Preliminary Analysis of BDS-3 Data Based on iGMAS

The Beidou Navigation Satellite Demonstration System and Beidou Regional Satellite Navigation System are accomplished, and the Beidou Global Satellite Navigation System (BDS-3) is under construction. Nineteen networking satellites have been launched by the end of November 2018. To assess the new and old signals of new networking satellites, the data quality is analyzed from the observation noise and pseudo-range multipath two aspects based on the data of iGMAS stations from 18th to 28th May 2018. The accuracies of BDS-3/GPS combined orbit determinations with B1I/B3I and B1C/B2a are compared. The results show that the data quality of the new signal B2a is similar to that of B1I/B3I, and the data quality of B1C is slightly worse than that of B1I/B3I. Comparing the orbit and clock correction accuracies of overlapped arcs (48 hours), the results with B1C/B2a are slightly better than those with B1I/B3I. The averaged 3D position accuracies (3D RMSs) of precise orbit determinations with B1C/B2a and B1I/B3I are around 35 cm. The RMS values of clock corrections are less than 0.5 ns.     

BDS-3新频点信号双频精密单点定位精度分析

北斗卫星导航系统(BeiDou navigation satellite System, BDS)已于2020年7月正式建成并开通, 北斗三号(BDS-3)在旧信号B1I和B3I的基础上, 增加了B1C、B2a新信号. 为了全面评估BDS-3的新信号B1C、B2a的定位性能, 试验了GPS (Global Positioning System)、BDS-3、BDS-2/BDS-3新旧信号的定位性能和BDS系统不同频点与GPS组合定位性能, 对BDS (B1I+B3I、B1C/B2a)+GPS (L1+L2)组合静态PPP (Precise Point Positioning)定位性能进行分析, 并与单卫星系统对比分析. 试验结果表明: BDS-3 (B1C/B2a)在East (E)、\lk North (N)、Up (U)方向的定位精度优于1.25cm、0.89cm、1.67cm, BDS-3新旧频点在E、N方向上定位精度与GPS L1/L2在同一水平上, U方向上新频点定位精度高于GPS L1/L2和BDS-3旧频点, 较旧频点定位精度提升了34.2%, 新频点收敛时间25.9min比旧频点提升了12.7%; 相较于BDS、GPS单系统, 组合系统BDS/GPS定位精度和收敛时间有了明显的提高, BDS-3 (B1C/B2a)+GPS在E、N方向上与BDS-3 (B1I/B3I)+GPS定位精度相当, 在U方向上定位精度前者较后者有了明显的提升, 提升了17.2%, 组合系统新频点收敛时间20.1min比旧频点提升了17.6%.     

电离层延迟改正模型综述

电离层延迟改正模型通常可以分为广播星历用的预报模型、广域差分用的实时模型、后处理模型3类,不同应用要求需要选择不同的模型。主要比较分析了几种常用的电离层延迟改正模型: 用于广域差分中生成格网模型的三角级数模型、多项式模型、低阶球谐函数模型等都可以获得很好的改正效果,且这3种模型基本上是等价的;电离层延迟谐函数展开模型可以用来分析电离层长时间系列的变化特征;国际电离层参考模型IRI的改正精度一般可以达到60％的效果;而GPS 星历采用的Klobuchar模型的参数设置存在一些不足,对此提出了一些改进措施。     

An ionosphere-plasmasphere global electron density model

Global Positioning System (GPS) satellites now provide a major source of data used in the study of the ionized regions that surround the Earth. With the greater use of GPS satellites in this role, understanding the effect the plasmasphere has on GPS radio signals has become increasingly important. The plasmasphere, with similar properties to the underlying ionosphere, can potentially contribute a significant fraction of the overall ionospheric error in radio signals transmitted from a GPS satellite to ground or space based receivers. A global plasmasphere-ionosphere electron density model has been developed to investigate this contribution. A variety of output examples that the model can produce are shown.     

