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Objectives: Global navigation satellite system-reflectometry (GNSS-R ) is a new method for measuring soil moisture with rich signal sources, which utilizes GNSS reflected signals for detecting the physical parameters of the reflector with cheap cost and high spatial resolution. Methods: Considering the fact that in the traditional GNSS-R soil moisture retrieval, the data of land cover types is ignored or simpli fied, this paper makes detailed labels about the land cover types based on the definition of international geo sphere biosphere programme (IGBP) considering the similarities and differences between different land cover types, and uses back propagation (BP) neural network to establish soil moisture inversion models. The main work is to replace the IGBP number labels with refined feature labels. After the extraction, matching and quality control of cyclone global navigation satellite system (CYGNSS) and soil moisture active passive (SMAP) data, the dataset is randomly divided into training set and testing set. Then BP algorithm is used for training neural networks to establish soil moisture retrieval model. Results: The correlation coefficient between retrieved soil moisture and reference value is 0.85, and the root mean square error (RMSE) is 0.060 cm3 /cm3 . Meanwhile, the soil moisture obtained by CYGNSS and SMAP have good spatial and tem poral consistency, but the inversion performance varies on the different land cover types. On the barren or sparsely vegetated land type, the RMSE is smallest and on the evergreen ncedlcleaf forest land type, the RMSE is largest. Meanwhile on the shrublands, grasslands and croplands land types, the inversion effect is better than permanent wetlands, urban/built-up and cropland/natural vegetation mosaic land types. Com‐ pared with the existing IGBP numbering and labeling method, the accuracy has been improved by 7.7%. The precision improvement of deciduous broadleaf forest, closed shrublands and woody savannas types is significant. Conclusions: The proposed method of refined land type digitization can effectively retrieve soil moisture without relying on other auxiliary data such as vegetation opacity and surface roughness, and it is feasible and effective. [ABSTRACT FROM AUTHOR]

Objectives: Global navigation satellite system-interferometric reflectometry can be taken advantage of identifying soil moisture of land surface. Methods: Aiming at the problem of multi-GNSS combined soil moisture inversion, we use the phase extracted from the signal-to-noise ratio observation data of global positioning system (GPS), BeiDou satellite navigation system (BDS), GLONASS and Galileo system, and solve soil moisture inversion within consideration of satellite track clustering by using the empirical model. The estimation of multi-GNSS combination is obtained by weighted average method. Results: The inversion accuracy of BDS and Galileo is equivalent and superior to GPS and GLONASS. The root mean square error of the method in this paper is 0.041 4 cm3 /cm3, which is about 16.3% and 5.2% lower than that of single navigation satellite system and the optimal frequency band respectively. Conclusions: The results indicate that the multi-GNSS reflection signal estimation method based on track clustering can effectively monitor changes in soil moisture. [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]

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

Objectives: 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: 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: 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]

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

Objectives: 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: Network real-time kinematic (RTK) technology can provide real-time and high-precision positioning service for users over the wide range, however, the current technology depends on high density of satellite navigation and positioning (SNAP) reference station network. To satisfy the high-precision positioning requirements based on the sparse reference station network, this paper recommends a net- work RTK service method based on virtual atmosphere constrait (VAC).Methods: Firstly, a non-combined double- difference observation model is constructed to solve and fix the network RTK baseline ambiguity quickly. Then, the baseline atmospheric delay is extracted and the slant path ionosphere and zenith trop- sphere are established respectively. Finally, the interpolated atmospheric delay and its accuracy information are regarded as virtual observations to improve the positioning performance of the terminal RTK. Based on a 205.1 km station spacing SNAP network from a northwest region， China, six rover stations inside and outside the network are used for termininal RTK verification. Results: The results show that the VAC net- work RTK service method can meet the needs of high-precision positioning users over large-scale reference networks. The terminal positioning accuracy and initialization speed by the VAC method are significantly improved by 61.64% and 9.96% compared with the traditional virtual reference station method. In this service mode, the average horizontal and vertical root mean squares of the fixed solution for the six terminals are 1.19 cm and 2.73 cm. Initializing RTK solution by a lot of times, 88.78% of periods can be fixed within 2 epochs for the six rover staions. Conclusions: The VAC service mode has good adaptability to both users inside and outside the large-scale SNAP base station network. [ABSTRACT FROM AUTHOR]

The BeiDou navigation satellite system have three attitude control modes-yaw-steering(YS) mode, orbit-normal(ON) mode and continuous yaw-steering(CYS) mode. The orbit accuracy and best orbit determination projects for different satellites were a hot topic at present in different time and different attitude control mode, which are researched and whose the dynamical mechanisms is analyzed in this paper. The results are as follows. For BeiDou-2 satellites, multi-satellite precise orbit determination by fixing satellite/station clock offset and extended CODE model (ECOM) 5 SRP should be the best project, user equivalent range errors (UERE) root mean square (RMS) in ON mode is 2.08 m,global satellite laser ranging (SLR) data fitting RMS is ablout 1 m. For BeiDou-3 satellites, multi-satellite precise orbit determination and ECOM 5 SRP should be the best project, UERE RMS in CYS mode is 1.22 m,SLR data fitting RMS is 0.23 m, which is comparable to that in YS mode. For GEO satellites near spring or autumn equinox, multi-satellite precise orbit determination by fixing satellite/station clock offset and ECOM 9 SRP should be the best project, UERE RMS is 0.72 m. [ABSTRACT FROM AUTHOR]

In this paper, the traditional objective function, the quadratic of difference of float ambiguity and integer ambiguity, is replaced by a modified function called sum of Li and L2 pure carrier phase residua squares when searching for a correct integer ambiguity. The integer ambiguities search space is obtained by a floating transformation and building observation model with DD (double difference) C/A code range and wide lane carrier phase. The calculation results from 18-hour static GPS data indicate that the modified is better than the traditional objective function for reliability and usability with a higher success rate [ABSTRACT FROM AUTHOR]

Based on 138 evenly distributed IGS coordinate time series spanning more than 10 years, site-to-site coefficents in large scale GPS network were analyzed. Their site-to-site coefficients indicate significant corelationship even when the between-site distance more than 5 000 km. Therefore, this paper proposes a new method introducing coefficients as weight factor to calculate common mode error, and it improved the correlation-based spatial filtering technique. Time series from global and regional networks were used to verify this new method. The results show that this improved coefficient-based spatial filtering technique overcomes the spatial scale limit and effectively extracts common mode error of GPS coordinates time series. [ABSTRACT FROM AUTHOR]

A new GNSS time synchronization system is designed and presented in this paper, the new system place hydrogen maser on the GEO satellite, establishing a high-precision inter-satellite link between the GEO satellites to maintain a high-precision time. The proposed two scenarios were simulated and compared, simulation results show that the proposed new system is feasible and can significantly enhance the accuracy of time synchronization, especially the accuracy of independent time synchronization. And the relationship of time synchronization accuracy with the clock and ISLs is analyzed. The simulation results show that the accuracy of ISLs is more important. [ABSTRACT FROM AUTHOR]

Copyright of Geomatics & Information Science of Wuhan University is the property of Geomatics & Information Science of Wuhan University and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)