Ionospheric threat simulation for GNSS using the Spirent hardware signal simulator

Ionospheric disturbances present a considerable hazard to single-frequency satellite navigation systems for airborne users. We discuss our implementation of three ionospheric threat models in the DLR “multi-output advanced signal test environment for receivers” global navigation satellite system simulator, which is based on Spirent GSS 7780/7790 signal generator. These threat models include the standard front-based threat model developed for the integrity assessment of ground-based augmentation systems (GBAS), a simplified plasma bubble model, and ionospheric scintillation, which can be combined with either of the two previously mentioned models. These effects can now straightforwardly be simulated at the German Aerospace Center’s research facilities. As an example, we simulate a GBAS ground facility with code–carrier divergence monitoring, affected by an ionospheric front, and we show the results of a simulation with coincidental occurrence of a plasma bubble and scintillation with an S ₄ index of 0.4.     

Ionospheric calibration of single frequency VLBI and GPS observations using dual GPS data

Summary The ionospheric effect is one of the main sources of error in Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) high precision geodesy. Although the use of two frequencies allows the estimation of this effect, in some cases dual observations are not possible due to the available equipment or the type of observation. This paper presents the ionospheric calibration of single frequency VLBI and GPS observations based on the ionospheric electron content estimated from dual frequency GPS data. The ionospheric delays obtained with this procedure and the VLBI baseline length results have been compared with those obtained with dual frequency data. For the European geodetic VLBI baselines, both solutions agree at the 3–5 parts in 10^–9 level. The noise introduced by the GPS-based calibration is in the order of 3 cm for the VLBI observables and of 10 cm for the GPS observables.     

Ionospheric delay corrections for single-frequency GPS receivers over Europe using tomographic mapping

The majority of navigation satellites receivers operate on a single frequency and experience a positioning error due to the ionospheric delay. This can be compensated for using a variety of approaches that are compared in this paper. The study focuses on the last solar maximum. A 4D tomographic imaging technique is used to map the ionospheric electron density over the European region during 2002 and 2003. The electron density maps are then used to calculate the excess propagation delay on the L1 frequency experienced by GPS receivers at selected locations across Europe. The excess delay is applied to correct the pseudo-range single frequency observations at each location and the improvements to the resulting positioning are calculated. The real-time tomographic technique is shown to give navigation solutions that are better than empirical modelling methods and approach the accuracy of the full dual-frequency solution. The improvements in positioning accuracy vary from day to day depending on ionospheric conditions but can be up to 25 m during mid-day during these solar maximum conditions at European mid-latitudes.

Correlation between ROTI and Ionospheric Scintillation Indices using Hong Kong low-latitude GPS data

The correlation between the rate of TEC index (ROTI) and scintillation indices S ₄ and σ _Φ for low-latitude region is analyzed in this study, using data collected from a Global Positioning System (GPS) scintillation monitoring receiver installed at the south of Hong Kong for the periods June–August of 2012 and May 2013 and July–December of 2013. The analysis indicates that the correlation coefficient between ROTI and S ₄/σ _Φ is about 0.6 if data from all GPS satellites are used together. If each individual satellite is considered, the correlation coefficients are above 0.6 on average and sometimes above 0.8. The analysis also shows that the ratio of ROTI and S ₄ varies between 1 and 4. The ratio ROTI/σ _Φ, varies between 2 and 9. In addition, it is also found that there is a good consistency between the temporal variations of ROTI with scintillation activity under different ionospheric conditions. ROTI has a high correlation relationship with scintillation indices on geomagnetically disturbed days or in solar active months. Moreover, the data observed at low elevation angles have weak correlation between ROTI and scintillation indices. These results demonstrate the feasibility of using ROTI derived from GPS observations recorded by common non-scintillation GPS receivers to characterize ionospheric scintillations.     

