errors on the repeatability of VLBI positions

Within the International VLBI Service for Geodesy and Astrometry (IVS) Monte Carlo simulations have been carried out to design the next generation VLBI system (“VLBI2010”). Simulated VLBI observables were generated taking into account the three most important stochastic error sources in VLBI, i.e. wet troposphere delay, station clock, and measurement error. Based on realistic physical properties of the troposphere and clocks we ran simulations to investigate the influence of the troposphere on VLBI analyses, and to gain information about the role of clock performance and measurement errors of the receiving system in the process of reaching VLBI2010’s goal of mm position accuracy on a global scale. Our simulations confirm that the wet troposphere delay is the most important of these three error sources. We did not observe significant improvement of geodetic parameters if the clocks were simulated with an Allan standard deviation better than 1 × 10⁻¹⁴ at 50 min and found the impact of measurement errors to be relatively small compared with the impact of the troposphere. Along with simulations to test different network sizes, scheduling strategies, and antenna slew rates these studies were used as a basis for the definition and specification of VLBI2010 antennas and recording system and might also be an example for other space geodetic techniques.     

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.     

差分VLBI测量的一种实现方案

差分VLBI通过交替观测目标天体和参考天体,将共同的误差因素从观测量中扣除,能够实现高精度的相对定位,因而在深空探测中有重要作用。然而,差分VLBI高精度的实现要求目标天体和参考天体的角距很近,这大大限制了其应用。讨论设计了差分VLBI测量的一种实现方案,利用多颗参考源的观测内插出目标源的非几何时延修正,放宽了对目标源和参考源的角距限制。该方案在S波段对目标源非几何时延的修正精度可以达到1 ns水平。     

差分VLBI测量的一种实现方案

差分VLBI通过交替观测目标天体和参考天体,将共同的误差因素从观测量中扣除,能够实现高精度的相对定位,因而在深空探测中有重要作用.然而,差分VLBI高精度的实现要求目标天体和参考天体的角距很近,这大大限制了其应用.讨论设计了差分VLBI测量的一种实现方案,利用多颗参考源的观测内插出目标源的非几何时延修正,放宽了对目标源和参考源的角距限制.该方案在S波段对目标源非几何时延的修正精度可以达到1 ns水平.     

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.