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Objective: Navigation context comprises observation environment and carrier activity. While global navigation satellite system (GNSS) can provide high-precision navigation, positioning, and timing services for users, users can also annotate the types of navigation scenarios they are in by distinguishing the quality and features of terminal observation values. Navigation context clustering is important for adaptive seamless navigation and location services, which can significantly promote context adaptability, configuration flexibility, and system robustness. Methods: In terms of current issues that the division of water/underwater context is simple and has insufficient granularity, the quality and characteristics of GNSS observation signals in different water depth layers are analyzed by collecting GNSS observations in different water regions. Results and Conclusions: The results show that the underwater navigation context has remarkable layering. Considering that the navigation context has inherent properties such as segmentation, merging, and connectivity, this paper subdivides the underwater navigation context into three layers: Submerged water (about less than 6.5 cm), shallow water (about greater than 6.5 cm and less than 8.5 cm), and deep water (about greater than 8.5 cm) through K-means clustering, and conducts navigation context classification experiments using various GNSS signal features. It reveals that the clustering accuracy rate is 90.4%, indicating the effectiveness of submerged-shallow-deep navigation context division. [ABSTRACT FROM AUTHOR]

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]

Soil moisture (SM), as an important component of global water and energy cycles, plays an essential role in the Earth's water cycle, energy balance, and climate change. Traditional field measurement methods for SM are difficult to obtain the spatial distribution of large-scale and long-term SM and have high measurement costs. Microwave remote sensing, with its all-weather and penetrating capabilities, is one of the most effective means of obtaining soil moisture at a large scale and long-term manner, particularly the L-band global navigation satellite system-reflectometry (GNSS-R) technology with advantages of both active and passive microwave remote sensing, which has been paying a lot of attention in SM measurements. Based on the investigation and analysis of domestic and international progress, this paper systematically introduces the principle and characteristics of GNSS-R technology for obtaining SM, and gives the specific steps of SM measurement from both forward and backward retrieval aspects, the main faced difficulties, and key issues to be solved. Taking the latest cyclone GNSS (CYGNSS) satellite estimation as the main part, the theoretical model and method based on CYGNSS SM estimation are deeply expounded. In addition, the opportunities and challenges in the development of GNSS-R SM remote sensing are briefly analyzed, aiming to provide a reference for future ground-based, airborne, and satellite-based GNSS-R SM remote sensing observation and application in China. [ABSTRACT FROM AUTHOR]

Continuous and dense global navigation satellite system (GNSS) observations of surface deformation can be used to recover regional terrestrial water storage (TWS) changes with high spatiotemporal resolution, which is of great significance for the study of regional water cycle and climate changes and has become a hot topic in the field of hydro-geodesy. This paper introduces the basic theory and inversion algorithms of the loading Green's function method and Slepian basis function method, and the stability and performance of these two methods in retrieving regional TWS changes by using GNSS surface deformations are evaluated. Meanwhile, the latest progress in using GNSS-derived TWS changes to study regional water cycles and extreme climate changes is reviewed. The existing problems and future development trends of the inversion of GNSS observations (e. g. refined GNSS data processing and joint inversion of multi-source data) are summarized and analyzed. It provides important insights into the use of GNSS observations to investigate regional TWS changes and related applications in hydro-geodesy. [ABSTRACT FROM AUTHOR]

Objectives: Latest release of international terrestrial reference frame has been generated with an enhanced modeling of nonlinear station motions, while more investigations on the strategies applied for the terrestrial reference frame modeling nonlinear station motions still needed. Methods: A global GPS (global positioning system) network has been reprocessed, using the latest GNSS (global navigation satellite system) data processing models and an integrated strategy, and then accurate and consistent input data are obtained; the post-seismic deformation models of 13 CMONOC (crustal movement observation network of China) stations are established and used as the input data, and FREQ2016 based on the function model, and GFZ2016 and EOST2016 considering the environmental loading correction are realized; these GPS terrestrial reference frames with different models are compared in terms of the goodness for fit, velocities and seasonal signals, and then the optimal model is explored. Results: The results show that the weighted root mean square averages of the postfit residuals of FREQ2016 are smaller than those of GFZ2016 and of EOST2016; FREQ2016 shows more significant seasonal signals, especially for semi-annual signals; the horizontal velocity field differences of these three are almost negligible (<0.08 mm/a), and the large vertical velocity differences, for some stations, reach 1.3 mm/a; the reduction of FREQ2016 horizontal velocity formal errors is about 10%. Conclusions: This paper suggests that the strategy based on the function model is better than that of considering the environmental loading correction. [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]

Global navigation satellite system(GNSS)⁃acoustic seafloor positioning is a technology proposed and developed for deformation monitoring of plates in the submarine subduction zone. As an important technology for the construction of marine space time reference network, it has broad application prospects. Although the current research results of GNSS⁃acoustic seafloor positioning cannot meet the construction re⁃quirement of marine space time reference network, its data processing method, especially the fine processing method of sound speed error, has important reference significance for the construction of seafloor part (seafloor geodetic reference station) of marine space time reference network. This paper firstly introduces the origin of GNSS ⁃acoustic seafloor positioning and summarizes it into two categories: Static measurements and dynamic measurements. At the same time, the sound speed error processing method is used as the development line to refine three development stages of the technology: Only assuming the vertical stratification of ocean sound speed, considering the temporal variation of sound speed, and considering the horizontal gradient of sound speed. For the stage of only assuming vertical stratification of ocean sound speed, foreign scholars used geometric structure symmetry to weaken the influence of sound speed error; domestic scholars mainly studied the error sources besides the sound speed (lever arm offset, time tag deviation, attitude angle error, etc.) and used the stochastic model to weaken the influence of system error on the positioning results. For the stage of considering the time⁃domain variation of sound speed, foreign scholars used fitting method (polynomial fitting or cubic spline fitting) combined with parameter smoothing constraint to solve the time⁃domain variation of sound speed, and improve the stability of positioning results. Based on this, domestic scholars have refined the parameter fitting method (considering the variation characteristics of the long period term of parameters). In addition, domestic scholars have proposed the underwater differential positioning algorithm to innovate the seafloor positioning technology. For the stage of considering the sound speed horizontal gradient, scholars have solved the sound speed horizontal gradient parameters in the GNSS⁃acoustic seafloor positioning processing, which improves the stability of the horizontal positioning results, and verified the reliability of the results by using the marine numerical model.This paper presents the prospect of high precision data processing methods of GNSS acoustic seafloor positioning for the seafloor reference station construction, and introduces the concept of small spatial⁃temporal scale sound speed tomography (sound speed error processing method based on ocean positioning reference network) in order to solve the problem that numerical prediction model cannot provide small spatial⁃temporal scale products, so as to provide high⁃precision sound speed error correction services for underwater vehicles. [ABSTRACT FROM AUTHOR]

Objectives: On July 31, 2020, China's BeiDou-3 satellite navigation system (BDS-3), which operates independently, has been fully completed and officially opened for service. The signal system of BDS-3 has been redesigned to provide public service signals of five frequencies: B1I, B3I, B1C, B2a and B2b. Methods: Based on the observations of international global navigation system monitoring and assessment service and multi-GNSS experiment (MGEX) tracking stations, this paper analyzes the characteristics of pseudoranges multipath error, signal to noise ratio, geometry-free ionosphere-free phase(GFIFP) combined observations of the new signal system, as well as the precise orbit determination (POD) performance evaluation of BDS-3 satellites. Results and Conclusions: The results show that the multipath noise level of BDS-3 satellites signal is better than that of BeiDou-2 navigation satellite system satellites, and there is no systematic deviation related to altitude angle, B1C is more significantly affected by multipath and noise. The GFIFP sequences of different signal combinations of BDS-3 satellites show periodic systematic error related to satellite, and the peak value is about 2 cm. For BDS-3 satellites, the one-step POD method is used. The two frequencies ionospheric free combination of B1I & B3I and B1C & B2a is adopted respectively. The orbit boundary discontinuity and satellite laser ranging carried out an orbit accuracy check, and the results show that when the number of available observations is less than B1I & B3I, the orbit accuracy of B1C & B2a is equivalent to that of B1I & B3I, and the radial internal coincidence accuracy is 6.1 cm and 6.6 cm respectively. [ABSTRACT FROM AUTHOR]

