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。     

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。     

VGOS天线地平遮挡角计算及其对UT1测量的影响EI北大核心CSCD

上海新建的2台VGOS(VLBI global observing system)天线被其附近原有的大天线遮挡,为了开展VGOS站网的联合观测,提出了一种基于天线几何位置关系的地平遮挡角计算方法。根据台站相对位置、天线口径及天线参考点到边缘点的距离等参数,即可得到VGOS天线的地平遮挡角。天马VGOS天线的最大遮挡角为22.3°,遮挡天区面积占5°以上天区面积的比值为2.19%;佘山VGOS天线的最大遮挡角为23.4°,遮挡天区面积占5°以上天区面积的比值为3.14%。通过在观测纲要中添加天线的地平遮挡角信息,成功实施了VGOS站网的联测。利用VieVS软件仿真分析了天马-维泽尔基线上VGOS天线的UT1测量精度,结果表明,天马VGOS天线的地平遮挡对UT1的测量精度没有显著的影响。     

VLBI观测双线极化条纹拟合方法

提出了一种可用于下一代VLBI观测系统(VGOS)的双线极化条纹拟合方法。现有的VLBI观测模式采用的是右圆极化(RCP),而VGOS系统采用的是双线极化。本文方法包括校正和组合条纹拟合两部分。校正部分选择一颗强源作为参考源,分别得到不同极化方式下的通道时延及相位校正数据,用于目标源的校正。组合条纹拟合部分将4种极化分量的可见度数据组合成伪Stokes分量,通过搜索差分星位角使伪Stokes分量的幅值达到最大,从而获得最终的时延观测量。与单极化条纹拟合相比,组合极化获得的条纹具有更高的信噪比(SNR)及更小的条纹相位弥散度。     

ΔVLBI用于“嫦娥一号”地月转移轨道段定轨及EOP解算

推导了基于相对论时空理论的"嫦娥一号"地月转移轨道段差分VLBI(ΔVLBI)的数学模型,在此基础上利用"嫦娥一号"实测的VLBI时延观测量和模拟的河外射电源时延观测量组成了ΔVLBI时延观测量,在参数最优先验精度下解算了不同弧段长度的"嫦娥一号"轨道及地球定向参数(EOP)等未知参数,并根据各参数的解算精度及外符合程度确定了最优观测弧段长度,并分析了该条件下的参数解算精度。     

上海佘山VLBI站的钟差补偿精度分析

甚长基线干涉测量(very long baseline interferometry, VLBI)台站钟差的补偿误差将导致VLBI时延观测量参考历元的偏离,进而对世界时(universal time,UT1)的准确估计产生影响。台站钟差包含格式器钟差和设备时延,设备时延是影响钟差补偿精准度的关键因素。上海佘山VLBI站是全球测地参考框架的基准站,有必要评估该站的钟差补偿精准度。利用该站2002—2017年国际测地观测数据的相关处理结果,分析了相关处理机对佘山站钟差的补偿精度。结果表明,不同VLBI相关处理机对佘山站模拟终端的设备时延补偿具有较好的一致性和长期稳定性,满足UT1测量精度需求,但是上海相关处理机对佘山站数字化终端的设备时延补偿存在大于1μs的系统差,有必要进一步改进钟差搜索方法和提高钟差补偿精度。     

Ka波段天测与测地VLBI观测的利弊分析

分析了天测与测地甚长基线干涉测量（very long baseline interferometry,VLBI）观测频率设置的必要性,重点分析了Ka波段观测的利弊。相比于X波段,Ka波段观测的优点包括河外源更致密、核移效应更小,更有利于提高射电天球参考架的实现精度以及与天体物理学全球天体测量干涉仪（global astrometric interferometer for astrophysics,Gaia）计划准惯性光学参考架的连接精度。在深空探测方面的优点表现为遥测数据率高,VLBI跟踪测量精度高,更有利于减小大气电离层和太阳等离子体对测量时延的不利影响。缺点包括河外源相对较弱,天线反射面精度和指向精度要求较高,大气吸收和辐射效应的影响更大等,但是这些因素所造成的系统灵敏度的降低有望通过高数据率采样而得到补偿。结合我国VLBI技术现状提出了发展建议。     

VLBI和GPS观测联合解算地球自转参数和日长变化

利用GAMIT软件处理了2005年9月、2006年1月和2月每天17个IGS国际GNSS服务(International GNSS Service,IGS)站的GPS观测数据,获得了地球自转参数(Earth rotation parameter,ERP)和日长变化(ΔLOD),并与IGS综合解进行了对比。利用OCCAM 6.2软件处理了相同时间内的甚长基线干涉测量(very long baseline interferometry,VLBI)观测数据,将解算结果与国际VLBI服务(International VLBI Service,IVS)结果进行了对比。分别采用基于内符合精度和IERS 08C04序列的定权方法对VLBI解算结果与GPS解算结果进行了加权平均,获得了VLBI和GPS技术对ERP和ΔLOD的协议结果。研究结果表明,VLBI解算结果与采用的插值方法有较大的关系,基于IERS 08C04序列的加权平均方法达到了利用VLBI解算结果对GPS解算结果进行修正的目的。VLBI和GPS技术的联合弥补了VLBI观测数据密度不够和GPS解算结果不稳定的缺陷,使解算结果的稳定性和可靠性有所提高。     

