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Objectives: Global navigation satellite system (GNSS) coordinate time series provide important data support for the study of crustal movement and deformation, and plate tectonics. Due to the noise caused by various external factors, the GNSS coordinate time series cannot reflect the real motion information of the station well. To effectively reduce the noise in the GNSS time series, we adopted a noise-reduction method combining genetic algorithm (GA) and variational mode decomposition (VMD-GA-VMD). Methods: First, the genetic algorithm was used to optimize VMD parameters, and the envelope entropy of the input signal was used as the fitness function of the genetic algorithm to find the optimal VMD parameter combination suitable for the signal. According to the optimized parameters, the signal was decomposed by VMD to obtain a series of modal components. Then we calculated the multi-scale permutation entropy (MPE) of each component and then regarded the MPE as the criterion of the noise component. Finally, according to the MPE, the noise components were identified and removed, and the remaining components were reconstructed to obtain the noise-reduced signal. In this paper, the noise reduction effect of GAVMD was analyzed through the example of noise reduction of analog signal and observation data, and compared with wavelet denoising (WD) and empirical mode decomposition (EMD) methods. Results: The experiment results show that: (1) The noise reduction results from the analog signals show that WD and EMD have the incomplete and excessive troubles on the noise reduction, respectively. However, GA-VMD can effectively eliminate noise and retain effective signals. From the evaluation index, compared with WD and EMD, the signal-to-noise ratio were increased by 5.18 dB and 2.91 dB, the correlation coefficient by 0.05 and 0.02, respectively, when using GA-VMD. (2) For the complex observation, we used the noise and velocity uncertainty as accuracy indicators to evaluate the noise reduction effects of the three methods. The results show that WD can only extract a part of the white noise, while EMD and GA-VMD can completely remove the white noise. GA-VMD can reduce the flicker noise to the range of 0 to 6 mm/a0.25. For the velocity uncertainty, the average gain rates of GA-VMD relative to the WD and EMD are 69% and 15.33%, respectively. GA-VMD has an average correction rate of 79.89% and 84.46% for the velocity uncertainty and flicker noise of GNSS coordinate time series. Conclusions: Therefore, GA-VMD is the most effective one among the three noise reduction methods, which can effectively reduce the noise in the GNSS time series and improve its accuracy. However, in this paper, we only discussed the effect of GA on VMD parameter optimization without comparing it with other methods. Hence, it will be the key for studying the advantages and shortcomings of those optimization algorithms in the selection of VMD, and improving the accuracy on the GNSS time series in the future. [ABSTRACT FROM AUTHOR]

Objectives: Atmospheric water vapor is one of the important components in troposphere. It is of great significance to study the factors and accuracy of influencing water vapor. This paper mainly studies the influencing factors of precipitable water vapor (PWV) in the Loess Plateau and evaluates its actual accuracy. Methods: Firstly, the pressure, temperature data of the fifth-generation atmospheric reanalysis data of EC-MWF (ERA5) and global navigation satellite system (GNSS) derived zenith troposphere delay (ZTD) are evaluated. Secondly, according to the pressure and temperature data of ERA5 and ZTD data of crustal movement observation network of China (CMONOC), the PWV with 1-hour time resolution is calculated, and the theoretical error of PWV is deduced by using error propagation theory, and then the accuracy of PWV in the Loess Plateau is analyzed by comparing with the actual calculation error of PWV. Results: The results show that the average root mean square(RMS) and bias of GNSS-derived ZTD calculated by GAMIT/GLOBK and PANDA are 4.05 mm and −0.46 mm, respectively. The average RMS and bias of ERA5-provided pressure and temperature are 3.36 hPa/1.97 K and − 0.01 hPa/0.04 K, respectively. The theoretical error of PWV in the Loess Plateau is 1.51 mm, and the actual error is 1.94 mm. Conclusions: The accuracy of PWV calculated in this paper is high, which is of great significance to the investigation of water vapor distribution and climate monitoring. [ABSTRACT FROM AUTHOR]

Objectives: Global navigation satellite system (GNSS) technology is widely used in deformation monitoring such as dams, bridges, and landslides. In traditional monitoring methods, the reference stations need to be built, which leads to the increased monitoring cost. Methods: This paper proposes a GNSS deformation monitoring method based on virtual reference station (VRS), which uses the existing continuously operating reference stations (CORS) observation data to generate VRS instead of physical reference station to achieve high accuracy deformation monitoring. Firstly, the existing CORS observation data are used to obtain the atmospheric error correction models and generate VRS. Secondly, the baselines between VRS and each monitoring point are calculated. In order to further improve the accuracy of data processing, Delaunay triangulation networks are formed, and the baseline vector solution results are checked according to the criterion that the triangle closure error is zero. Finally, the checked baseline solution results are adjusted to obtain the final three-dimensional position coordinates of the monitoring points. Results: This paper uses about 30 monitoring points in Pingbian Miao Autonomous County, Yunnan Province for experimental analysis. The results show that the interpolation accuracis of the double-difference tropospheric and ionospheric delay are better than 10 mm if the distance between CORS reference stations is less than 40 km. The observation data longer than 8 hours can realize deformation monitoring with horizontal accuracy better than 5 mm and vertical accuracy better than 10 mm. Conclusions: The proposed method can realize the practical application of landslide monitoring. In the future, we can study how to improve the atmospheric error model and further improve the accuracy of data processing [ABSTRACT FROM AUTHOR]

