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.     

脉冲微波式氢原子钟初期电路设计

上海天文台时间频率研究室以原有被动型氢钟物理部分为基础,开展了脉冲微波式氢原子钟的研究。设计电路产生2个相干微波脉冲,连续激励氢原子跃迁,模拟双腔共振,使氢原子发生Ramsey干涉,压缩氢原子跃迁谱线宽度,以期提高氢原子钟短期稳定度指标。具体做法为:用DDS产生扫频电路,混频生成1.420 405 GHz激励信号后,再用CPLD产生脉冲时序控制数字衰减器,将激励信号衰减为脉冲形式,激励氢原子发生Ramsey干涉,导出微波信号并进行相关处理就可以产生Ramsey条纹。已观测到Ramsey干涉条纹,其中心峰宽度为1.2 Hz,相比传统被动型氢原子钟压缩了60%。     

流动VLBI站氢原子钟的研制与性能测试

氢脉泽是射电天文所必需的频率标准.文中对为流动vLBI站研制的新一代氢脉泽的物理结构特点进行了讨论，包括对腔泡结构、磁屏蔽、原子束光学系统等.同时性能测试表明在80秒到1天之间频率稳定度优于1×10     

Research on the Methods of Performance Monitoring and Evaluation for Active Hydrogen Maserstwo

Active hydrogen maser is the main frequency standard for establishing and maintaining the time scale. It has the characteristics of high short-term stability and low phase noise. At present, it plays an increasingly important role in the international atomic time (TAI) and various local time scales. Firstly, in combination with the internal state parameters of an active hydrogen maser, the correlation between the internal state parameters and the comparison data of a hydrogen maser is analyzed, and a method for the performance monitoring of a hydrogen maser is proposed. Secondly, according to the characteristics of hydrogen maser performance, a method for evaluating the performance of a hydrogen maser is given. The hydrogen maser performance includes mainly two aspects, namely the frequency stability and predictability. The performances of two types of active hydrogen masers (CH1-75 and MHM-2010) are evaluated by this method. The correlation analysis of the atomic clock state parameters and comparison data shows that the state parameter monitoring can effectively predict the variation of the hydrogen maser performance. The evaluation results of atomic clock frequency stability and predictability show that the atomic clock with a higher medium-and-long term frequency stability has a better predictability. There are two methods for the predictability evaluation, one is based on the data published by BIPM (Bureau International des Poids et Measures), and another one is based on the quadratic model. Both methods are consistent with the weights published by BIPM. Therefore, the two methods can be used as a quantitative method to evaluate the predictability of a hydrogen maser.     

Electron content near the lunar surface using dual-frequency VLBI tracking data in a single lunar orbiter mission

In VLBI observations of Vstar, a subsatellite of the Japanese lunar mission SELENE, there were opportunities for lunar grazing occultation when Vstar was very close to the limb of the Moon. This kind of chance made it possible to probe the thin plasma layer above the Moon's surface as a meaningful by-product of VLBI,by using the radio occultation method with coherent radio waves from the S/X bands.The dual-frequency measurements were carried out at Earth-based VLBI stations. In the line-of-sight direction between the satellite and the ground-based tracking station where VLBI measurements were made, the effects of the terrestrial ionosphere, interplanetary plasma and the thin lunar ionosphere mixed together in the combined observables of dual-frequency Doppler shift and phase shift. To separate the variation of the ionospheric total electron content(TEC) near the surface of the Moon from the mixed signal, the influences of the terrestrial ionosphere and interplanetary plasma have been removed by using an extrapolation method based on a short-term trend. The lunar TEC is estimated from the dual-frequency observation for Vstar from UT 22:18to UT 22:20 on 2008 June 28 at several tracking stations. The TEC results obtained from VLBI sites are identical, however, they are not as remarkable as the result obtained at the Usuda deep space tracking station.     