Objectives: Non-ideality of navigation satellite signals can cause ranging bias between different receivers, which is an important factor that may degrade the accuracy and integrity performance of global navigation satellite system. In civil aviation and other high integrity services, it is necessary to consider the non-ideality characteristics of the signal and define the design constraints of the user receiver, so as to reduce the impact of non-ideality and ensure service safety. Methods: The B1C and B2a signals of BeiDou satellite navigation system (BDS) are planned to join the international civil aviation standard, so it is necessary to study their non-ideality characteristics and define the receiver design constraints. This paper analyzed the non-ideality characteristic of BDS B1C and B2a signals. In order to avoid the influence of noise and multipath, a large aperture antenna was used to collected all the in-orbit satellite broadcasting B1C and B2a signals (including 27 satellites) to obtain pure signal samples. Then, the software receiver was used to process the signal samples under various receiving parameters to obtain the ranging biases under different receiver front-end bandwidth and code discriminator space, and the ranging bias range and variation with the receiving parameters are evaluated. Furthermore, taking the application in dual frequency multi-constellation satellite based augmentation service as an example, the receiver design constraints of the two signals were analyzed. Results and Conclusions: The experimental results show that the range biases introduced by the non-ideality of B1C and B2a signals are less than 0.68 m and 0.44 m, respectively, under the parameters range commonly used by receivers. Under the requirement of range bias less than 0.1 m, the applicable parameter range of receiver design constraints for B1C and B2a signals is better than the relevant requirements of the international civil aviation organization draft standard. There is sufficient margin to further consider other constraints. [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]

Objective: The research on topography and landform of the Yangtze estuarine waters is of great significance to human social and economic activities such as river management, river construction and water transportation. Establishing an accurate and seamless chart datum and its transformation model with other vertical datums will directly affect the acquisition of high ⁃precision topographic and geomorphic data at the water-land junctions and the unified comprehensive management and analysis. Thus, this paper studies the establishment of seamless chart datum and its transformation model with other vertical datums for estuarine waters, combining with geoid, sea surface topography and three-dimensional numerical simulation of tidal wave motion. Methods: Firstly, the unstructured-grid, finite-volume community ocean model (FVCOM) was used to simulate tidal level and calculate the tidal range at each grid node with the underwater topographic data and the open boundary water level forcing data. According to the correlation between tidal range and chart datum, the chart datum value of each grid node was deduced using tidal range ratios from tide gauge stations with known datums, and a regional seamless chart datum was then established. Secondly, the mean sea surface topography corresponding to a long time scale at each grid node was calculated with the daily sea surface topography product provided by the Copernicus marine environment monitoring service (CMEMS). The mean sea surface topography model was further established by the minimum curvature interpolation. Meanwhile, the geoid in the corresponding area was calculated by the global ultra ⁃high ⁃ order gravity field model Earth gravitational model 2008 (EGM2008) with a spatial resolution of 5′ × 5′. Thirdly, the geoid model was established through bi ⁃ quadratic interpolation. Finally, according to these three constructed models and the geometric relationship of three kinds of vertical datums in space, unification to a consistent spatial resolution was performed and the separation value between chart datum and the ellipsoid reference datum at each grid node was calculated. The vertical datum transformation model be⁃ tween the seamless chart datum and ellipsoid reference datum was established by the minimum curvature interpolation. Results: Using this method, we carried out the experiment at the typical estuarine waters, south branch of the Yangtze Estuary, and established the seamless chart datum and its vertical datum trans⁃ formation model. The global navigation satellite system (GNSS) leveling data from three long-term tide gauge stations were used for verification and accuracy evaluation of the model. The results show that the ab⁃ solute errors of the vertical datum transformation model at these three long-term tide gauge stations were − 19.8 cm, −25.8 cm and −27.0 cm respectively, which were all higher than the evaluated error (12.4 cm) of the model. The main reason is that the Yangtze estuarine waters have complicated tidal wave characteristics and they are nearshore waters, which leads to poor accuracy of the mean sea surface topography model in this region, thus further reducing the accuracy of the transformation model as a whole. Despite this, the accuracy of the transformation model still meets the requirements of International Hydrographic Organization standards for hydrographic surveys. Conclusions: Therefore, the research method of this paper are feasible and reliable, especially in far-sea areas. When the accuracy of satellite altimetry is further improved in the nearshore areas, the proposed method in this paper can be extended to nearshore and inland waters. [ABSTRACT FROM AUTHOR]

In order to meet the requirements of high-precision positioning in offshore complex construction environment far from the coast, it is necessary to assist the offshore survey platform to carry out the survey and positioning. In this paper, a floating state precise positioning method based on the combination of GNSS and total stations is presented, in which a working vessel is used as a carrier and a GNSS receiver and an electronic total station can be set up directly on the deck of the working vessel to effectively control the influence of measurement error caused by wave and current fluctuation to realize the accurate survey and location of the construction plan. A floating-state precise positioning system with the combination of GNSS and total station is developed based on the project cases and the feasibility of the system useing for accurate survey and location of the construction plan with floating state survey is verified by project practice. [ABSTRACT FROM AUTHOR]

Objectives: As BeiDou satellite navigation system‑3 （BDS‑3） starts to provide service for global users, it is possible to get global‑coverage and all‑time positioning service for space application using BDS alone. The performance of space‑borne BDS positioning is thoroughly analyzed with the broadcast ephemeris and the in‑orbit data of global navigation satellite system occultation sounder (GNOS) aboard FengYun‑3D (FY‑3D) satellite. Methods: Firstly, the visibility and position dilution of precision of BDS satellites in different low‑erath‑orbit （LEO） are calculated based on broadcast ephemeris. Moreover, the orbit, clock error of broadcast ephemeris and signal‑in‑space range error (SISRE) are studied. Secondly, the real visible satellite number, signal strength, precision of pseudo range and position accuracy are verified with the measurement data of GNOS. The code biases of BDS‑2 satellites are focused on. Finally, the total electron content above 836 km LEO orbit is measured using BDS dual‑frequency measurements. Results: The results show that the global coverage usability from ground to 2 000 km height orbit has already been 100%. The mean visible BDS satellite number across the world is 50% larger than that of global positioning system （GPS） . For BDS broadcast ephemeris, the 3D orbit error is 1.5 m and clock error is 2.4 ns. SISRE is about 0.79 m and the clock accuracy of BDS‑3 has reached the same level of GPS. The in‑orbit data results show that GNOS in FY‑3D could get 100% positioning with BDS‑2 signals in service areas, and 3D position accuracy is 5.53 m. All BDS‑2 satellites including geosynchronous earth orbit （GEO）, inclined geosynchronous orbit and medium earth orbit satellites have code biases. As GEO satellite is static relative to ground users, the code biases of GEO is firstly measured directly, in which the maxium code bias can reach about 0.6 m. The total electron content above 836 km can cause relative pseudo range delay of about 0.6 m. Conclusions: The research in this paper verifies the usability and performance of BDS for space‑based users . [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]

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]

Objectives: BeiDou-3 navigation satellite system (BDS-3) has been fully completed and pro‐ vides reliable positioning, navigation, and timing services to global users. In order to achieve compatibility and interoperability with other global navigation satellite systems (GNSS), B1C and B2a signals have been modulated by BDS-3 based on BDS-2, which enable frequency multiplexing with E1 and E5a signals of Galileo. The inter-system biases (ISBs) are important for the fusion processing of different GNSS. Methods: This paper proposes an ISB estimation and application model based on the single-differenced observations and analyzes the ISBs between the overlapping frequencies of BDS-3 and Galileo. Based on several receivers of the new trackable BDS-3 signals, the characteristics of ISBs between BDS-3 and Galileo are revealed, and the real-time kinematic (RTK) positioning performance of BDS-3 and Galileo is analyzed. Results: The results show that the ISBs of B1C-E1 and B2a-E5a exist in different types of receivers but not in the same type of receivers. In addition, compared with BDS-3-only and Galileo-only, the combination of BDS-3 and Galileo has an improvement of more than 10% in both the success rate of ambiguity resolution and positioning accuracy. Conclusions: The multi-constellation RTK positioning performance with the introduction of ISBs is more advantageous than that of a single constellation [ABSTRACT FROM AUTHOR]