Objectives: The position time series of global navigation satellite system (GNSS) contain rich tectonic and non-tectonic deformation information, and the complex components are difficult to accurately model and effectively separate non-tectonic information. It is very important to remove the non-tectonic de‐ formation information for the accurate and effective application of the observation data. Methods: Empirical mode decomposition （EMD） is an adaptive time-frequency processing method to correct the period term of time series at 24 GNSS continuous stations and 2 mobile stations in Sichuan Province and Yunnan Province. Results: The results show that the correction of the period term is necessary, and the EMD method can ex‐ tract the different frequency and amplitude period components adaptively according to its own characteristics of signal at each station. Compared with the original time series, the average root mean square error after correction is reduced significantly in N, E and U directions. And EMD method is more accurate and effective than harmonic model modification. We use the modified continuous station time series to simulate the mobile observations, and find that relatively reliable motion velocities can be obtained after an observation period of 5 ‐6 years. The stability and reliability of EMD method are verified by correcting the period term of the mobile station with the actual continuous observation station at a close distance. Conclusions: This paper provides a reference and theoretical basis for the implementation of mobile GNSS observation and the correction of observation data. [ABSTRACT FROM AUTHOR]

The global constellation network of BeiDou-3 navigation satellite system (BDS-3) has been completed, which means that BDS-3 has entered a new era of providing high-quality positioning, navigation and timing service for global users.Methods: To comprehensively compare the performance of BDS-3 uncombined precise point positioning (PPP) with other global navigation satellite systems (GNSS), this paper focuses on three aspects, including the consistency of BDS-3 precise orbit and clock products among different analysis centers, the satellite availability of BDS-3/GNSS, the positioning performance of BDS-3/GNSS single-system and multi-system PPP.Results: Based on the precise orbit and clock products from five analysis centers, the three-dimensional root mean square error of BDS-3 static PPP is between 2.31 cm and 4.00 cm, and its convergence time is significantly slower than that of other GNSS. The intro‐ duction of GPS observations can achieve the most obvious improvement among BDS-3/GNSS dual-system joint PPP. Besides, the quad-constellation observations can effectively shorten the convergence time of PPP and improve the positioning accuracy on kinematic mode. Conclusions: With the development of BDS-3 and the improvement of its orbit and clock products, BDS-3 has a great potential to provide highquality positioning services as other GNSS. [ABSTRACT FROM AUTHOR]

Ionosphere is an important part of the near-earth space environment, and it has an important impact on radio communication, satellite navigation and positioning. Therefore, monitoring the ionospheric morphology is helpful to understand the spatiotemporal changes of the ionosphere and predict the ionospheric morphological characteristics. With the development of global navigation satellite system（GNSS）,ionospheric monitoring has become an important research and application direction. This paper systematically introduces the research progress of multidimensional GNSS ionospheric monitoring and its application in related aspects. It mainly includes space-based/ground-based GNSS joint inversion of ionospheric characteristic parameters, tomography inversion of ionospheric three-dimensional structure, ionospheric delay modeling, ionospheric abnormal disturbance monitoring and mechanism cognition. [ABSTRACT FROM AUTHOR]

Objectives: Mobile phone positioning is a widely used approach for navigation, which has broad application prospects. The global navigation satellite system (GNSS) is widely used in smartphone positioning, but its performance can be degraded in urban canyons because of signal reflections or blockages. Shadow matching (SM) based on the three‑dimensional (3D) city model can effectively improve positioning accuracy in cross ‑street direction. However, variation of signal‑to‑noise ratio (SNR) is large using smartphone for GNSS signal reception while the conventional method fails to distinguish neighboring streets, hence, greater cross ‑ street errors. Methods: This paper proposes an improved SM method together with SNR smoothing implemented in smartphones to improve the positioning accuracy in urban canyons. Firstly, a SNR smoothing method based on low‑pass filter is proposed to mitigate the variation, and further to improve the correctness and stability of the visibility classification based on observations. On this basis, an improved SM, namely cluster shadow matching (Cluster‑SM), is proposed, in which, the effective candidate points are clustered related to their locations. Results: Experiment results showed that SNR smoothing re‑ duces error rate of the SNR classification from 5%-30% to 0%-20%, while the implementation of optimization Cluster ‑ SM based on SNR filtering significantly improve the GNSS positioning accuracy from 19.4 m to 2. m in dynamic test, compared to conventional method. Conclusions: This shows the effective‑ ness of the novel approach in increasing positioning accuracy with the ability to distinguish neighboring streets, which provides opportunities to implement the smartphones in location‑based services applications, pedestrian positioning or vehicle navigation which requires a higher positioning accuracy. [ABSTRACT FROM AUTHOR]

The development and improvement of the global navigation satellite system (GNSS) provides an important basis for multi‐constellation’ s fusion positioning, navigation, and timing, and it also provides more favorable external conditions for the receiver’s autonomous integrity monitoring. Firstly, the protection level of GNSS single system and multi‐system combination under single‐frequency and dual‐frequency conditions were calculated respectively through the constellation simulation. Then, This paper analyzed the basic distribution of protection levels under different frequencies and different combinations of GNSS worldwide. Finally, the availability of receiver autonomous integrity monitoring (RAIM) in different aviation approaching phases under different frequencies and combinations was discussed. The calculation results show that the single system under single‐frequency or dual‐frequency combination cannot meet the requirements of the approach phase of localizer performance with vertical guidance 200ft (LPV‐200).Nevertheless, double‐ system under dual‐frequency can meet the requirement.The triple‐system under dual‐frequency combination can meet the requirements of approach with vertical guidance II (APV ‐II), but cannot meet the requirements of precision approaching. But the improvement of four‐system combination is not obvious com ‐ pared with that of triple‐system combination. [ABSTRACT FROM AUTHOR]