Tropospheric Delay from VLBI and GNSS Measurements

Using an updated version of the QUASAR software package developed at the Institute of Applied Astronomy of the Russian Academy of Sciences, we have processed the VLBI observations within the international CONT14 program (May 6–20, 2014), in which a global network of 17 stations was involved (a total of ~250 000 observations). The package update concerned the optimization of data structure and the refinement of stochastic models for the random variations in wet tropospheric delay and atomic clock difference. The main goal of this paper is to compare the VLBI determinations of the tropospheric delay with its independent determinations using global navigation satellite systems (GNSS). We show that both these determinations agree well between themselves only in the case of a global analysis of the VLBI observations, where the VLBI station coordinates are also refined, along with the tropospheric delay and the clock synchronization and Earth orientation parameters. If, alternatively, the station coordinates are insufficiently accurate and are not refined from VLBI observations, then it is appropriate not to determine the tropospheric delay from these observations, but to take it from the publicly accessible independent GNSS data. However, this requires that the VLBI and GNSS techniques operate simultaneously at a common observing site. We have established the shortcomings of the universally accepted method of stabilizing the global solution associated with the absence of a criterion for choosing reference stations and radio sources. Two ways of their elimination are proposed: (i) introducing a coordinated list of weight factors for the errors in the coordinates of such stations and sources into the stabilization algorithm and (ii) adopting a coordinated list of stations and sources the refinement of whose coordinates is not required at all for a certain time.     

空间VLBI的天体脉泽观测研究

在本文中较全面地述了天体脉泽源的ＶＬＢＩ观测方法，简单地提及ＶＬＢＩ观测研究的新进展，着重述评空间ＶＬＢＩ用脉泽的观测，并提及了地面ＶＬＢＩ的一些预备观测情况。     

被动型氢原子钟原子跃迁谱线特性分析

被动型氢原子钟原子跃迁谱线特性直接决定了整机的性能指标。本文理论分析了影响原子跃迁谱线特性的各项因素,并对3个主要影响因素进行了实验研究。     

脉冲星对氢原子钟的频率驾驭算法研究

氢原子钟具有较高的短期稳定度,将其作为主钟可在短期内产生高精度的本地时间信号.但氢钟存在频率漂移现象,导致其长期稳定度较差,从而影响本地时间的准确性.毫秒脉冲星自转高度稳定,借助于其长期稳定度高的特性,可定期实现对氢原子钟的频率驾驭,并对实时信号加以控制.首先分析了国际脉冲星计时阵(International Pulsar Timing Array,IPTA)第二批发布数据中四颗毫秒脉冲星的稳定度随时间的变化,同时采用哈达玛方差分析了中国科学院国家授时中心(National Time Service Center,NTSC)一台氢钟的频率稳定性能,最终给出了利用脉冲星驾驭氢原子钟频率的方法.     

氢原子频标的小型化

小型化是目前氢钟的发展方向之一，氢钟的小型化主要是谐振腔的改进，它的结构尺寸决定了氢钟的最小尺寸和重量，目前要用的小型腔主要有电极负载腔，磁控管腔，介质负载腔和TE111腔，新型的谐振腔虽然减小了体积减轻的重量，但量由于它本身已经很难满中立起振条件，所以在伺服电路上也不同于主动型氢钟，根据电路原理的不同，将小型化的氢钟分为反馈振荡型和被动型，反馈振荡型氢钟使激射器工作在反馈振荡状态，通过正反馈维持激射器振荡，被动型氢钟的激射器工作在振荡阈值之下，其作用与揩振放大器相似。     

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．     

数字调频和数字伺服在被动型氢原子钟上的应用

简单介绍了被动型氢原子钟的组成及原理,阐述了基于数字调频和数字伺服的电子电路在被动型氢原子钟上的应用(目的是改善钟性能),并给出了设计的最终测试结果及其分析,数据表明该系统的稳定度比原有系统有很大提高。为了进一步改善钟性能,又对伺服系统提出了以数字信号处理器(DSP)和现场可编程门阵列(FPGA)为主体的新的方案。     

空间VLBI的研究现状

评述了过去近三十年中空间VLBI的发展．其内容包括空间VLBI的简短发展历史,空间VLBI与地面VLBI的不同、空间VLBI的目前状况和空间VLBI的未来展望。着重介绍了作为首次空间VLBI的VSOP的各个方面情况,并给出了VSOP的连续谱、谱线普查样本,供参考。     

Peculiarities of processing VLBI observations of pulsars

We determined the features of pulsars that were disregarded in standard amplitude-calibration procedures for VLBI observations. We suggest additional amplitude-calibration methods. These methods were used to process the VLBI observations of the pulsar PSR B0329+54 carried out with the HALCA ground—space interferometer. Data from the space radio telescope are corrected for a nonuniform reception band. We estimated the diameter of the scattering disk for this pulsar at a frequency of 1600 MHz: \( < 1\mathop .\limits^{''} 8 \times 10^{ - 3}\).     

