Accuracy analysis of GNSS-IR snow depth inversion algorithms

In recent years, with the continuous development of Global Navigation Satellite System (GNSS), it has been applied not only to navigation and positioning, but also to Earth surface environment monitoring. At present, when performing GNSS-IR (GNSS Interferometric Reflectometry) snow depth inversion, Lomb-Scargle Periodogram (LSP) spectrum analysis is mainly used to calculate the vertical height from the antenna phase center to the reflection surface. However, it has the problem of low identification of power spectrum analysis, which may lead to frequency leakage. Therefore, Fast Fourier Transform (FFT) spectrum analysis and Nonlinear Least Square Fitting (NLSF) are introduced to calculate the vertical height in this paper. The GNSS-IR snow depth inversion experiment is carried out by using the observation data of P351 station in PBO (Plate Boundary Observatory) network of the United States from 2013 to 2016. Three algorithms are used to invert the snow depth and compared with the actual snow depth provided by the station 490 in the SNOTEL network. The observations data of L1 and L2 bands are respectively used to find the optimal combination between different algorithms further to improve the accuracy of GNSS-IR snow depth inversion. For L1 band, different snow depths correspond to different optimal algorithms. When the snow depth is less than 0.8 m, the inversion accuracy of NLSF algorithm is the highest. When the snow depth is greater than 0.8 m, the inversion accuracy of FFT algorithm is higher. Therefore, according to the different snow depth, a combined algorithm of NLSF + FFT is proposed for GNSS-IR snow depth inversion. Compared with the traditional LSP algorithm, the inversion accuracy of the combined algorithm is improved by 10%. For L2 band data, the results show that the accuracy of snow depth inversion of various algorithms do not change with the variations of snow depth. Among the three single algorithms, the inversion accuracy of FFT algorithm is better than that of LSP and NLSF algorithms.     

Total electron content monitoring using triple frequency GNSS: Results with Giove-A/-B data

Triple frequency GNSS will be fully operational within the next decade, opening opportunities for new applications. Dual frequency GNSS already allow to study the ionosphere through the estimation of Total Electron Content (TEC). However, the precision is limited by the ambiguity resolution process. This paper studies a triple frequency TEC monitoring technique in which the use of Geometry-Free and Iono-Free linear combinations improves the ambiguity resolution process and therefore the precision of TEC. We have tested it on a set of triple frequency Giove-A/-B data from January and December 2008. The conclusions achieved are (1) TEC values are affected by an error of about 2–2.5 TECU produced through the ambiguity resolution process; (2) the error caused by the Geometric Free phase combination delays (hardware, multipath, noise, antenna phase center) on TEC is about 0.2 TECU; (3) the total error on TEC approximately reach 2–3 TECU.     

不同GNSS在高纬度地区时间传递性能分析

全球卫星导航系统（GNSS）载波相位时间传递技术是高精度时间传递领域的主要研究方法之一，但目前关于该部分的研究主要集中在中低纬度地区，在高纬度地区并不多见。不同GNSS由于星座设计不同，在高纬度地区结构差异较大，因此需要对不同GNSS在高纬度地区的时间传递性能进行分析。实验结果表明，在高纬度地区时间传递中，Galileo稳定度最高，GPS和BDS次之，GLONASS最差。同时因在高纬度地区卫星的高度角普遍偏低，为合理平衡低高度角时可视卫星多和多路径误差大的矛盾，对不同截止高度角下获取的链路时间传递性能进行了分析。结果表明，在5°截止高度角下，高纬度地区的时间传递链路稳定性最好。     

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     

全球导航卫星系统的测高研究

以美国全球定位系统为代表的全球导航卫星系统具有广泛的应用领域，特点是能够在全球范围和近地空间，全天候地提供飞行器的位置、速度和时间信息。     

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.     

An unbiased carrier-phase anti-interference filter based on mirror frequency amplitude compensation

Anti-interference and high-precision measurement are two important indicators of the performance of a satellite navigation receiver. However, current receiver designs do not simultaneously satisfy these two criteria. While the carrier-phase ranging technique is necessary for high-precision receivers, frequency domain interference suppression (FDIS) results in tracking error biases for nonideal analog receiver channels. Importantly, as the FDIS filter is adaptive, the bias will vary with the jamming pattern, particularly when the frequency of interference varies. For precision navigation applications, this bias must be mitigated. Therefore, a new FDIS filter based on the mirror frequency amplitude compensation (MFAC) method is proposed in this paper. The amplitude at the symmetry position of the notch frequency is doubled in the MFAC method to mitigate this carrier-phase bias. The simulation results showed that the MFAC method can reduce the range of the carrier-phase bias by more than 60% for different interference bandwidths, which substantially exceeds that achieved using the conventional FDIS and calibration filter methods of 20 orders.     

发动机机匣、支板系统结构动力分析

发展了在环向只能处理连续变量的常规有限条法。分析在环向由若干个离散分布的支板和支架支承的发动机机匣回转壳。各离散支板和支架回转壳刚度的贡献, 是通过支板和壳体连接处位移相等的变换, 将各支板的刚度阵叠加到回转壳的总体刚度阵上实现的。同时证明了位移在环向的Fourier级数展开式中, 只有n=0, n=1项对轴系的支承刚度有贡献。支板及支板附近的壳体的局部应力, 是由局部子结构空间板壳有限元分析得到, 该子结构的边界位移由整体的回转壳分析给出。算例表明, 本文提出的方法能有效地计算轴系的支承动刚度和壳体的局部应力。