Enhancing mobile applications by combination of GNSS and W-CDMA

The location requirements for emergency callers outside urban areas can hardly be fulfilled without global navigation satellite systems (GNSS). Consequently, interest in positioning techniques based on use of a GNSS such as GPS or on the cellular network infrastructure itself is growing rapidly in the mobile-telephone community. Moreover, the increasing demand for commercial location-based services (LBS) has driven cellular-phone and network manufacturers to focus on positioning solutions which are even more accurate than the regulatory mandates for positioning of emergency callers. One example of these upcoming LBS is our PARAMOUNT project, which aims at improving user-friendly info-mobility services for hikers and mountaineers by combining wireless communications (GMTS), satellite navigation (GNSS) and geographic information systems (GIS), based on a mobile client/server architecture. The availability of mobile phones or PDAs with combined GNSS and cellular network-based wireless communication on a high integration level is one primary demand of such LBS applications. Based on this, we will give some initial answers to the question of whether mobile handset architecture synergies exist for the combination of GNSS with wireless location in CDMA cellular wireless networks. In order to identify synergies, we will outline similarities and differences between wireless communication and satellite navigation. In this respect, we pay particular attention to the so-called RAKE receiver architecture employed in mobile CDMA cellular handsets. Our initial investigations will show that the RAKE receiver architecture, on which mobile CDMA cellular handsets are based, will most likely be the one most suitable for achieving synergies between the two positioning techniques within the same mobile handset architecture. Consequently, several receiver components could be used to handle both types of signals (navigation and communications), resulting in a reduction of manufacturing costs and in a decrease in energy consumption. Electronic Publication     

GPS/GNSS卫星定位原理与应用双语课程建设初探

主要介绍GPS/GNSS导航定位双语教学课程建设的设计思想和体会。针对测绘类专业卫星导航定位课程教学大纲内容,选取一些主要的GPS/GNSS英文原版参考书和网络资料,对其涵盖的主要内容及其更新性、读者对象、可读性等方面进行详细分析。对于采用原版教材、自编教材等方式进行讨论,提出采用以Internet网络资源为主、原版参考书为辅的GPS/GNSS双语教材选取模式。     

Application of the TaiWan Ionospheric Model to single-frequency ionospheric delay corrections for GPS positioning

The performance of a three-dimensional ionospheric electron density model derived from FormoSat3/COSMIC GPS Radio Occultation measurements, called the TaiWan Ionosphere Model (TWIM), in removing the ionospheric delays in single-frequency pseudorange observations is presented. Positioning results using TWIM have been compared with positioning results using other ionospheric models, such as the Klobuchar (KLOB) and the global ionospheric model (GIM). C/A code pseudoranges have been observed at three International GPS Service reference stations that are representative of mid-latitude (BOR1 and IRKJ) and low-latitude (TWTF) regions of the ionosphere. The observations took place during 27 geomagnetically quiet days from April 2010 to October 2011. We perform separate solutions using the TWIM, KLOB, GIM ionospheric models and carry out a solution applying no ionospheric correction at all. We compute the daily mean horizontal errors (DMEAN) and the daily RMS (DRMS) for these solutions with respect to the published reference station coordinates. It has demonstrated that TEC maps generate using the TWIM exhibit a detailed structure of the ionosphere, particularly at low-latitude region, whereas the Klobuchar and the GIM only provide the basic diurnal and geographic features of the ionosphere. Also, it is shown that even for lower satellite elevations, the TWIM provides better positioning than the Klobuchar and GIM models. Specifically, using TWIM, the difference of the uncorrected solution (no ionospheric correction), and the other solutions, relative to the uncorrected solution, is 45 % for the mean horizontal error (DMEAN) and 42 % for the horizontal root-mean-square error (DRMS). Using Klobuchar and GIM, the percent for DMEAN only reaches to about 12 % and 3 %, while the values for the DRMS are only 12 and 4 %, respectively. In the vertical direction, all models have a percentage of about 99 and 70 % for the mean vertical error (VMEAN) and vertical root-mean-square error (VRMS), respectively. These percentages show the greater impact of TWIM on the ionospheric correction compared to the other models. In at least 40 % of the observed days and across all stations, TWIM has the smallest DMEAN, VMEAN, DRMS, and VRMS daily values. These values reach 100 % at station TWTF. This shows the overall performance of TWIM is better than the Klobuchar and GIM.     