Observation quality is one of the key issues to assess the performances of satellite navigation system. As the main error of positioning and navigation, the multipath delay mitigation has be‐ come the focus of the research in global navigation satellite system. Because the traditional multipath model‐ ling strategy cannot obtain the global optimal solutions, this paper proposes a real-time multipath correction model for BeiDou navigation satellite system (BDS) observation with information constraints. Methods: Firstly, the regularization algorithm combined with total variation term is used to denoise and solve the mul‐ tipath delay series based on the estimated BDS multipath delays. Secondly, the trend and random parts of multipath delays are separately processed and together modelled, in which the strategy of least square plus autoregression is used to estimate model coefficients of multipath delay model by one-step solution. Finally, the multipath mitigation model is further optimized by introducing the estimated model coefficients as the prior information constraints in the BDS real-time applications.Results: The results of real-time experiments of BDS precise point positioning show that, compared with the traditional strategy, the position ac‐ curacy of BDS-2 PPP in east (E), north (N) and up (U) directions can be improved with 10.6%−64.9%, 0.0%−59.1% and 12.6%−67.2% (B1I+B3I), respectively. Moreover, the position accuracy of BDS-3 PPP can be optimized with at least 13.9%, 60.0%, 45.9% (B1I+B3I) and 19.1%, 46.5%, 23.9% (B1C+B2a) in E, N and U directions, respectively. Conclusions: Therefore, the proposed mitigation multipath delays method can significantly improve the BDS observation quality in the BDS real-time positioning services. [ABSTRACT FROM AUTHOR]

Agriculture is crucial for economic development. Losses in agricultural production were caused by a variety of circumstances, but pests and illnesses are among the most important. The use of plant protection machinery to spray pesticides is still the most effective method of pest control at present. Traditional agricultural sprayers were inefficient and time-consuming, which caused pesticide waste and environmental contamination. In the process of spraying pesticides, farmers are directly exposed to pesticides, which is easy to cause bodily harm or even poisoning. With the steady progress of China's agricultural modernization, robots are widely used in agricultural plant protection work. Intelligent pesticide spraying robots are an effective method to solve the problem of applying pesticides in complex environments such as hills, orchards, fields, and greenhouses, which is more efficient and accurate. Intelligent spraying robot not only saves labor but also reduces the improper use of resources and environmental pollution. It is conducive to our better reaching of the goal of agricultural sustainable development. In this study, the research progress analysis of an intelligent pesticide spraying robot focuses on four respects: mobile chassis design, spraying the device design, navigation technology, and target detection technology. This paper discusses the factors limiting the development of intelligent pesticide-spraying robots for the primary technologies applied to various operation scenarios. Firstly, the mobile chassis design of the spraying robot was analyzed from the ground spraying robot and aerial spraying robot. The study and development of wheel-track composite robot chassis can integrate the advantages of wheeled chassis as well as a tracked chassis, which will be the research trend of ground intelligent spraying robot. Plant protection UAV is unrestricted by terrain, and can be applied to a variety of scenarios, which will be the key orientations for future research. Secondly, the spray devices required by the spraying robot in different spraying scenarios were summarized. It mainly includes an adjustable boom sprayer device, electrostatic spraying device, targeted spraying device, air-assisted spraying equipment, and profile modeling spray equipment. The most commonly used navigation methods are analyzed, including Global Positioning System (GPS) navigation, visual navigation, inertial navigation, multi-sensor fusion navigation, laser radar navigation, odometer, geomagnetic navigation, etc. Additionally, the detection methods of various pesticide spraying robots are evaluated from the perspectives of crop detection and pest detection. The deep learning algorithm can automatically extract the image features of crops, make use of the information, and exactly identify the location of crop pests and diseases, to quickly respond to achieve accurate spraying. Finally, the development trend of intelligent pesticide application robots for precise application is presented. Six requirements or prospects were proposed for the intelligent pesticide spraying robot development in the future, including to improve the response speed of the spraying robot, to improve detection accuracy, to design lightweight chassis, to reduce the development cost of the spraying robot, and to integrated air-ground spraying technology. In conclusion, intelligent and precise application robots have the potential to significantly improve the quality of precision agriculture operations by facilitating intelligent decision-making on operational parameters before actual operation. [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]

Unmanned Aerial Vehicles (UAV) have widely been served as a new technology in agricultural aviation plant protection and pest control in China. However, it is highly demanded to improve the accuracy of UAV spraying pesticides and operational efficiency. In this study, a Variable Rate Application (VRA) of a centrifugal spraying system was designed for precise placement and timing of pesticide application in specific field conditions using the network Real-Time Kinematic (RTK) technology. STM32F103 single-chip microcomputer was used for the core controller. A serial port of controller was selected to obtain the global position system (GPS) information. A network modular Data Terminal Unit (DTU) was connected to realize the network RTK technology via the serial port of the controller. The control voltage was used to tailor the speeds of the centrifugal nozzle motor and peristaltic pump. A Pulse Width Modulation (PWM) was used to control the output voltage from the core controller to the armature of motors in this system. The speed of the centrifugal nozzle motor determined the output droplet size, while the speed of peristaltic pump determined the flow rate and amount of spray fog. The agricultural dataset before the system working was acquired using the airborne multispectral camera, ground object spectrometer, and handheld GPS. A prescription chart was also constructed using ArcGIS software. In the system working, the GPS modular was used to capture the location longitude and latitude data for the system to read and analyze. The system was always real-time searching the UAV geographical position during the spraying pesticides process using the GPS data, and then matching the position using the orthogonal grid. Meanwhile, the system was used to real-time tailor the changes in the duty cycle of output PWM, after matching the decision of the prescription map. The speeds of the centrifugal nozzle motor and the peristaltic pump were then to control the particle size and application amount of the UAV. The operation data was finally uploaded to the monitoring platform for real-time display and storage. Several spraying experiments were carried out in the research base of South China Agricultural University in Zengcheng City, Guangdong Province of China. The airborne spraying device was carried on the MG-1p plant protection UAV using a developed DJI Drone. The sampling points of droplets were set to match the planting density of crops, where the water-sensitive paper was used to collect the droplet data. After UAV operation, the water sensitive paper was collected into a plastic bag for later use. An hp4678 scanner was selected to map the collected water-sensitive paper after the experiment. A DepositScan software was utilized to analyze the water-sensitive paper in each sampling area after image acquisition. A detailed dataset was obtained, including the deposition amount, coverage density, and particle size of droplets. The data demonstrated that the accuracy of the device was within 0.6 m at the operating speed of 2 m/s. The application effect was finally stabilized within 2.2 m in the case of continuous operation. Particle size varied smoothly in the boundary area of prescription, as the rotating speed of the centrifugal nozzle increased. The findings can provide a sound basis for the improvement of variable application technology of plant protection UAV in precise agriculture. [ABSTRACT FROM AUTHOR]

Objectives: Aiming at the mixed observation data of multiple distribution forms, a expectation-maximum (EM) combined p-norm distributed model(EM_p) is established.Methods: Considering that the mixed number in the mixture model belongs to incomplete data, the EM algorithm is introduced to estimate the parameters of the mixture model and the p-model mixture model parameters are derived in detail. The estimated iteration formula and the corresponding iteration steps are given.The mixture Gaussian distribution data, Laplace distribution and Gaussian distribution mixture data, and the residual data of measured global positioning system(GPS) observations are used to verify the correctness and adaptability of the formula in this paper. Results and Conclusions: The results of the calculation examples show that, compared with the single probability distribution, the p-norm mixture model can accurately reflect the actual situation of the data distribution, and the model parameters estimated by the EM algorithm have higher accuracy. [ABSTRACT FROM AUTHOR]

