GNSS geodetic techniques for time and frequency transfer applications

We performed an initial analysis of the pseudorange data of the GIOVE-B satellite, one of the two experimental Galileo satellites currently in operation, for time transfer.¹ For this specific aim, software was developed to process the GIOVE-B raw pseudoranges and broadcast navigation messages collected by the Galileo Experimental Sensor Stations (GESS) tracking network, yielding station clock phase errors with respect to the Experimental Galileo System Time (EGST). The software also allows processing the Global Positioning System (GPS) P1 and P2 pseudorange data with broadcast navigation message collected at the same stations to obtain the station clock phase errors with respect to the GPS system time (GPST). Differencing these solutions between stations provides two independent means of GNSS time transfer. We compared these time transfer results with Precise Point Positioning (PPP) method applied to GPS data in combined carrier-phase and pseudorange mode as well as in pseudorange-only mode to show their relative merits. The PPP solutions in combined carrier-phase and pseudorange mode showed the least instability of the methods tested herein at all scales, at few parts in 10¹⁵ at 1 day for the stations processed, following a tau^−½ interval dependency. Conversely, the PPP solutions in pseudorange-only mode are an order of magnitude worst (few parts in 10¹⁴ at 1 day for the stations processed) following a tau⁻¹ power-law, but slightly better than the single-satellite raw GPS time transfer solutions obtained using the developed software, since the PPP least-squares solution effectively averages the pseudorange noise. The pseudorange noise levels estimated from PPP pseudorange residuals and from clock solution comparisons are largely consistent, providing a validation of our software operation. The raw GIOVE-B time transfer, as implemented in this work, proves to be slightly better than single-satellite raw GPS satellite time transfer, at least in the medium term. However, one of the processed stations shows a combined GPS P1 and P2 pseudorange noise level at 2 m, a factor 2 worst than usually seen for geodetic receivers, so the GPS time transfer results may not be at their best for the cases processed. Over the short term, the GPS single-satellite time transfer instability outperforms the GIOVE-B by an order of magnitude at 1 s interval, which would be due to the different characteristics of the tracking loop filters for GPS P1 and P2 on one hand and the GIOVE-B signals on the other. Even at this preliminary stage and using an experimental satellite system, results show that the GIOVE-B (and hence Galileo) signals offer interesting perspectives for high precision time transfer between metrological laboratories.     

The impact on tsunami detection from space using GNSS-reflectometry when combining GPS with GLONASS and Galileo

The devastating Sumatra tsunami in 2004 demonstrated the need for a tsunami early warning system in the Indian Ocean. Such a system has been installed within the German-Indonesian Tsunami Early Warning System (GITEWS) project. Tsunamis are a global phenomenon and for global observations satellites are predestined. Within the GITEWS project a feasibility study on a future tsunami detection system from space has therefore been carried out. The Global Navigation Satellite System Reflectometry (GNSS-R) is an innovative way of using GNSS signals for remote sensing. It uses ocean reflected GNSS signals for sea surface altimetry. With a dedicated Low Earth Orbit (LEO) constellation of satellites equipped with GNSS-R receivers, densely spaced sea surface height measurements could be established to detect tsunamis. Some general considerations on the geometry between LEO and GNSS are made in this simulation study. It exemplary analyzes the detection performance of a GNSS-R constellation at 900 km altitude and 60° inclination angle when applied to the Sumatra tsunami as it occurred in 2004. GPS is assumed as signal source and the combination with GLONASS and Galileo signals is investigated. It can be demonstrated, that the combination of GPS and Galileo is advantageous for constellations with few satellites while the combination with GLONASS is preferable for constellations with many satellites. If all three GNSS are combined, the best detection performance can be expected for all scenarios considered. In this case an 18 satellite constellation will detect the Sumatra tsunami within 17 min with certainty, while it takes 53 min if only GPS is considered.     