Global ionosphere map constructed by using total electron content from ground-based GNSS receiver and FORMOSAT-3/COSMIC GPS occultation experiment

Effects of rapidly changing ionospheric weather are critical in high accuracy positioning, navigation, and communication applications. A system used to construct the global total electron content (TEC) distribution for monitoring the ionospheric weather in near-real time is needed in the modern society. Here we build the TEC map named Taiwan Ionosphere Group for Education and Research (TIGER) Global Ionospheric Map (GIM) from observations of ground-based GNSS receivers and space-based FORMOSAT-3/COSMIC (F3/C) GPS radio occultation observations using the spherical harmonic expansion and Kalman filter update formula. The TIGER GIM (TGIM) will be published in near-real time of 4-h delay with a spatial resolution of 2.5° in latitude and 5° in longitude and a high temporal resolution of every 5 min. The F3/C TEC results in an improvement on the GIM of about 15.5%, especially over the ocean areas. The TGIM highly correlates with the GIMs published by other international organizations. Therefore, the routinely published TGIM in near-real time is not only for communication, positioning, and navigation applications but also for monitoring and scientific study of ionospheric weathers, such as magnetic storms and seismo-ionospheric anomalies.     

Precise estimation of residual tropospheric delays using a regional GPS network for real-time kinematic applications

 A new method called Trop_NetAdjust is described to predict in real time the residual tropospheric delays on the GPS carrier phase observables using the redundant measurements from a network of GPS reference stations. This method can not only enhance the effectiveness and reliability of real-time kinematic users within the network, but also provide a valid approach to tropospheric parameter variation forecasting. Trop_NetAdjust is theoretically based upon LS prediction criteria and enables the prediction of residual tropospheric delays remaining after a standard model has been applied to the raw GPS measurements. Two cases are analyzed, namely a first case when the delay is required for an existing satellite at a new point within the network and a second case when the delay is required for a new satellite. Field tests were conducted using data collected in a network of 11 reference stations covering a 400×600 km region in southern Norway. The results were analyzed in the measurement domain (ionospheric-free double-difference residuals) and showed improvements of 20 to 65% RMS errors using Trop_NetAdjust. The estimates of the Trop_NetAdjust prediction accuracy were also obtained using the covariance analysis method. The agreement was consistently better than 30% when compared with data from a real network. Received: 28 February 2000 / Accepted: 9 January 2001     

哈尔滨测绘院GNSS系统进行GPS静态四等水准测量的研发

哈尔滨测绘院利用GNSS系统进行GPS静态四等水准控制测量的设计与实现是哈尔滨市GNSS系统进行GPS高等级静态水准控制测量的历史性突破,比对常规四等水准测量检测成果,成果精度优良.利用GNSS系统进行GPS静态四等水准控制测量,大幅度地缩短了工期,降低了外业劳动强度,整体组织科学,实现了良好的社会效益,对哈尔滨市及黑龙江省以至于全国GNSS系统进行GPS静态高等级水准控制测量起到了带头和推动作用.     

A semivariance-based index for spatially correlated errors in rapid differential GNSS applications

In this work, a regional network of permanent Global Navigation Satellite Systems (GNSS) receivers is used to estimate the decorrelation of the spatially correlated errors in differential GNSS positioning. Emphasis is laid on the dispersive errors (i.e. mainly the ionosphere). A new index, based on variance as function of station separation (semivariance) is proposed and compared to the existing I95 index. This study uses data from the 29–30th October 2003, a period with severe ionospheric activity. The proposed index is shown to give realistic predictions of differential measurement accuracy, and has potential for further development towards use in RTK-networks.     