Objectives: It is difficult to effectively evaluate and weaken the impact of observation environment change on data quality. To solve the problem, this paper studies a multi-point hemispherical grid model which can be applied to various satellite navigation systems.Methods: It integrates the observations of different global navigation satellite system (GNSS) satellite pairs in different observation periods to establish a global optimal multipath model. GNSS multipath error can be estimated effectively using multi-point hemispherical grid model (MHGM), image-based inverse of multipath effect and reflect the orientation of interference sources in the observation environment around stations. Results: To a certain extent the observation environment around stations can be modelled. The test under deliberate high multipath environment reveals that using MHGM significantly improved the root mean square error (RMSE) of the double-differenced carrier phase observation residuals in the period of fixed ambiguity, reducing the mean RMSE from 0.983 cm to 0.318 cm, a 67.7% improvement. In addition, the abnormal areas with large value of correction in the MHGM results is consistent with the direction of the baffle mounted at the corresponding stations. This model indicates multipath effects on GNSS carrier phase observations in different directions from the station. The test with International GNSS Service historical observations shows MHGM can effectively reflect the influence of changing multipath interference around stations on carrier phase observations, with an average improvement of 29.5% in the RMSE of residuals over the past 18 years. Conclusions: This model can be used to evaluate the changes in observation environments surrounding stations over time. It has potential application value to improve the accuracy and reliability of augmentation system products. [ABSTRACT FROM AUTHOR]

Objectives: In precise point positioning (PPP) technology, the correct fixing of ambiguity is al⁃ ways an essential part of achieving high-precision positioning. However, the wrong fixing of ambiguity will lead to serious positioning errors. Therefore, it is necessary to optimize the selection of the ambiguity sub⁃ set to ensure a more reliable fixing of PPP ambiguity. Methods: This paper proposes a PPP partial ambiguity resolution (PAR) method that combines quality control and Gram-Schmidt orthogonalization. By utilizing the multi-GNSS(global navigation satellite system) experiment (MGEX) observations in the un-differenced and un-combined PPP model, the PAR method based on Gram-Schmidt orthogonalization was com⁃ pared and analyzed with the selected reference satellite method with maximum elevation, and the performance of ambiguity resolution and positioning was assessed under the multi-GNSS conditions. Results: Experimental results show that the average epoch fixing rate of each day and each station based on the Gram Schmidt method is improved by 7.74% and 11.46% in the static mode, and 7.90% and 7.78% in the pseudo-kinematic mode, respectively, compared with that of the selected reference satellite method considering the maximum elevation. Meanwhile, time-to-first-fix (TTFF) of each day and each station based on the Gram-Schmidt orthogonalization can be significantly reduced by 22.30% and 25.42% in the static mode, corresponding to an improvement of 20.44% and 19.65% in the pseudo-kinematic mode, respectively, compared with that of the selected reference satellite method considering the maximum elevation. As far as multi-GNSS PPP AR is concerned, multi-GNSS PPP AR has a significant improvement on the convergence time of BDS (BeiDou navigation satellite system) only. The convergence time of horizontal components is shortened by 20.00 min on average at 95% confidence level, and the vertical components are shortened by 19.00 min on average. For the positioning accuracy aspects, the positioning accuracy of horizontal components is improved by 1.50 cm on average, while that of vertical components is improved by 1.12 cm on average. Conclusions: The PPP PAR algorithm based on the Gram-Schmidt method can significantly improve the performance of ambiguity resolution and positioning accuracy. [ABSTRACT FROM AUTHOR]

Objectives: Revealing the characteristics of deformation field related to earthquake preparation process is an important way to distinguish earthquake preparation stage and earthquake risk. In order to reveal the significance of crustal deformation observation, the deformation field related to the Wenchuan Ms 8.0 earthquake preparation and the relationship between the deformation field and the seismic activity are discussed from a large spatial scale. Methods: The original data of crustal tilt observed at Wenchuan station in the epicenter area of the Wenchuan earthquake since 2002 are collected, and the phased characteristics of crustal tilt change are extracted by using wavelet analysis method and piecewise fitting method. Based on the daily solution data of global navigation satellite system (GNSS) station coordinates since 2001, the baselines between Luzhou station and the surrounding stations are calculated. By comparing the baseline change with the Benioff creep, the synergistic characteristics between them are found. Results: The tilt rate in EW direction at Wenchuan station began to decrease significantly at the end of 2004, from 230 ms/a to 130 ms/a, and it decreased to 60 ms/a until the earthquake occurrence. There was no significant exponential acceleration change from the end of 2007 to the earthquake occurrence. The rapidly shortening anomaly of Guangzhou-Luzhou GNSS baseline in early 2005 has a high correlation with the earthquake creep loss anomaly, and the correlation coefficient is 0.89. The duration of rapidly shortening is about 2.5 years, and the magnitude of the corresponding earthquake is estimated to be 7.9, which is close to the magnitude of the Wenchuan earthquake. The Sumatra Ms 8.9 earthquake in 2004 has a significant impact on the stress-strain field in the eastern Tibet Plateau and accelerates the preparation process of the Wenchuan earthquake. Conclusions: The different influence of surrounding earthquakes should be considered in the analysis of crustal deformation and earthquake risk. The research results of this paper have reference value for understanding the significance of crustal deformation measurements and innovating earthquake prediction ideas. [ABSTRACT FROM AUTHOR]

Objectives: North China is an important social and economic area in China but suffers from severe surface deformation. Monitoring the surface displacement effectively and timely can provide data basis for understanding the deformation mechanism and predicting disasters. However, the surface vertical displacement in North China shows significant seasonal fluctuations. Hence, this paper aims to analyze the spatiotemporal feature of seasonal vertical displacement and determine the driving sources. Methods: Firstly, the vertical time series of 20 global navigation satellite system (GNSS) stations from January 2011 to November 2019 are decomposed by independent component analysis. Secondly, the first six independent components (ICs) are derived and compared with the vertical displacements caused by environmental loadings. Results: The results show that GNSS-IC1 presents obvious annual variation, as well as a relative uniform spatial response, which is highly consistent with the displacement time series derived from atmospheric loading, and the average correlation coefficient at all stations is 0.60. GNSS-IC2 and GNSS-IC3 are both the mixed signals constrained by annual and intra-annual periods, while the spatial responses reflect locally clustered characteristics, which corresponds to the distribution of water resources in North China. After combining GNSS-IC2 and GNSS-IC3 as GNSS-IC2/3, the average correlation coefficient between GNSS-IC2/3 and the hydrological loading displacement time series at all stations is 0.50. Conclusions: The temporal component of each IC is generally less clustered by different time cycles and the spatial response shows distinct localized characteristics. Atmospheric and hydrological loadings are the primary driving sources of seasonal vertical displacement in North China. However, the seasonal displacements induced by other effects, including non-tide ocean loading and thermal expansion effect of bedrock and observation pillar, are not extracted and need further analysis. [ABSTRACT FROM AUTHOR]

This paper briefly reviews the temporal and spatial characteristics of crustal deformation and its relationship with strong earthquakes in Chinese mainland based on geodetic observations. It also introduces the progress and understanding of extracting multi-scale crustal deformation characteristics associated with seismogenic process and various deformation anomalies before earthquakes in Chinese mainland. However, it is still insufficient that the effective basis for the crustal deformation can give the prediction of strong earthquakes in a clear time scale of 10 years and shorter. In order to promote the progress of continental strong earthquake prediction in a short time scale, it is necessary to effectively identify the process of strong source approaching earthquake in late seismogenic stage. Based on the analysis that the crustal deformation information observed by global navigation satellite system(GNSS) before several strong earthquakes, the crustal deformation state in the focal region of the late pregnancy earthquake could be transferred to the inelastic deformation state. Considering the factors such as the progress of observation technology, the possibility and related problems of the effective identification of the late pregnancy earthquake approaching to the strong source are discussed. [ABSTRACT FROM AUTHOR]