基于1Mcps/s码速率的亚欧卫星双向时间比对性能分析

卫星信道租赁费是目前卫星双向时间传递(Two-Way Satellite Time and Frequency Transfer, TWSTFT)的主要成本之一.在2017年5月以前,参与UTC (Coordinated Universal Time)计算的亚洲-欧洲实验室之间进行Ku波段卫星双向时间频率传递一直使用2.5 Mcps/s码速率,带宽为2.5 MHz的伪随机码.为了在不影响时间频率传递性能的前提下降低成本,在欧亚间首次尝试采用1 Mcps/s码速率,带宽为1.7 MHz的伪随机码,进行亚欧卫星双向时间传递.并使用已校准的GPS PPP (Global Position System Precise Point Positioning)链路为双向链路进行间接校准.选择2018年12月的TWSTFT链路数据,分析链路性能发现,通过ABS-2A卫星,使用1 Mcps/s码速率构建的卫星双向时间比对链路的日频率稳定度达到10~(-15),时间稳定度优于0.3 ns.与已校准的GPS PPP链路数据进行验证分析,结果表明,使用1 Mcps/s码速率进行超长距离卫星双向时间传递与已校准的GPS PPP时间传递结果一致,与传统手段相比,其系统造价低,时间传递性能可以满足国际原子时计算的需求.     

大口径射电望远镜电磁兼容评估技术研究

电磁兼容性是设备或系统的重要性能指标, 也是保障系统的工作效能和提高系统可靠性的重要因素. 大口径射电望远镜运行阶段, 台址周围无线电业务及内部潜在的电磁干扰会降低观测系统灵敏度、影响天文观测的质量. 本论文针对拟建的新疆110 m全向可动射电望远镜(Qi Tai raido Telescope, QTT)开展了系统电磁兼容评估技术及控制方法研究, 具有重要的工程应用价值. 首先, 依据现有电波环境测量方法的不足, 深入分析了仪器设备的关键参数配置方法及测量时间计算方法, 采用Y因子法校准测量数据, 提出一种准实时电波环境测量方法. 面向高重复性宽带频谱, 分析了宽带频谱信号和噪声特征, 结合标准差理论, 提出一种基于邻值比较的信噪分离方法, 并采用邻值统计方法优化关键参数, 提高信噪分离精度. 针对QTT台址, 开发了自动化电波环境监测系统, 该系统6 GHz以下频段系统增益大于40 dB, 系统噪声系数小于2 dB, 测量不确定度小于1.49 dB, 具有极高的系统灵敏度和测量精度; 分析了频谱监测数据流, 设计了基于HDF5 (Hierarchical Data Format version 5)的数据存储格式, 开发了自动化电波环境测量和监控软件及数据处理软件. 依据QTT台址长期监测数据, 评估分析了台址电磁环境、主要干扰源特征及其影响. 其次, 提出大口径射电望远镜馈源口面干扰电平限值量化方法, 建立了基于台址地形的电波传播模型, 分析了现有电波传播模型的优缺点及适应性, 结合QTT台址实际地形及地质特征, 采用Longley-Rice和Two-Ray电波传播模型, 预测分析了QTT台址潜在干扰区域电磁干扰达到射电望远镜的电波路径衰减, 结合大口径射电望远镜天线增益量化方法, 提出设备所在位置干扰电平限值量化方法, 运用该方法对QTT台址潜在干扰区域的干扰电平限值进行量化. 依据设备所在位置干扰电平限值, 调研分析了国内外军用、民用电磁兼容测量标准, 结合电磁干扰对射电天文观测的影响, 提出一种大口径射电望远镜电磁兼容控制方法, 解决了现有电波暗室测量系统无法直接测量评估电子设备电磁兼容的问题, 该电磁兼容控制方法计划应用于QTT建设及运行阶段, 确保系统拥有良好的电磁兼容性. 最后, 依据QTT台址潜在干扰区域干扰电平限值, 结合典型电子设备电磁辐射频谱, 分析了QTT电磁兼容设计需求, 提出电磁兼容设计初步方案. 另外, 针对台址建筑设施内的中低电磁辐射干扰源, 提出一种低成本建筑屏蔽方法, 应用于QTT台址现有建筑.     

Sun-induced variations in time in the global positioning system

Sun-induced effects on the measurement and transfer of time information reported by orbiting satellites are reexamined. The problem of transforming the frequency of coherent radio signals to the Geocentric Inertial Coordinate frame (as required during the transfer of time information) involves an implicit, dependence on earth-sun-clock orientation that has been omitted, except in Very-Long-Baseline Interferometry (VLBI) studies. This known VLBI effect leads to an important second order Doppler effect. The effect explains systematic deviations between the time reported by satellites in the Global Positioning System (GPS) relative to Universal Time (UT) and can be used to improve existing bounds for the validity of the equivalence principle.     