GPS三频非差观测数据周跳的自动探测与改正研究

在GPS三频非差观测数据的处理中,由于伪距噪声的影响,利用原始的伪距和载波相位观测数据估计的模糊度误差比较大,不能用于探测和改正周跳。对原始观测数据进行平滑或适当的组合处理,可降低观测噪声的影响。因此,本文选择合适的经过平滑或组合处理后的观测数据作为探测周跳的检验量,探测并改正单个频点上的周跳。在分析了一般周跳的特点并在研究双频周跳自动探测与改正方法基础上,提出了选取检验量的四条基本原则。最后,依此原则选取了三个观测值组合作为周跳检验量,利用该组检验量实现三频非差观测数据周跳的自动探测与改正。     

A case study of using Raman lidar measurements in high-accuracy GPS applications

This paper investigates the impact of rapid small-scale water vapor fluctuations on GPS height determination. Water vapor measurements from a Raman lidar are used for documenting the water vapor heterogeneities and correcting GPS signal propagation delays in clear sky conditions. We use data from four short observing sessions (6 h) during the VAPIC experiment (15 May–15 June 2004). The retrieval of wet delays from our Raman lidar is shown to agree well with radiosonde retrievals (bias and standard deviation (SD) were smaller than 1 and 2.8 mm, respectively) and microwave radiometers (from two different instruments, bias was 6.0/−6.6 mm and SD 1.3/3.8 mm). A standard GPS data analysis is shown to fail in accurately reproducing fast zenith wet delay (ZWD) variations. The ZWD estimates could be improved when mean post-fit phase residuals were removed. Several methodologies for integrating zenith lidar observations into the GPS data processing are also presented. The final method consists in using lidar wet delays for correcting a priori the GPS phase observations and estimating a scale factor for the lidar wet delays jointly with the GPS station position. The estimation of this scale factor allows correcting for a mis-calibration in the lidar data and provides in the same way an estimate of the Raman lidar instrument constant. The agreement of this constant with an independent determination using radiosonde data is at the level of 1–4%. The lidar wet delays were derived by ray-tracing from zenith pointing measurements: further improvement in GPS positioning is expected from slant path lidar measurements that would properly account for water vapor anisotropy.     

A grid-based tropospheric product for China using a GNSS network

Tropospheric delay accounts for one source of error in global navigation satellite systems (GNSS). To better characterize the tropospheric delays in the temporal and spatial domain and facilitate the safety-critical use of GNSS across China, a method is proposed to generate a grid-based tropospheric product (GTP) using the GNSS network with an empirical tropospheric model, known as IGGtrop. The prototype system generates the GTPs in post-processing and real-time modes and is based on the undifferenced and uncombined precise point positioning (UU-PPP) technique. GTPs are constructed for a grid form (\(2.0{^{\circ }}\times 2.5{^{\circ }}\) latitude–longitude) over China with a time resolution of 5 min. The real-time GTP messages are encoded in a self-defined RTCM3 format and broadcast to users using NTRIP (networked transport of RTCM via internet protocol), which enables efficient and safe transmission to real-time users. Our approach for GTP generation consists of three sequential steps. In the first step, GNSS-derived zenith tropospheric delays (ZTDs) for a network of GNSS stations are estimated using UU-PPP. In the second step, vertical adjustments for the GNSS-derived ZTDs are applied to address the height differences between the GNSS stations and grid points. The ZTD height corrections are provided by the IGGtrop model. Finally, an inverse distance weighting method is used to interpolate the GNSS-derived ZTDs from the surrounding GNSS stations to the location of the grid point. A total of 210 global positioning system (GPS) stations from the crustal movement observation network of China are used to generate the GTPs in both post-processing and real-time modes. The accuracies of the GTPs are assessed against with ERA-Interim-derived ZTDs and the GPS-derived ZTDs at 12 test GPS stations, respectively. The results show that the post-processing and real-time GTPs can provide the ZTDs with accuracies of 1.4 and 1.8 cm, respectively. We also apply the GTPs in real-time kinematic GPS PPP, and the results show that the convergence time of the PPP solutions is shortened. These results confirm that the GTPs can act as an efficient information source to augment GNSS positioning over China.     