Objectives: Deep space antennas are widely used in deep space and radio astronomical observations, and their precise position coordinates are required for research needs. In many cases, this precise position is given by very long baseline interferometry (VLBI) based on an initial position with millimeter-scale accuracy. Global positioning system (GPS) alone is merely not sufficient to obtain the initial position of newly-built antennas. Therefore, a method using other objects as reference is needed to calibrate observatory positions, especially antenna arrays, and this paper presents such a method for deep space and radio astronomical observations. Methods: This method employs an azimuth-elevation telescope with an accurate position as a reference object to measure the center position of a target telescope. Other reference objects with satisfying position accuracy can also be used in this method. GPS receivers are adopted to establish a reference network with given scale and orientation by differential measurement. The next step is measuring the local positions of two telescopes (or objective and reference) in the reference network using total station instruments deployed in several different locations. Calculations are induced during the transfer of positions in the reference system to unify data from those measurements. The position of the target to the reference object in a local coordinate system is obtained. The final step is the calculation of the coordinates of the tar⁃ get telescope through the accurate position of the reference telescope. A series of processes are used to con⁃ vert coordinate positions from the local reference system to the international terrestrial reference frame 2014 (ITRF2014) reference system. Results: Experimental measurement was carried out to obtain the coordinates of the 40 m diameter radio telescope in national observatory of China (NAOC) Miyun observatory with the 50 m diameter radio telescope also there as a reference object. Total station measurement was con⁃ ducted at three locations around the reference telescope and four locations around the target telescope. Two reflecting prisms calibrated by GPS devices were used as scale and orientation reference. Total station targets were placed on azimuth and elevation structures of both telescopes. The target position data were obtained when the telescope rotated and tilted at multiple angles. More than 400 times of measurement were taken during a two ⁃ day field experiment. Round ⁃ fitting and sphere ⁃ fitting achieved by the trust-region method were used to process measured points, and the coordinates of the 40 m diameter radio telescope in NAOC Miyun observatory were given with a calibration error of 2.312 mm (root mean square error， RMSE) in the ITRF2014 system. Conclusions: This method provides center point location in mm or sub-mm-scale precision in complex terrain situations with simple tools and low time cost. The measurement of total station instruments towards reference objects or target objects could be the direct or centering measurement. This method can be applied to obtain positions of single or multiple antennas for observation and further calibration. Observatories can apply this method in complex terrain locations such as mountains, rocky fields, and even a meteor crater on an extra-earth planet. Subsequent observations show that this calibration has met the need. [ABSTRACT FROM AUTHOR]

Objectives: On 14th and 15th January 2022, the Hunga Tonga‑Hunga Ha̓apai (HTHH) underwater volcano in Tonga in the South Pacific Ocean violently erupted and caused a tsunami, which has attracted widespread international attention. In this paper, we attempted to rapidly investigate this oncein‑a‑millennium massive event using integrated remote sensing techniques. Methods: We comprehensively used multi‑temporal satellite optical images, radar images, global navigation satellite system (GNSS) observations and other datasets to quickly interpret and analyze the volcanic eruption process and its impacts. The damage assessment in parts of Tonga was also performed. Based on the integrated remote sensing techniques, we presented a technical framework for the rapid interpretation and analysis of volcanic eruptions, which comprises of six components, namely multi‑source data acquisition, geomorphology monitoring, deformation monitoring, environmental response detection, damage assessment and recovery decision. Results: Our results show that the HTHH underwater volcano in Tonga had exhibited obvious surface displacements in the satellite radar line of sight direction with a maximum accumulated displacement of 6.0 cm since June 2020, and erupted since around 22nd December 2021. During the volcanic eruption, the local magnetic field and ionosphere exhibited obvious abnormal signals, and the continuous GNSS data in Tonga suggested clear surface displacements with an uplift up to 50.2 cm. Most areas of Tonga were blanketed in ash, but it appeared that, in northern part of Tonga̓s capital, little damage was observed in the coastlines and the main buildings and roads remained intact. Conclusions: This study shows how to use integrated remote sensing techniques to quickly respond to underwater volcanic eruptions, including the topography evolution of the HTHH volcano and the damage assessment of its eruption. It is believed that the remote sensing integrated technical framework shall not only contribute to the speedy recovery and reconstruction of the society after the HTHH volcanic eruption, but also to the mitigation of future geohazard events. [ABSTRACT FROM AUTHOR]

Objectives: There are obvious seasonal variations in the GPS height time series, which affect the improvement of precision and can be corrected by both mathematical modelling and geophysical mechanisms. Compared to least square fitting, singular spectrum analysis (SSA) can extract random seasonal signals effectively through signal reconstruction, which is unaffected by the assumed sinusoidal waves. According to the elastic loading theory, the gravity recovery and climate experiment (GRACE) can be used to calculate the vertical surface displacement caused by changes in terrestrial water storage. Methods: This paper mainly studies the feasibility of correcting the seasonal variations in GPS heights using SSA and GRACE inversion results. The height time series of 27 GPS stations in Australia with a time span of from 5 to 10 years were chosen and combined with GRACE simultaneous inversions. Results: Because the spatial resolutions of GRACE are coarse and the loading displacement is much more sensitive to near‑field mass changes than far‑field ones, the amplitudes of GRACE ‑inferred hydrological loading deformations are significantly smaller than GPS. The weighted root mean square (WRMS) are reduced at 22 stations after GRACE ‑inferred displacement corrections, and the correlation coefficients between deformations estimated by GPS and GRACE range from 0.12 to 0.78 with a mean value of 0.43, indicating that GPS and GRACE results have good consistency and correlation. SSA is used to extract the annual signals of vertical displacements derived from GPS and GRACE, and contribution rates of singular spectral variance of annual signals are 21.60% and 34.48%, respectively, expressing that annual signals are the main components of GRACE inferred results. Geographical climatic conditions have a significant impact on the consistency of annual signals derived from GPS and GRACE. Compared with the arid areas in central and western Australia, the amplitude and phase of annual signals derived from GPS and GRACE are more consistent in the northern region with seasonal rainfall. Furthermore, cross wavelet transform (XWT) finds that the vertical displacement series derived from GPS and GRACE of each station have a significant resonance period of one year. The circular average phase angles of GPS/GRACE at the period closet to 1 cycle per year (cpy) outside the cone of influence range from − 74.03° to 67.23° . The mean XWT ‑ based semblances range from 0.28 to 0.99 with an average value of 0.79, showing that there is a significant positive correlation between the annual variations derived from GPS and GRACE. Conclusions: Overall, GRACE‑inferred deformations can explain the annual variations of GPS‑derived displacements, particularly in areas with high hydrological loading. It is possible to correct the annual signals of GPS heights by GRACE inversions, but the effect is not as good as the SSA‑filtered annual signals. [ABSTRACT FROM AUTHOR]

Objectives: The global navigation satellite system (GNSS)radio occultation (RO) signals can be reflected on the earth surface during the propagation process. It is of great value to explore atmospheric information contained in the reflected occultation signal for improving the accuracy of weather forecast and climate monitoring. Methods: Using the constellation observing system for meteorology, ionosphere and climate (COSMIC) occultation data collected from January 2011 to December 2015 in China, this paper extracted the occultation profile containing the reflected signals, which were strong at oceans and weak at inland and also showed seasonal changes. It was found that the inland snowy areas and the ocean surface were easier to reflect the occultation signals. Further, the offshore areas of China were set into 5°×5° grids and the correlation between the percentage of occultation reflected signals and the reanalysis data from European Centre for Medium‑Range Weather Forecasts in each grid was analyzed. Results: Around the offshore areas to the north of 10°N, the water vapor and temperature are negatively correlated with the percentage of occultation profile reflected signal, with a correlation coefficient from − 0.28 to − 0.58. However, around the offshore areas to the south of 10°N, affected by the monsoon at the South China Sea, the percentage of occultation profile reflection signal is positively correlated with the reanalyzed water vapor data, with a correlation coefficient from 0.32 to 0.34, but not significantly correlated with the reanalyzed temperature data. Conclusions: The RO reflected signals associate with atmospheric conditions, and it can be applied to the direct signals as supplementary data. [ABSTRACT FROM AUTHOR]

