Tide variation monitoring based improved GNSS-MR by empirical mode decomposition

Global Navigation Satellite System multipath reflectometry (GNSS-MR) technology has great potential for monitoring tide level changes. GNSS-MR techniques usually extract signal-to-noise ratio (SNR) residual sequences using quadratic polynomials; however, such algorithms are affected considerably by satellite elevation angles. To improve the stability and accuracy of an SNR residual sequence, this study proposed an SNR signal decomposition method based on empirical mode decomposition (EMD). First, the SNR signal is decomposed by EMD, following which the SNR residual sequence is obtained by combining the corresponding intrinsic mode function with the frequency range of the coherent signal. Second, the Lomb–Scargle spectrum is analyzed to obtain the frequency of the SNR residual sequence. Finally, the SNR frequency is converted into the tide height. To verify the validity of the SNR residual sequence obtained by EMD, the algorithm performance was assessed using multigroup satellite elevation angle intervals with measured data from two station, SC02 in the United States and RSBY in Australia. Experimental results demonstrated that the accuracy of the improved algorithm was improved in the low-elevation range. The improved algorithm demonstrated high monitoring accuracy, and the effective number was not less than 80% of the total in SC02, which means it could effectively describe the trend of the tide with accuracy of approximately 10 cm, meanwhile, the RMS error of RSBY could be reduced by 30 cm, to the maximum extent. The EMD method effectively expands the range of available GNSS-MR elevations, avoids the loss of effective information, enhances considerably the utilization rate of GNSS data, and improves the accuracy of GNSS-MR tide level monitoring.     

Statistical evaluation of GLONASS amplitude scintillation over low latitudes in the Brazilian territory

The ionospheric scintillation, generated by the ionospheric plasma irregularities, affects the radio signals that pass through it. Their effects are widely studied in the literature with two different approaches. The first one deals with the use of radio signals to study and understand the morphology of this phenomenon, while the second one seeks to understand and model how much this phenomenon interferes in the radio signals and consequently in the services to which these systems work. The interest of several areas, particularly to those that are life critical, has increased using the concept of satellite multi-constellation, which consists of receiving, processing and using data from different navigation and positioning systems. Although there is a vast literature analyzing the effects of ionospheric scintillation on satellite navigation systems, the number of studies using signals received from the Russian satellite positioning system (named GLONASS) is still very rare. This work presents for the first time in the Brazilian low-latitude sector a statistical analysis of ionospheric scintillation data for all levels of magnetic activities obtained by a set of scintillation monitors that receive signals from the GLONASS system. In this study, data collected from four stations were used in the analysis; Fortaleza, Presidente Prudente, São José dos Campos and Porto Alegre. The GLONASS L-band signals were analyzed for the period from December 21, 2012 to June 20, 2016, which includes the peak of the solar cycle 24 that occurred in 2014. The main characteristics of scintillation presented in this study include: (1) the statistical evaluation of seasonal and solar activity, showing the chances that an user on similar geophysical conditions may be susceptible to the effects of ionospheric scintillation; (2) a temporal analysis based on the local time distribution of scintillation at different seasons and intensity levels; and (3) the evaluation of number of simultaneously affected channels and its effects on the dilution of precision (DOP) for GNSS users are also presented in order to alert the timetables in which navigation will be most susceptible to such effects, as well as statistics on simultaneously affected channels. Relevant results about these statistical characteristics of scintillation are presented and analyzed providing relevant information about availability of a navigation system.     

中国民航实施卫星导航的当前策略探讨

论述了全球卫星导航系统（ＧＮＳＳ）将取代所有传统的陆基无线电导航系统的背景和目标以及当前的策略、在实施早期应用的同时应采取必要的增强措施和安全保障     

一种GNSS双频信号跟踪的新方法

在双频接收机双频L1辅助L2跟踪方法基础上提出一种GNSS双频信号载波和差联合跟踪新方法。该算法能够克服传统的L1辅助L2跟踪方法L2信号能得到L1信号辅助信息、而L1信号得不到L2信号任何辅助信息的固有缺点。仿真结果表明：该和差联合跟踪方法相比传统的双频信号跟踪算法能够耦合双频信号实现其相互辅助跟踪,具有提高GNSS双频信号的跟踪稳健性和灵敏度等优点。     

一种卫星导航模拟器置信度评估方案及应用

置信度评估能够为卫星导航模拟器的研制与改进提供依据。提出了一种基于对数最小二乘的模糊层次分析法和相似度法相结合的置信度评估方案。该方案利用基于对数最小二乘的模糊层次分析法将卫星导航模拟器系统分解为若干子系统或模型,并利用相似度法计算各个子系统或模型的置信度,同时使用模糊层次分析法计算各子系统或模型的权重向量,而后再逐级综合得到模拟器系统的总置信度。该方案能够将复杂系统逐层细化,且求解的子系统或模型的权重向量与置信度较为客观真实,具有重要的工程参考价值。     