SOHM-4型氢原子钟的设计改进与初步性能

氢原子钟是一种最稳定的 (除极短测量时间间隔之外 )频率标准 ,但是环境温度变化及微波谐振腔老化会引起原子钟输出频率的变化 ,从而导致氢原子钟长期性能变差。为了减小这些影响 ,可借助一种自动调谐器来确保谐振腔的频率始终工作在所需的频率上 ,并采用新的温度控制系统来改善氢原子钟的长期性能。针对这些年来许多氢钟出现的有关问题 ,上海天文台在借鉴国外氢钟实验室经验的基础之上 ,对原有氢钟进行了技术改造 ,并为国家授时中心研制了SOHM - 4型氢原子钟。对该型氢原子钟技术改造特点作了介绍 ,并给出了期望的性能指标及初步的测试结果     

一种新的守时型原子钟综合时间尺度方法研究

在这篇文章中, 提出了一种基于改进的指数平滑和Vondrak_Cepek联合平滑的氢铯综合时间尺度产生方法. 以最小误差方法为理论基础, 动态估计氢原子钟频率漂移参数, 提升氢原子钟钟差预测准确度; 基于改进的二次指数平滑产生氢原子钟组时间尺度、加权平均方法产生铯原子钟时间尺度, 同时设计Vondrak_Cepek滤波器以结合两类时间尺度长短期稳定度优势, 提升综合时间尺度性能. 实验结果表明, 所提方法产生的氢铯综合时间尺度时稳可达1.60x10^-15,天稳可达3x10^-15,优于ALGOS、AT1和Kalman滤波3种经典方法产生的时间 尺度性能.     

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.     

ALSEP-quasar differential VLBI

A program of ALSEP-Quasar Very Long Baseline Interferometry (VLBI) is being carried out at the Jet Propulsion Laboratory. These observations primarily employ a 4-antenna technique whereby simultaneous observations with two antennas at each end of an intercontinental baseline are used to derive the differential interferometric phase between a compact extragalactic radio source (usually a quasar) and a number of ALSEP transmitters on the lunar surface. A continuous ALSEP-quasar differential phase history over a few hour period will lead to milliarcsecond angular accuracy in measuring the lunar position against the quasar reference frame if suitable calibration measurements are obtained. Development of this application of the 4-antenna technique has been underway at JPL for more than a year and is now producing high quality data utilizing Deep Space Network (DSN) stations in Australia, Spain, and Goldstone, California as well as the STDN Apollo station at Goldstone. These high accuracy observations are of value to tie the lunar ephemeris to a nearly inertial extragalactic reference frame, to test gravitational theories, and to measure the Earth-Moon tidal friction interaction.This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.     

环月飞行器精密定轨的模拟仿真

以中国正在实施的探月计划“嫦娥1号”工程为背景，分析了在中国联合S波段(USB)测控网和甚长基线射电干涉(VLBI)跟踪网的现有空间分布、观测精度水平下的环月飞行器精密定轨．采用的方法是模拟仿真计算，即首先模拟观测数据，然后在计入各误差源的影响后进行求解，并对解算结果进行比较．模拟仿真的工具是美国宇航局哥达德飞行中心的空间数据分析软件系统GEODYN．环月飞行的主要误差源是月球重力场，为此首先讨论了目前精度最高的月球重力场模型JGL165P1的(形式)误差．在模拟了测距、测速以及VLBI的时延、时延率数据后，计入月球重力场的误差进行精密轨道确定．定轨时采用了减缩动力学(reduced dynamic)方法，即选用合适的经验加速度参数吸收重力场误差对定轨的影响．结果表明对于一个不将月球重力场作为主要科学目标的探月计划(如“嫦娥1号”)，减缩动力学方法是一个简单、有效地提高环月飞行器定轨精度的方法．     

Space–VLBI observations of the OH maser OH 34.26+0.15: low interstellar scattering

We report on the first space–VLBI observations of the OH 34.26+0.15 maser in two main-line OH transitions at 1665 and 1667 MHz. The observations involved the space radio telescope on board the Japanese satellite HALCA and an array of ground radio telescopes. The map of the maser region and images of individual maser spots were produced with an angular resolution of 1 mas, which is several times higher than the angular resolution available on the ground. The maser spots were only partly resolved and a lower limit to the brightness temperature     was obtained. The maser seems to be located in the direction of low interstellar scattering, an order of magnitude lower than the scattering of a nearby extragalactic source and pulsar.