卫星导航系统发展动态

卫星导航系统是20世纪60年代中期发展起来的一种新型导航系统，90年代，卫星导航时入全运行和盛行时期，除了用于陆上，海上和空中的航行引导外，几乎扩展到军事和经济的各个方面，目前，除了已经运行的美国的GPS（全球定位系统）和俄罗斯的GLONASS（全球导航卫星系统）外，欧盟也正在积极开发伽利略系统。本文分别介绍GPS、GLONASS和伽利略系统的发展状况。     

我国卫星导航系统相关技术发展分析

卫星导航系统应用广泛,有利于促进国民经济的快速发展。本文在介绍国内卫星导航系统及其国内市场状况基础上,结合北斗卫星导航相关产业发展趋势,从组合导航、增强系统、时钟同步和定向解算等方面分析了北斗卫星导航系统的技术成果,并讨论了有助于提高北斗卫星导航系统精度的相关辅助技术的发展情况。最后结合我国国情和国内卫星导航系统相关技术特点,给出我国第二代卫星导航定位系统发展的若干建议。     

多星座组合导航系统自主完好性监测方法研究

提出一种“VGC BIT”的方法分析多故障情况下多星座组合导航系统的自主完好性监测性能。该方法基于现有GPS观测数据构建“准现实”的GPS/Galileo组合导航环境,可有效实现对多故障情况下GPS/Galileo组合系统完好性监测性能的分析。应用该方法计算GPS/Galileo组合导航系统中同时存在1~4个故障时的故障检测(FD)和排除(FDE)可用率。计算结果表明,在北京和上海两地,即使出现分布情况最坏的4个故障星,航路阶段FD和FDE的可用性仍大于99%。     

GNSS全球导航卫星系统的新发展

GNSS全球导航卫星系统是20世纪90年代中期欧盟提出的一种综合星座系统,而不是一个单一星座系统。GNSS主要包括GPS、GLONASS、Galileo、Compass和IRNSS及它们的星基增强系统,例如,WAAS、EGNOS和GDGPS等。文中主要分析了GPS、GLONASS、Galileo的新发展及Compass和IRNSS的建设进展。     

伽利略卫星定位系统三载波姿态确定技术研究

利用伽利略卫星定位系统来确定载体的姿态是伽利略系统的重要应用领域。目前利用GPS确定姿态的算法比较复杂，初始化时间较长，本文详细地介绍了伽利略定位系统三载波模糊度解算技术（TCAR）的要点，分析了TCAR技术求解模糊度的概率分布特点，提出了基于小模糊度搜索空间的单差模糊度搜索策略，克服了传统TCAR技术模糊度确定错误率较高的缺点。以低轨卫星姿态确定为应用对象，利用STK软件工具与Matlab进行了联合仿真，仿真结果表明，该方案完全可行，效果理想，适合单历元姿态确定。     

伽利略系统的当前频率和信号设计方案

本文简要介绍伽利略系统最新提出的频率和信号设计方案,涉及方案设计基本要求、频率和信号基本方案、各种导航信号的调制体制、所用扩频测距伪码、性能参数、时间与坐标参考系等。     

Talking space: The European Commission's changing frames in defining Galileo

This paper traces the way in which the European Commission has framed and reframed the issue of EU satellite navigation over 20 years. It investigates how the EU's agenda-setter has ‘talked about’ space policy, with a particular focus on Galileo, and how its own institutional discourse – as revealed in its communications throughout the agenda-setting stage of Galileo's ‘definition’ phase – evolved in the 1990s through the use of ‘frame sets’. In so doing, it illustrates the ways in which, over time, the EU's executive has ‘projected’ the issue of independent satellite navigation capabilities as being politically and economically desirable for Europe, and has sought to persuade decision makers of its cross-policy relevance and potential economic, social and security benefits. The article deconstructs official documents and engages in a close-up analysis of policy formulation, to identify nascent, evolving and mature frames in the definition of Galileo.     

GPS/Galileo组合导航的模糊度搜索空间

为了组合导航的载波相位模糊度固定,将目前在GPS中常用的模糊度固定方法--最小二乘降相关平差（LAMBDA）法直接应用于GPS/Galileo组合模糊度固定,发现其搜索空间的确定方法并不能很好地适应GPS/Galileo组合中模糊度维数较高的情况。通过对常规LAMBDA搜索空间确定方法的分析比较,在传统方法的基础上提出了一种专门针对高维模糊度固定的搜索空间确定方法--修正法确定模糊度搜索空间。通过对修正法进行仿真试验,证明该方法能保证在GPS/Galileo组合定位模式下实际备选模糊度个数基本与预先设定的备选模糊度个数一致,进而能在不降低模糊度固定成功率的基础上有效提高LAMBDA模糊度固定的搜索效率,其性能优于传统的模糊度搜索空间确定方法。