VLBI在GEO卫星精密定轨中的应用

影响GEO轨道确定精度的原因主要有两方面:一是高轨卫星的几何跟踪条件受到局部测轨网的限制;二是卫星相对于地面的动力学约束信息较弱。利用一个针对GEO卫星的简化动力法定轨的协方差分析模型,研究了联合测距跟踪网和甚长基线干涉测量(VLBI)对定轨精度的改善情况。指出测距系统的校正误差是常规测距跟踪网定轨精度的主要误差源;当附加一条东西向VLBI基线时,仅利用不定期的少量VLBI高精度数据就能够显著改善测距偏差对轨道的影响,从而保证了卫星的整体位置解算精度。     

关于脉冲星参数的我国VLBI网测量

根据毫秒脉冲星计时测量和深空导航应用的基本原理,分析了对脉冲星位置、自行和视差等天体测量参数的VLBI测量精度需求,讨论了我国VLBI网的现有观测能力和不足及可能的改进途径,包括天线后端配置、时钟同步和数据互相关处理等。     

Ultra-rapid earth rotation determination with VLBI during CONT11 and CONT14

We present earth rotation results from the ultra-rapid operations during the continuous VLBI campaigns CONT11 and CONT14. The baseline Onsala–Tsukuba, i.e., using two out of the 13 and 17 stations contributing to CONT11 and CONT14, respectively, was used to derive UT1-UTC in ultra-rapid mode during the ongoing campaigns. The latency between a new observation and a new UT1-UTC result was less than 10 min for more than 95% of the observations. The accuracy of the derived ultra-rapid UT1-UTC results is approximately a factor of three worse than results from optimized one-baseline sessions and/or complete analysis of large VLBI networks. This is, however, due to that the one-baseline picked from the CONT campaigns is not optimized for earth rotation determination. Our results prove that the 24/7 operation mode planned for VGOS, the next-generation VLBI system, is possible already today. However, further improvements in data connectivity of stations and correlators as well in the automated analysis are necessary to realize the ambitious VGOS plans.     

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.     

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.     

基于联邦卡尔曼滤波的月球车联合定位方法

针对月球车定位,设计和推导了一种基于甚长基线干涉测量（very long baseline interferometry,VLBI）和天文导航系统（celestial navigation system,CNS）相结合的月球车联合定位方法,并采用联邦卡尔曼滤波来实现位置信息的最优估计以增强系统的可靠性和容错能力。最后,通过嫦娥-3号（CE-3）实测数据的解算证明了该方法较使用最小二乘法进行联合解算和单独采用VLBI方法定位,可以获得更高的月球车定位精度。同时,也有效地保障了月球车定位的可靠性和稳定性。     

A comparison of GPS, VLBI and model estimates of ocean tide loading displacements

In recent years, ocean tide loading displacements (OTLD) have been measured using the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). This study assesses the accuracy of GPS measurements of OTLD by comparison with VLBI measurements and estimates derived from numerical ocean tide models. A daily precise point positioning (PPP) analysis was carried out on ∼11 years of GPS data for each of 25 sites that have previous OTLD estimates based on data from co-located VLBI sites. Ambiguities were fixed to integer values where possible. The resulting daily estimates of OTLD, at eight principal diurnal and semi-diurnal tidal frequencies, were combined to give GPS measurements of OTLD at each site. The 3D GPS and VLBI measurements of OTLD were compared with estimates computed (by convolution with Green’s functions) from five modern ocean tide models (CSR4.0, FES2004, GOT00.2, NAO99b and TPXO6.2). The GPS/model agreement is shown to be similar to the VLBI/model agreement. In the important radial direction, the GPS/model misfit is shown to be smaller than the VLBI/model misfit for seven of the eight tidal constituents; the exception being the K2 constituent. Fixing of GPS carrier-phase ambiguities to integer values resulted in a marginal improvement to the GPS/model agreement. Statistically, it is shown there is no significance to the difference between the fit of the GPS and VLBI measurements of OTLD to modelled values. Equally, differences in fit of either the complete set of GPS or VLBI estimates to the five sets of model-derived values cannot be identified with statistical significance. It is thus concluded that, overall, we cannot distinguish between GPS and VLBI measurements of OTLD, and that at the global scale, present ocean tide models are accurate to within the current measurement noise of these techniques.     

VLBI在探月卫星定位中的应用分析

中国实施的"嫦娥"探月工程中,探月卫星的定轨测控系统由我国现有的S频段航天测控网(USB)和甚长基线干涉测量(VLBI)系统组成。系统中,VLBI技术主要为绕月卫星定轨提供卫星的角位置。本文分析了在探月项目中,VLBI单点定位的必要性。探讨了VLBI技术用于探月卫星单点定位的基本原理及其实现方法。通过算例对模拟数据进行处理,检验了方法的正确性。对结果进行分析,得出一些结论。     

ERP精度对“嫦娥一号”差分VLBI定位精度的影响

推导了差分VLBI（ΔDOR）用于月球探测器定位的数学模型,利用嫦娥一号实测数据对探测器的位置和地球自转参数（ERP）同时进行了解算,并分析了ERP先验精度对定位精度的影响。     

深空探测器同波束相位参考成图相对定位方法

针对我国后续深空探测任务中高精度相对定位需求,提出了采用天文同波束相位参考成图技术进行深空探测器相对定位的方法。首先建立适用于深空探测器信号特点的同波束相位参考成图相对定位模型,然后分析信号带宽和UV覆盖两个因素对测量精度的影响。最后利用我国VLBI观测网(CVN)在2013年15、20和21日接收的嫦娥三号着陆器和玉兔号巡视器下行同波束数据,处理得到了巡视器在5个环拍点相对着陆器的位置,验证了本文方法的可行性及其对探测器下行信号的强适应性。与视觉定位结果的对比表明,巡视器相对着陆器定位精度达到米级。