基于CGGTTS的北斗3号系统时间性能评估

依据国际时间频率咨询委员会(Consultative Committee for Time and Frequency, CCTF) GNSS (Global Navigation Satellite System)时间比对工作组制定的时间传递标准(Common GNSS Generic Time Transfer Standard Version2E, CGGTTS_V2E), 针对GNSS接收机观测到的伪距信号开发了数据处理软件, 用于生成标准格式的CGGTTS文件, 并对其可靠性进行了验证. 结果表明, 与sbf2cggtts软件生成的CGGTTS文件相比, 在同一历元下, 分别利用相同GPS和BeiDou-2卫星观测值计算的星地钟差值基本一致, 互差绝对值不超过0.5ns的差值分别占总数的96%、94%. 以中国标准时间UTC(NTSC) (Coordinated Universal Time (National Time Service Center))为参考, 利用数据处理软件分别对BeiDou-2和BeiDou-3卫星的B1I和B3I双频消电离层组合观测值处理并生成标准格式的CGGTTS文件, 通过分析其星地钟差参数对BeiDou系统时间的性能进行评估. 结果表明, 与BeiDou-2相比, BeiDou-3系统时间的内符合精度提高约28%, 且1 d以上中长期频率稳定度明显优于BeiDou-2.     

基于实时多系统PPP模糊度固定的时间传递算法

随着RTS (Real-Time Service)工程的发展,时频用户可以运用实时精密单点定位(Precise Point Positioning, PPP)技术进行时间传递研究.作为RTS工程的主要参与者,CNES (Centre National d’Etudes Spatiales)分析中心开展PPPWIZARD (Precise Point Positioning with Integer and Zero-difference Ambiguity Resolution Demonstrator)工程验证实时PPP模糊度固定技术.为了探究多系统观测值和实时PPP模糊度固定对时间传递的性能提升,在综合GPS (Global Positioning System)、GLONASS (GLObal NAvigation Satellite System)、 BDS (Bei Dou navigation System)和Galileo的多系统观测值的基础上,使用CNES分析中心播发的实时产品开展PPP时间传递验证实验,检验了4种不同PPP模式的工作性能.实验结果证明,在多种不同工作模式当中,综合运用多系统观测值和GPS模糊度固定技术进行PPP时间传递的标准差结果最小,标准差相比于传统GPS PPP时间传递平均下降38.1%.     

The Performance Evaluation of BeiDou-3 System Time Based on CGGTTS

According to the Common GNSS Generic Time Transfer Standard Version2E (CGGTTS_V2E) developed by the GNSS (Global Navigation Satellite System) Working Group of the International Consultative Committee for Time and Frequency (CCTF), the data processing software is developed by using the pseudorange signal measured by the GNSS receiver, which is used to generate the CGGTTS files in the standard format, and its reliability is verified. The results show that, compared with the CGGTTS files generated by sbf2cggtts software, the offset between the GNSS system time and local time scale calculated by the same GPS and BDS satellite observations in the same epoch is identical, and the difference with the absolute value of the difference less than 0.5 ns accounts for 96% and 94% of the total, respectively. Taking Chinese standard time UTC(NTSC) (Coordinated Universal Time (National Time Service Center)) as the reference time scale, the data processing software is used to process the observations of the B1I and B3I dual-frequency ionospheric combination of BeiDou-2 and BeiDou-3 satellites, and generate the CGGTTS files in the standard format, and the performance of BeiDou system time is evaluated by analyzing the parameter of offset between the GNSS system time and local time scale. The results show that, compared with BeiDou-2, the internal precision of BeiDou-3 system time is increased by about 28%, and the frequency stability of medium and long-term is obviously better than BeiDou-2 after one day.     

Time Transfer Algorithm Using Multi-GNSS PPP with Ambiguity Resolution

With the development of real-time service (RTS) project, timing users can apply the real-time precise point positioning (PPP) technique for time transfer. As a participant in the RTS project, the Centre National d’Etudes Spatiales (CNES) implements the PPPWIZARD (Precise point positioning with Integer and Zero-difference Ambiguity Resolution Demonstrator) project to validate the PPP with ambiguity resolution. In order to analyze the contribution of multiple global navigation satellite system (multi-GNSS) and real-time ambiguity resolution to time transfer, our experiment used the observation from multi-GNSS, including GPS (Global Positioning System), GLONASS (GLObal NAvigation Satellite System), BDS (BeiDou navigation System), and Galileo for data processing. Meanwhile, the real-time products from CNES were utilized to examine the performance of four different PPP processing modes. The experimental results indicated that, of all the processing modes, the time transfer using multi-GNSS PPP with GPS ambiguity resolution had the smallest standard deviations (STDs). The STD value was decreased by 38.1$\%$, compared with the traditional time transfer results using GPS PPP.     