Objectives：Continuous and dense global navigation satellite system (GNSS) surface deformation data provide an effective tool to invert refined regional surface mass variations. However, the factors influencing the reliability of GNSS inversion results need to be further studied, such as regularization (including the construction of regularization matrix and determination of optimal regularization parameter), observation noise and distribution of GNSS stations. Methods: First, we proposed an improved regularized Laplacian constraint matrix and discussed the adaptability of the generalized cross⁃validation (GCV) method in selecting the regularization parameter of ill⁃posed equations for inversion of regional surface mass variations based on the loading Green's function theory. Second, we compared the effects of different constraint matrices and constraint methods on the GNSS inversion results. Third, we further investigated the influences of different noise levels of GNSS vertical displacement, the number and distribution of GNSS stations on the inversion results. Results: (1) The regularized Laplacian matrix in this paper can better suppress the edge effects than the traditional Laplacian matrix. (2) The GCV method can effectively determine the optimal regularization parameter, and the inversion results are in good agreement with those solved by the root mean square error (RMSE) criterion. (3) If there are enough GNSS stations and the observation accuracy is high enough in the studied area, the inversion results will be more reliable. Meanwhile, the accuracy of inversion results for uniformly distributed stations is comparable to that of randomly distributed stations when the number of stations is large enough. Conclusions: The improved regularized Laplacian matrix and the GCV method can improve the reliability of GNSS inversion results, which can guide the inversion of surface mass variations using measured GNSS data. [ABSTRACT FROM AUTHOR]

The ERP (Earth rotation parameters) obtained by different technologies and different analysis centers are often different. In order to provide unified ERP for users, it is often necessary to fuse the ERP by using a special method. In this paper, an additional boundary constraints and internal constraints fusion model based on the ERP results of multi-analysis center is proposed. Firstly, all the analysis center results are converted to the same time by using the parameter transformation model and the boundary constraints are applied to the same characteristic points on the boundary point of the adjacent observation period. Then the inner constraints of the transformation parameters are applied to the long-term solutions of the analysis centers, and the fusion solutions of the ERP are obtained. Using SINEX (solution independemt exchange format) files from seven GNSS (Global Navigation Satellite System) analysis centers of six years from 2005 to 2011, their results are integrated and analyzed. Compared with the results of IERS C04 (International Earth Rotation and Reference Systems Service Combined 04), the results of the fusion method proposed in this paper have been significantly improved. [ABSTRACT FROM AUTHOR]

Global navigation satellite system (GNSS)/acoustic positioning precision is comprehensively de- termined by both the configuration of GNSS buoys array and ranging precision. Thus, optimizing the GNSS buoys array is meaningful to improve the positioning accuracy and reliability. This paper proposes an analytical method for optimizing the GNSS buoys array with regard to the cutoff angle constraints for obser- vations. By using the latest geometric dilution of precision (GDOP) minimization method developed by this paper and introducing a set of constrains considering the coplanar GNSS buoys and the cutoff angle limita- tion, we discuss the position dilution of precision (GDOP′) minimization for underwater positioning circum - stances and propose an algorithm for producing the configuration with the smallest GDOP. Moreover, we develop a method for searching the best configuration to achieve an isotropic positioning coverage within a given region by employing the concepts about the GDOP mean value and variance. In the experimental test, we take five GNSS buoys as an example to give full solutions based on the centralized GDOP′ minimi- zation as well as a unique solution for minimizing the GDOP′ mean value. The effectiveness of the proposed method is verified by the simulation experiment. Our experiments show that within a region the centralized GDOP minimization may be equivalent to the GDOP′ mean value minimization as well as the GDOP′ vari- ance minimization. [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]

Objectives: With the development of the global navigation satellite system (GNSS), the number of visible satellites has increased, and the constellation configuration has been improved. While improving user observation information, multi ‑ system GNSS positioning increases the risk of multiple gross errors, posing a threat to the integrity of the system and restricting the application of GNSS in complex environments. A common method, the receiver autonomous integrity monitoring (RAIM) to ensure the reliability of positioning based on single error, but the reliability of positioning would decline in the case of multiple gross errors. The problem of poor detection and recognition, based on the correlation analysis method of the post‑test residual vector, the residual vector is affected by multiple gross errors, showing that the correlation with the observed feature vector of gross errors is weakened. The phenomenon makes gross error detection distortion. The aim of the study is to improving the accuracy of multiple gross errors detection by a new RAIM algorithm based on the density center of observed dataset. Methods: This paper proposes a correlation analysis method based on the density center of the observation data set to realize the detection and identification of multiple gross errors. Firstly, we construct the observation dataset through the QR calibration method. Then, we estimate the density center by the improved Mean Shift model. Finally, we test the correlation distance between the observation feature points and the density center for detection and recognition of multiple gross errors was compared. Result: The gross errors are simulated by the factual observation data, and the correlation distance of density center to gross error satellite and normal satellite, In the case of a single gross error, two gross errors, and three gross errors, the average difference correlation distance are 1.122 m and 1.516 m, 1.021 m and 1.266 m, 1.177 m and 1.588 m respectively. Compared the correlation distance of QR test vector to gross error satellite and normal satellite, the average difference correlation distance are 0.639 m and 1.142 m, 0.497 m and 0.510 m, 0.108 m and 0.198 m respectively. Conclusions: The new RAIM algorithm overcomes the problem of gross error detection distortion caused by the reduced correlation between the calibration vector and the observation vector in the presence of multiple gross errors, which can effectively improve the reliability of multi‑GNSS positioning. [ABSTRACT FROM AUTHOR]

Objectives: With the rise of location service applications and the popularization of mobile smart terminals, the demand for high ‑ precision navigation and positioning is extending from isolated areas to seamless whole areas, from professional groups to mass users. However, real‑time, continuous, and complete pedestrian navigation still faces many challenges, such as occlusion of global navigation satellite system (GNSS) signals, frequent updates of Wi‑Fi/ bluetooth low energy (BLE)/geomagnetic fingerprint data‑ bases, and smoothly switching in the transitional area of indoor and outdoor. In addition, the positioning sensors of popular mobile smart terminals are limited by cost and power consumption, and the observation data is generally noisy and poor in stability. Methods: Therefore, this paper proposes a pedestrian indoor and outdoor seamless positioning technology based on cloud‑end collaboration of smartphone, which breaks through the crowd sourced collection and rapid update of the beacon fingerprint database, the precise positioning of the smart phone multi‑sensor information fusion, and the indoor and outdoor seamless switching. We have developed a smartphone positioning application for pedestrians. The collaborative precision positioning enhancement information is provided by the location service platform and the multi‑source observations data (GNSS, miniature inertial measurement unit (MIMU), Wi ‑ Fi, BLE, magnetometer, barometer) are obtained by the smartphone. Results: The feild test results show that the outdoor positioning accuracy of ordinary users is better than 1.5 m, and indoor accuracy is about 1 ‒ 3 m. Conclusions: It supports the demand of mass‑market users for seamless and precise positioning indoors and outdoors. [ABSTRACT FROM AUTHOR]

The secondary outfitting work of the immersed tube needs to be carried out in a floating state. The measuring tower is an important outfitting part in the secondary outfitting. Its calibration of the precise position of pipe sections is related to the success of the floating installation of the immersed tube. Since the pipe sections are floating on the sea surface, it poses a huge challenge to the calibration of the measuring tower. In this paper, the calibration methods of measuring towers in the floating state of five pipe sections were introduced, and their advantages and disadvantages were analyzed, and the calibration methods used in the Dalian Bay subsea tunnel project were analyzed. The results show that the accuracy of the GNSS synchronization calibration method can reach 2 cm on the plane, and that of total station floating calibration method can reach 1 cm on the plane, which meets engineering needs. The results can provide reference for other similar marine engineering calibration measurements. [ABSTRACT FROM AUTHOR]

Objectives: The global navigation satellite system (GNSS) coordinate time series can be used to analyze the motion of GNSS station, and the colored noise in coordinate time series affects the estimation accuracy of the motion parameters. Therefore, in order to improve the reliability of the motion analysis, it is necessary to take effective measures to eliminate the colored noise.Methods:This paper proposes a method to eliminate colored noise by combining wavelet packet transform and information entropy theory. First, the simulation time series containing flicker noise is estimated to verify the estimation accuracy of the flicker noise. Then, discrete wavelet transform and wavelet packet transform are respectively carried out on the residual series of the simulation time series, and the flicker noise is eliminated by using information entropy. On this basis, the denoising effects of these two methods are compared. Finally, the coordinate time series at KMIN station is used as the measured data to further verify the denoising effect of wavelet packet coefficient information entropy.Results: The experimental results show that the power spectrum and the time series of colored noise eliminated by using wavelet packet coefficient information entropy are in good agreement with the original colored noise.Conclusions: The proposed method can effectively eliminate colored noise in the residual series and only retain white noise. [ABSTRACT FROM AUTHOR]