LEO enhanced Global Navigation Satellite System (LeGNSS) for real-time precise positioning services

Global Navigation Satellite System (GNSS) has been widely used in many geosciences areas with its Positioning, Navigation and Timing (PNT) service. However, GNSS still has its own bottleneck, such as the long initialization period of Precise Point Positioning (PPP) without dense reference network. Recently, the concept of PNTRC (Positioning, Navigation, Timing, Remote sensing and Communication) has been put forward, where Low Earth Orbit (LEO) satellite constellations are recruited to fulfill diverse missions. In navigation aspect, a number of selected LEO satellites can be equipped with a transmitter to transmit similar navigation signals to ground users, so that they can serve as GNSS satellites but with much faster geometric change to enhance GNSS capability, which is named as LEO constellation enhanced GNSS (LeGNSS). As a result, the initialization time of PPP is expected to be shortened to the level of a few minutes or even seconds depending on the number of the LEO satellites involved. In this article, we simulate all the relevant data from June 8th to 14th, 2014 and investigate the feasibility of LeGNSS with the concentration on the key issues in the whole data processing for providing real-time PPP service based on a system configuration with fourteen satellites of BeiDou Navigation Satellite System (BDS), twenty-four satellites of the Global Positioning System (GPS), and sixty-six satellites of the Iridium satellite constellations. At the server-end, Precise Orbit Determination (POD) and Precise Clock Estimation (PCE) with various operational modes are investigated using simulated observations. It is found out that GNSS POD with partial LEO satellites is the most practical mode of LeGNSS operation. At the user-end, the Geometry Dilution Of Precision (GDOP) and Signal-In-Space Ranging Error (SISRE) are calculated and assessed for different positioning schemes in order to demonstrate the performance of LeGNSS. Centimeter level SISRE can be achieved for LeGNSS.     

Responses of the African equatorial ionization anomaly (EIA) to some selected intense geomagnetic storms during the maximum phase of solar cycle 24

This study investigates the morphology of the GPS TEC responses in the African Equatorial Ionization Anomaly (EIA) region to intense geomagnetic storms during the ascending and maximum phases of solar cycle 24 (2012–2014). Specifically, eight intense geomagnetic storms with Dst ≤ ?100 nT were considered in this investigation using TEC data obtained from 13 GNSS receivers in the East African region within 36–42°E geographic longitude; 29°N–10°S geographic latitude; ± 20°N magnetic latitude. The storm-time behavior of TEC shows clear positive and negative phases relative to the non-storm (median) behavior, with amplitudes being dependent on the time of sudden commencement of the storm and location. When a storm starts in the morning period, total electron content increases for all stations while a decrease in total electron content is manifested for a storm that had its sudden commencement in the afternoon period. The TEC and the EIA crest during the main phase of the storm is significantly impacted by the geomagnetic storm, which experiences an increase in the intensity of TEC while the location and spread of the crest usually manifest a poleward expansion.     

基于GNSS散射信号的双站雷达系统干扰抑制的分析与研究

利用GNSS(全球卫星导航系统)卫星作为照射源,接收机在飞行器的收发分置的GNSS-R双站雷达探测系统作为一门新兴遥感技术越来越受到关注,但由于GNSS散射信号功率太弱,其接收机对GNSS干扰信号的抑制一直是个难点.根据直达波和邻近卫星信号干扰的特点,提出r改进的分块子空间投影算法.即首先对干扰信号进行捕获与跟踪,根据得到的干扰信号参数构筑相应的干扰子空间,利用改进的分块子空间投影进行干扰抑制,该方法既降低了跟踪误差对干扰性能的影响,又使计算量大幅下降,适合多变环境和多数据样本情况.通过对仿真结果的分析,验证了方法的有效性,也为后续系统的优化设计提供了依据.     

分步北斗三频部分模糊度解算方法

随着月球探索进入一个新的时代,为确保未来月球任务的实时高精度定位,大幅提高登月航天器的自主性,且减少对地球基站的依赖,选取合适的轨道构建月球导航星座,并且实现月球导航星座与地球GNSS的通信链路同步尤为重要。为评估月球导航星座轨道中卫星接收GNSS信号的性能,首先对椭圆形月球冻结轨道(ELFO)、近直线晕轨道(NRHO)、顺行圆形轨道(PCO)和低月球轨道(LLO)4种轨道进行仿真。然后基于天线方向图等指标仿真了GNSS卫星信号,并依据主旁瓣波束宽度和载噪比等仿真结果评估了4种不同轨道卫星对GNSS的可见性。结果表明,NRHO和ELFO对GNSS星座有较好的可视效果,最高可见时间占比达99.57%,保障了绝大部分时间内能够稳定接收GNSS信号,有助于实现月球导航星座轨道卫星的轨道和时钟校正,并保证轨道的定位性能。