Solar radio filtering algorithm based on improved long short-term memory

The effective observation of burst events in solar radio research has been impeded by various interference signals,especially interference signals with a wide frequency range and high intensity,as they can partially or completely obscure the observation of burst events.Image processing methods that directly remove the interference signal channels and subtract the average of the interference signal channel are not suitable for processing all types of interference signals.This paper proposes the use of a specific kind of recurrent neural networks,called long short-term memory networks,to predict the value of the radio frequency interference signals with high intensity of the burst event in the solar radio spectrum.The predicted interference can then be removed in accordance with the principle that signals can be linearly added.Therefore,predicted value is subtracted from the data containing the burst event signals and the RFI signals(The radio frequency interference signals to be processed in this article refer to the signal of the broadcast signal that can be received in the frequency range,the signal transmitted by the mobile phone,and the signal transmitted by the sea vessel,and the like) to remove the interference.Then,in order to reduce the error caused by the stepwise prediction in the network and further improve the prediction accuracy,this paper analyzes the characteristics of the value of the radio interference and applies the digital mapping method to convert the prediction problem into the classification problem in the time series.The experimental results show that the proposed method can effectively remove the radio interference in the solar spectrum and clearly show the burst events.     

GPS掩星折射率剖面一维变分同化

近年来,GPS／LEO(全球定位系统／低地球轨道)卫星无线电掩星技术给出了地球大气探测的新途径．从LEO卫星观测到的掩星数据可以反演的地球大气的气压、水汽、温度等剖面;它们对气象和大气科学研究,是具有潜在价值的数据资源．掩星数据资料的同化技术可以有效地改进这些气象参数的剖面,从而改进目前的数值天气预报模式．在当前采用的一维变分同化反演技术中,可以用掩星观测资料的大气折射率或弯曲角剖面进行同化,来反演大气水汽和温度剖面以及海平面压强．作为独立自主开发的GPD／LEO掩星技术系统的一部分,以欧洲中尺度天气预报分析(ECMWF)资料为背景场,CHAMP 掩星观测得到的折射率剖面为观测值,采用Levenberg—Marquardt方法实行GPS掩星资料一维变分同化．在讨论中,用掩星观测点附近相应的探空气球资料来检验CHAMP掩星资料变分同化的结果．     

对称模式下的CHAMP弯曲角掩星数据同化

简单介绍了无线电掩星技术探测行星大气的发展史,列举了该技术中存在的若干问题。从 Eyre提出的统计学的最优估计反演方法,比较了用相位、弯曲角和折射率作为同化因子时出现的问题和各自的优缺点。对弯曲角同化因子,以欧洲中期天气预报中心(ECMWF)资料为背景场,运用一维变分技术,进行CHAMP掩星观测资料变分同化反演,从而获得水汽和温度剖面。将反演获得的气象剖面与非同化的剖面作比较,并且采用相应的探空气球资料作为验证,可以看出变分同化技术比传统的标准反演技术反演误差小。证实掩星数据资料的一维变分同化技术可以改进目前的数值天气预报模式。     

Analysis of the wide area differential correction for BeiDou global satellite navigation system

The regional BeiDou Satellite System, or BDS2, broadcasts a differential correction as Equivalent Satellite Clock Correction to correct both orbit and satellite clock errors. For the global BDS, or BDS3, satellite orbit and clock corrections conforming with RTCA standards will be broadcast to authorized users. The hybrid constellation and regional monitoring network pose challenges for the high precision separation of orbit and satellite clock corrections. Three correction models of kinematic,dynamic and Two-way Satellite Time Frequency Transfer(TWSTFT)-based dynamic were studied to estimate the satellite orbit and clock corrections. The correction accuracy of the three models is compared and analyzed based on the BDS observation data. Results show that the accuracies(root mean square, RMS) of dual-frequency real-time positioning for the three models are about 1.76 m, 1.78 m and 2.08 m respectively, which are comparable with the performance of WAAS and EGNOS. With dynamic corrections, the precision of Precise Point Positioning(PPP) experiments may reach about 23 cm after convergence.     