Objectives: For global navigation satellite system (GNSS), the ranging performance of observation would influence the system service capability.Methods: Parabolic antenna with large⁃diameter could greatly reduce the impact of the ground equipment and the environment on signal acquisition. Based on data from parabolic antenna, observation characteristics and ranging performance of BeiDou⁃3 global satellite navigation system (BDS⁃3) are analyzed.Results: The results show that, regarding the pseudorange deviation on BeiDou⁃3 satellite, no systematic deviation of the same magnitude, compared with BeiDou⁃2, is found on the signal of B1I, B3I, B1C, B2a, and B2b. There are fluctuations on some signals. However, the correlation with elevation angle is not strong, and the average magnitude of all satellite fluctuations is 0.1 m. This paper analyzes the multipath variation and noise level of carrier observation for BeiDou⁃3 satellite of each signal using triple⁃frequency combinations. The fluctuations of carrier phase triple combination are of the millimeter level, and those fluctuations do not closely correlate with the elevation or time. The root mean square of the triple⁃frequency carrier combination from parabolic antenna is mostly less than 0.6 mm.Conclusions: The results indicate that no systematic bias occurs in code and phase observation of BDS⁃3, and the system has a good ranging performance. [ABSTRACT FROM AUTHOR]

Objectives: With the popularization of smartphones and the evolution of global navigation satellite systems (GNSS), navigation contributes as an indispensable function for smartphones, and the demand of high‑precision positioning for mass‑market smartphones is becoming higher. From 2016, developers can obtain GNSS raw observations through Google Android application programming interface (API). Android 9.0 and above versions, provide the option to turn off the duty cycle, which makes it possible for smartphones to conduct precise point positioning (PPP). Methods: This paper conducts quality analysis of the GNSS observations for several common‑used dual‑frequency smartphones, Huawei Mate 20/30 and Xiaomi 8, and propose a set of quality control schemes that is suitable for smartphone PPP. Gross errors can be removed by data detection according to satellite elevation, signal‑to‑noise ratio, pre‑residuals, etc., while cycle slips can be detected by the accumulated delta range state (ADRS) identifiers as well as the time difference carrier phase (TDCP) observations. A real‑time PPP application based on Android platform was developed. It supports for singe‑ and dual‑frequency, real‑time and post‑processing PPP. To verify the performance of PPP with smartphones, several field experiments were performed with different smartphones in different scenarios. Results: Field results show that: (1) The pseudorange quality of Xiaomi 8 and Huawei Mate 20 are similar, while the carrier phase quality of Huawei Mate 20/30 is significantly worse than Xiaomi 8. (2) The ADRS identifier of Xiaomi 8 can better identify the cycle slip, while it is difficult for Huawei Mate 20/30 to directly determine the cycle slips through the ADRS identifier. By combining the ADRS and TDCP can detect most of cycle slips. (3) The positioning accuracy of single ‑frequency PPP for Xiaomi 8 and Huawei Mate 20/30 reach 0.5‑0.6 m in horizontal, and 1.0‑2.0 m in vertical. The positioning accuracy of dual‑frequency PPP is better than that of single‑frequency PPP after convergence, especially in the vertical direction. (4) For the real‑time and post‑processing PPP, they both have a similar positioning accuracy in the horizontal components, while the positioning accuracy of post‑processing PPP in the vertical direction is higher than that of real‑time PPP by 20% ‑40%. Conclusions: An accuracy of a few decimeters can be achieved for real ‑ time PPP with Android smartphones. Different from geodetic GNSS receivers, the widely‑used smartphones use low‑cost linearly‑polarized antennas. In addition, smartphones are often used in urban complex scenarios, thus the GNSS data they collect have more gross errors, cycle slips and multipath effects. Further study should focus on the multipath mitigation, multi ‑ frequency and multi ‑ GNSS combination, and fusion with other sensors such as gyroscopes, magnetometers, barometers, bluetooth, Wi‑Fi, etc. to realize continuous and seamless positioning both indoor and outdoor. [ABSTRACT FROM AUTHOR]

China’s Beidou-3 global satellite navigation system, short for Beidou system（BDS）, was fully operational on July 31, 2020, which has become the basis of the interconnection and intelligence of all things in the information society. Based on the analysis of the architecture and key technologies of Internet of Things （IoT）, the service types of BDS, and the main mode of BDS applications in IoT, this paper proposes a BDSIoT technology architecture, which takes BDS’s positioning and timing as the main perception information, takes the short message and other characteristic services as the main information transmission means, and integrates Internet and artificial intelligence technology. Then, the typical application of BDS services in the IoT field is further analyzed, and the application mode of“IoT + BDS”is put forward. Finally, taking smart city, intelligent transportation and other applications as examples, the challenge of BDS is analyzed, the development advice is given, and the new driving force of BDS for innovation driven development is proved. [ABSTRACT FROM AUTHOR]

Agricultural machinery is one of the important components of modern agriculture. In recent years, the number of agricultural machinery has continually increased, as well as the fuel consumption caused by agricultural machinery. The fuel consumption of agricultural machinery is directly related to agricultural production costs and the vital interests of farmers. Estimating the fuel consumption of agricultural machinery is of great significance in environmental governance, agricultural machinery operator evaluation, and agricultural cost input. Different from road vehicles, the factors affecting agricultural machinery fuel consumption seem to be more complex. Taking driving conditions for the only consideration cannot accurately predict the fuel consumption of agricultural machineries. Random forest, as a typical representative of ensemble learning, has many applications in various fields and has strong fitting ability for nonlinear data. It is widely used in the research of vehicle fuel consumption prediction. The purpose of this article is to realize the fuel consumption prediction of the grain harvester, WORLD 4LB-150AA, during working in the farmland. Based on the engine operating condition data and driving condition data collected by the harvester CAN(Controller Area Network) bus and GPS (Global Positioning System) terminal, seven indicators are constructed, including engine mean torque, engine mean speed, average speed, mean acceleration, mean deceleration, acceleration standard deviation and deceleration standard deviation. The acquisition frequency of the average data is 1.3 s, and the total number of the records is 130 788. Agricultural machineries that provided the data worked in six provinces including Liaoning, Jilin Province, Shandong, Jiangsu, Zhejiang, and Hubei. At the same time, by calculating the Spearman correlation coefficient, the correlations between seven indicators and fuel consumption were explored. According to China's agricultural divisions, the six provinces are divided into three regions: northeast region, plain region, and hilly region. Then, the fuel consumptions of the same grain harvesters in different regions were analyzed. Above the analysis, the fuel consumption prediction model of the harvester based on Random Forest was established, and compared with the one based on support vector machine. The results showed that the fuel consumption is correlated with all indicators. Among them, the fuel consumption is highly correlated to engine mean torque, engine mean speed and average speed, all with the correlation coefficient above 0.6, followed by mean acceleration, mean deceleration, acceleration standard deviation and deceleration standard deviation, whose correlation coefficients are above 0.4. There are significant differences in the fuel consumption of harvesters working in different regions. Among them, areas with high output per unit area are also relatively high in fuel consumption. Moreover, the Random Forest based model can realize the higher accurate prediction of fuel consumption during harvester operation. The root mean square error RMSE is 0.14, the average absolute error MAE is 0.24, and the coefficient of determination R2 is 0.84. The proposed method in this paper can provide a reference for the optimization of working conditions of agricultural machinery and precise fuel consumption monitoring. [ABSTRACT FROM AUTHOR]