GPS位置时间序列中温度变化驱动的非线性信号机制解释

GNSS连续运行基准站坐标时间序列(以下简称为时间序列)中由地球物理效应引起的非线性信号主要指由固体、海洋及大气潮汐、环境负载、热膨胀效应等造成的基准站周期性位移.由周期性温度变化驱动的基准站天线观测墩热效应(thermal expansion of the monument ,TEM )及其所在基岩的热弹性形变(th...     

Efficient interpolations to GPS orbits for precise wide area applications

For precise real time or near real time differential GPS positioning in a wide or global area, precise GPS orbits or, alternatively, precise orbital corrections with respect to a reference orbit, such as GPS broadcast ephemerides, must be used. This work tests orbit interpolation methods, in order to represent the GPS orbits and orbital corrections accurately and efficiently for these and other GPS applications. For precise GPS orbits given in the SP3 format at the 15 min interval, numerical tests were conducted using Lagrange and Chebyshev as well as trigonometric polynomial functions. The results have demonstrated that the 19- or 20-term trigonometric function is apparently the most efficient interpolator for a 12 h GPS orbital arc, achieving 1 cm level 3D interpolation accuracy that can meet the requirements of most precise applications. The test results also demonstrated that the 9-term trigonometric function always yields optimal interpolation for a 2 h GPS orbit arc, in terms of interpolation errors, compared to the results when using a different number of terms for the same function or one of the other tested polynomial functions. This is evident from the minimal performance degradation when using the 9-term trigonometric function to interpolate near or at the end of a data interval. By limiting interpolation to the center 15 min to 1.5 h of a 2 h orbit arc, thereby eliminating the need to interpolate near the ends of that interval, users can opt for more terms (11 and 13) or different interpolators to further improve interpolation accuracy. When interpolating the orbital corrections with respect to the GPS broadcast ephemeris, all the tested interpolation functions of 3- to 9-term yield the same suitably accurate results. Therefore, a 3- to 5-term trigonometric function is arguably sufficiently accurate and more efficient for GPS orbital correction messaging in wide area and real time positioning.     

GPS/VRS虚拟观测值生成与发布研究

虚拟参考站（VRS）技术是目前GPS网络RTK系统研究领域的热点之一。介绍VRS系统的组成及原理;论述VRS虚拟观测值的生成算法及发送给用户后随机商业软件的解算模型;最后介绍算法的软件系统实现,通过与真实观测值比较的方法验证虚拟观测值的可行性和稳定性。     

Forward modeling of GPS multipath for near-surface reflectometry and positioning applications

Multipath is detrimental for both GPS positioning and timing applications. However, the benefits of GPS multipath for reflectometry have become increasingly clear for soil moisture, snow depth, and vegetation growth monitoring. Most multipath forward models focus on the code modulation, adopting arbitrary values for the reflection power, phase, and delay, or they calculate the reflection delay based on a given geometry and keep reflection power empirically defined. Here, a fully polarimetric forward model is presented, accounting for right- and left-handed circularly polarized components of the GPS broadcast signal and of the antenna and surface responses as well. Starting from the fundamental direct and reflected voltages, we have defined the interferometric and error voltages, which are of more interest in reflectometry and positioning applications. We examined the effect of varying coherence on signal-to-noise ratio, carrier phase, and code pseudorange observables. The main features of the forward model are subsequently illustrated as they relate to the broadcast signal, reflector height, random surface roughness, surface material, antenna pattern, and antenna orientation. We demonstrated how the antenna orientation—upright, tipped, or upside-down—involves a number of trade-offs regarding the neglect of the antenna gain pattern, the minimization of CDMA self-interference, and the maximization of the number of satellites visible. The forward model was also used to understand the multipath signature in GPS positioning applications. For example, we have shown how geodetic GPS antennas offer little impediment for the intake of near-grazing reflections off natural surfaces, in contrast to off metal, because of the lack of diversity with respect to the direct signal—small interferometric delay and Doppler, same sense of polarization, and similar direction of arrival.