基于小波变换的射频干扰消除方法的研究

在射电天文观测中,射频干扰(Radio Frequency Interference, RFI)会以多种形式混入望远镜接收系统,给观测带来误判或者降低观测信噪比.近年来国内国际射电天文快速发展,国内国际大型射电望远镜和阵列先后建设,观测灵敏度大为提高,射频干扰的影响尤为突出.随着科技发展和人类活动的加剧,射频干扰日益严重且不可逆转.提出利用2维离散小波变换的方法分析射电天文观测的数据,对望远镜系统输出的时间频率序列进行小波变换,根据小波系数分离出原始信号中各分量,每个分量统计得到相应的阈值,将各分量与阈值相比较识别干扰成分并标记去除.利用该方法对实际观测数据进行了处理,结果表明该方法能够很好地标记并消减干扰信号,且提高了观测的信噪比.     

Study of Radio Frequency Interference Mitigation Method Based on Wavelet Transform

In radio astronomy observations, radio frequency interference (RFI) is mixed into the telescope receiving system in various forms. The existence of RFI brings misjudgment to the observation or reduces the observational signal-to-noise ratio. In recent years, the domestic and overseas radio astronomy has developed rapidly. Large-scale radio telescopes and telescope arrays at home and abroad have been constructed successively. Observation sensitivity is greatly improved, and the influence of RFI is particularly prominent. With the development of science and technology and the intensification of human activities, the RFI has become increasingly serious and irreversible. We propose to use the two-dimensional discrete wavelet transform method to analyze the data of radio astronomy observations, and to perform wavelet transform on the time-frequency sequence output by the telescope system. According to the wavelet coefficients, the every component in the original signal is separated, and the corresponding threshold value is obtained by the statistics on each component. Each component is compared with the threshold to identify the interference component, and to mark it for removal. This method is used to process the actual observation data. The results show that this method can well mark and eliminate the interference signal, and improve the observational signal-to-noise ratio.     

One-dimensional variational retrieval of refractivity profiles by GPS occultation

In recent years, the technique of Global Positioning System/Low Earth Orbit (GPS/LEO) radio occultation has opened up a new route for exploring the earth atmosphere. With this technique, the vertical profiles of pressure, temperature, water vapor of the earth atmosphere can be retrieved from GPS/LEO occultation data, thus making the latter a potentially valuable data resource for meteorological and atmospheric sciences. The technique may effectively improve the quality of the meteorological profiles, and so improve the current numerical weather forecasting model. In the one-dimensional variational (1DVAR) assimilation retrieval technique in current use, the profiles of atmospheric water vapor and temperature, as well as the sea level pressure are retrieved by assimilating the atmospheric refractivity or bending angle profile from GPS/LEO occultation data. In the system independently developed at Shanghai Observatory, the 1DVAR assimilation of CPS occultation data is accomplished by the Levenberg-Marquardt method with the analytical data from the European Center for Medium Range Weather Forecasting (ECMWF) as the background field, the refractivity profile obtained from CHAMP occultation data as the observational quantity. The result was checked with sounding balloon data near the occultation points.     

GPS定时噪声分析与控制

分析了选择可用性干扰的噪声特性,得到了选择可用干扰的数学模型。建立起时刻差方程的状态空间的描述,利用离散的状态方程,用卡尔曼滤波的方法减小ＳＡ干扰对校频,定时精度的影响。     

无人值守GNSS方法测定射电望远镜参考点

为解决常规射电望远镜归心测量工作耗时耗力的问题,引入GNSS (Global Navigation Satellite System)同步监测技术实现了一种针对射电望远镜参考点的无人值守监测方法.设计了针对GNSS靶标点观测数据的归算方法,包括数据匹配、数据检核以及后续精度评估等步骤,并对2018年佘山25-m射电VLBI (Very Long Baseline Interferometry)望远镜的GNSS靶标点实测数据开展了数据预处理、解析与归心解算等,证明了该方法的可行性.结果表明基于该方法,采用单日内部分(5%)数据(约7600个靶标点),所测定的VLBI望远镜参考点的点位形式精度可达3 mm.总结了针对射电望远镜采用GNSS开展无人值守归心测量先行试验中的一些经验教训,明确了利用该方法测量过程中应该注意的问题,为今后更高精度射电望远镜参考点无人值守归心监测提供重要参考.