Objectives: Global warming has become a significant climate change in the past 100 years. And the global mean sea level continues rising caused by the thermal expansion of sea water and the melting of land‐based glacier and polar ice. The Ny-Alesund area in Arctic has preserved a typical primitive polar ecosystem. The objective and accurate analysis of sea level change in this area can better provide a basis for monitoring and protection of natural ecological environment, and for the research of climate change. Methods: This paper analyzed the liner trend and seasonal variation of sea level change by using data of satellite altimeter, tide gauge station and global navigation satellite system (GNSS) along Ny‐Alesund coast from 1993 to 2018.Results: The observation data at tide gauge station show that the rate of crust rise is (8.09±0.19) mm/a, and the rate of relative sea level decline is (-7.31±0.36) mm/a. So the rate of absolute sea level rise is (0.78±0.41) mm/a, which is similar to the result from observation data of satellite altimeter with the difference of (0.23±0.46) mm/a, and is lower than the rate of global sea level rise.Conclusions: In the analysis of regional sea level change, we can get the absolute sea level rise with correction of tide gauge station by GNSS. The relative sea level change corrected by GNSS is characterized by significant seasonal features in Ny‐Alesund. The period of seasonal high sea level is from October to November, and the period of seasonal low sea level is from March to April. The correlation analysis between sea surface temperature and sea level shows that the sea level changes with sea surface temperature. [ABSTRACT FROM AUTHOR]

In order to effectively extract useful information from time series of the global naviga- tion satellite system(GNSS) sites, and improve the modeling accuracy of coordinate time series, this paper proposes a nonlinear motion modeling method combining wavelet decomposition and singular spectrum analysis. Experiments were carried out using global positioning system(GPS) vertical coordinate time series of 11 stations around the world from 1999 to 2018. Methods: Firstly, the coordinate time series is decom-posed into different scales by wavelet decomposition.And then the singular spectrum analysis(SSA) is per- formed on the high-frequency part and the low -frequency part of each layer. Finally, combine the fitting value that is the fitting coordinate time series, and the fitting effect of the new method was evaluated. Results: The results show that, compared with the simple singular spectrum analysis method, the new method can extract useful information such as trend and period more accurately from the limited scale of the time series with noise. And reduce the partial period items in the singular spectrum analysis method to some extent, for example, the seasonal period item and the monthly period item are regarded as the probability of noise rejec- tion, and the modeling accuracy is improved by 26%. Conclusions: The purpose is to propose a nonlinear motion modeling method based on wavelet decomposition and singular spectrum analysis, so as to improve the modeling accuracy of coordinate time series. [ABSTRACT FROM AUTHOR]

The basic construction of BeiDou global satellite navigation system (BDS) has played a key role in promoting the development of global navigation satellite system, which provides an important guarantee for multi⁃constellation fusion positioning, navigation and timing. Multi⁃system fusion positioning provides users with more navigation information, and also provides a better guarantee for the realization of navigation performance indicators. At present, in order to ensure integrity, it is necessary to warn users when navigation service is unavailable. Due to satellite failure, the protection level exceeds its alert limit. However, under the condition of multi⁃frequency and multi⁃constellation, the number of satellites is sufficient. After a few satellite faults are eliminated quickly, the influence on the overall navigation accuracy is very small and no alarm is needed. A protection level reconstruction method based on robust estimation is proposed. The protection level is reconstructed by using robust information and posterior variance. In order to verify the feasibility of the algorithm, the accuracy and protection limits required by localizer performance with vertical guidance 200ft (LPV⁃200) approach phase are selected as the standard of this paper, and the global positioning system (GPS)/BDS combination of dynamic and static data are used for calculation and analysis. The GPS adopts L1 pseudo range observation, and the BDS adopts B1 pseudo range observation. Each group of experiments is divided into two parts, fault detection and elimination and the method of adding abnormal error is calculated respectively, the vertical protection level is calculated by the protection level calculation method based on a posteriori variance and the protection level reconstruction method based on robust estimation. The results show that our proposed method can reduce the alarm frequency on the premise of ensuring the positioning accuracy. [ABSTRACT FROM AUTHOR]

In this paper, we evaluate regional geological environment stability of the Three Gorges area quantitatively, using global navigation satellite system (GNSS) and gravity continuous observation data, combined with hydrological meteorological and topographic data. The results are based on quantitatively detecting the influence of some environmental factors on the geological stability of different sites. We find out that geological environment of the Three Gorges reservoir area is generally stable, and the stability of the geological environment in the lower reaches of the Three Gorges Dam is generally good. However, there are 5 to 6 weak geological environment areas, with different shapes and areas along the Yangtze River in the Zigui-Yunyang section of the Yangtze River. There are also some areas with weak geological stability in Fang County and Zhenping County, outside the Three Gorges reservoir area. The quantitative evaluation method and results have important reference value for regional geological environment assessment and geological disaster prevention and control. [ABSTRACT FROM AUTHOR]

This paper introduces a method which utilizes global navigation satellite system (GNSS) based precision point positioning (PPP) for conducting long-term tilt and settlement monitoring of high-rise buildings in North China. Two-year continuous GNSS observations from two high-rise buildings near a deep foundation pit in Beijing are used for the case study. The method comprises four steps: ① Calculating the geocentric coordinates (XYZ) of the GNSS antenna with respect to a global reference frame (e.g., IGS08) with the PPP method; ② transforming the geocentric coordinates to a regional reference frame − the North China reference frame (NChina16); ③ converting the geocentric XYZ coordinates to a station-centric East -North- Up (ENU) coordinates; ④ removing seasonal ground-surface deformation from the ENU time series and restoring the true displacements (tilt and settlement) of the building. Experimental result indicates that the PPP method is able to achieve 2-3 mm horizontal accuracy(root mean square error) and 6-9 mm vertical accuracy (root mean square error)for 24 - hour continuous observations in North China. The combination of PPP, NChina16, and the North China seasonal ground deformation model can perform millimeter-accuracy, nearreal time, and long term static structural health monitoring of high-rise buildings and other large-scale infrastructure projects in North China. [ABSTRACT FROM AUTHOR]

Landslide evolution is a long‐term and complex process. Geo‐hazards have different characteristics in various data, and have certain applicability in different stages of landslide. This paper applies different characteristics and applicability of various data sources in landslide by the multi‐source data fusion method. The deformation and failure characteristics and temporal and spatial evolution law of Huangnibazi landslide during its dynamic evolution from pre‐sliding to mid‐sliding to post‐sliding are studied. The results show that the deformation and failure process of Huangnibazi landslide can be divided into four stages: startup stage, accelerated deformation (accelerated sliding) stage, front expansion (decelerated sliding) stage and gradual stabilization stage. Huangnibazi landslide is a creep ‐ pull ‐ split landslide formed under the influence of self‐weight, rainfall infiltration, earthquake and vibration caused by human engineering activities. The application of various data in different stages of landslide is summarized: interferometric synthetic aperture radar(InSAR) technology, optical image and topographic data are used to determine potential landslide body before sliding. Optical remote sensing image is used to observe the overall deformation and trend of landslide accumulation body during sliding. Global positioning system (GPS) is used to continuously observe local deformation after sliding. Field investigation is used to determine the characteristics of geological hazard body. [ABSTRACT FROM AUTHOR]

The Swarm satellite is equipped with the Global Positioning System (GPS) receiver and ac⁃ celerometer, which can be used to restore Earth’s temporal gravity field model. Firstly, the terrestrial wa⁃ ter storage (TWS) variability and 2015-2016 drought induced TWS deficits in the Amazon basin are esti⁃ mated from the Swarm time ⁃ variable gravity field model between December 2013 and December 2016. Then, the estimated results from Swarm data are compared to the results derived from Gravity Recovery and Climate Experiment (GRACE), three hydrological models （Global Land Data Assimilation System (GLDAS)， Climate Predication Center (CPC)， National Centers for Environmental Prediction (NCEP)) and 4 virtual stations (AMZ_076， AMZ_152， AMZ_ 228 and AMZ_215). Finally, the precipitation and temperature data are employed to study the origination of the drought event in the Amazon basin. The re⁃ sults indicate that the TWS derived from Swarm, GRACE, hydrological models, and virtual stations are in high consistency in the space/time distribution; Swarm provided a new perspective in detecting TWS, which has a potential to substitute GRACE satellite in studying the extremely drought and flood in the Ama⁃ zon basin. The results in this paper may provide a reference in understanding Earth’s mass transformation by Swarm. [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]