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Objectives: The BeiDou-3 satellite navigation system (BDS-3) can provide real-time precise point positioning (PPP) services to China and neighboring regions through PPP-B2b signals. Because PPPB2b signals do not contain ionospheric model products, the current positioning research mainly adopts ionospheric-free combination. The positioning accuracy can reach decimeter level, but requires about 30 min of convergence time. In this paper, aiming at the long convergence time of PPP-B2b positioning, the improvement of positioning convergence performance based on ionospheric model products is analyzed. Methods: Quasi-4-dimensional ionospheric modeling is introduced into regional ionospheric delay modeling in China and four schemes of 50,100,150 and 200 reference stations uniformly distributed in China (day of year 201—207) are adopted for ionospheric modeling. Results: The results show that when the number of stations used in ionospheric modeling increases from 50 to 200, the accuracy of ionospheric model products increases from 2-3 TECU to better than 0.8 TECU. Compared with the current ionospheric-free combined PPP, the convergence time of BDS-3 dynamic PPP can be deceased by 77%. In addition, the convergence time of GPS+BDS-3 dynamic PPP is reduced from 13.51 min to 4.45 min. Conclusions: High-precision ionospheric model products can significantly shorten the convergence time of PPP-B2b. [ABSTRACT FROM AUTHOR]

Objectives: Convolutional neural network (CNN) has been used in spaceborne global navigation satellite system-reflectometry （GNSS-R） sea ice detection, which has the advantages of simple data preprocessing and maximum retention of reflector information. However, the data sets used in previous studies of the GNSS-R CNN sea ice detection method have a small span in time and limited representativeness, and the influence of the delay-Doppler map (DDM) ratio of seawater and sea ice in the training set on the generalization ability of the method is not considered. Methods: To solve these problems, a method of screening out malformed DDM is proposed. The appropriate CNN structure and parameters are designed, and the dataset selection strategy is optimized through comparative tests of small samples. A large sample dataset from 2018 is used to evaluate the validity and reliability of the improved method in the case of large data volume and large time. Results and Conclusions: The results show that the proposed method can screen the wrong data effectively. The CNN model has high accuracy and the best generalization ability when the DDM ratio of seawater and sea ice in the training set is 1∶ 1, and the improved method is still effective and reliable in large data volume and large time span. The improved method improves the generalization ability and reliability of the CNN sea ice detection method by strengthening data quality control and optimizing the dataset selection strategy, to make it more applicable to practical application scenarios and provide a reference for studies on sea ice melting [ABSTRACT FROM AUTHOR]

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]

Objectives: The global BeiDou-3 navigation satellite system (BDS-3) along with regional BeiDou navigation satellite system (BDS-2) is providing users with high-precision positioning, navigation and timing services on three or more frequency signals. To deeply integrate the triple-frequency observations of BDS-2 and BDS-3, we establish the BDS-2/BDS-3 joint triple-frequency precise point positioning (PPP) models and analyze the characteristics of biases arisen in these models. Methods: Using the linear combinations of raw observations, the triple-frequency ionosphere-free and uncombined PPP models are developed. In particular, the mathematical expressions of receiver inter-frequency bias (IFB) and time delay bias (TDB) between BDS-2 and BDS-3 are derived. Results: The experimental results show that BDS-3 can significantly enhance the PPP performance. In static mode, the BDS-2/BDS-3 joint PPP is able to achieve the positioning accuracy of 1.2/1.0/1.8 cm after the convergence time of 15.8/7.3/22.3 min in the east/north/up components. As for kinematic PPP, the convergence time and converged positioning accuracy are 27.3/ 10.3/33.8 min and 2.2/1.7/4.4 cm, respectively. Conclusions: The convergence performance and positioning accuracy of triple-frequency ionosphere-free and uncombined PPP models are comparable, and the third frequency signal makes a marginal contribution to positioning performance. Both IFB and TDB show the good intra-day stability. The intra-day standard deviations (STD) of IFB can reach 7.9 and 3.6 cm for BDS-2 and BDS-3 signals, respectively, and the intra-day STD of TDB can reach 8.8 cm. [ABSTRACT FROM AUTHOR]

Objectives: The integration of low earth orbit (LEO) constellations into the BeiDou satellite navigation system has been under consideration, aiming to establish a global coverage space-time network through the fusion of high, medium, and low earth orbit constellations. However, it should be noted that the LEO constellation is currently in the construction phase, and there is a lack of comprehensive simulation and performance evaluation for the LEO-enhanced BeiDou system. Methods: Therefore, In this contribution, research is conducted centered around the highly realistic simulation and comprehensive performance evaluation of the LEO-enhanced BeiDou system, a sophisticated simulation model of the LEO constellation was developed by integrating actual measurement data and space physics theoretical models, and then a comprehensive performance evaluation method for the LEO-enhanced system was proposed, and an integrated space-ground platform for simulation and comprehensive performance evaluation of the LEO enhanced BeiDou system was designed and established, the research findings revealed the performance enhancement effects of the LEO constellation on the existing navigation system. Results: The results indicate that, on the system-terminal, the LEO augmentation improves the precision of BeiDou's high-precision products while simultaneously reducing the limit on the number and distribution of ground stations. On the user-terminal, the inclusion of LEO satellites holds the potential to reduce the convergence time of BeiDou precise point positioning to less than 10 minutes. Conclusions: The research results provide theoretical and practical support for the development of the LEO-enhanced navigation system. [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: BeiDou satellite navigation system (BeiDou) /global navigation satellite system (GNSS) can provide continuous and reliable high-precision navigation and positioning service in opensky, yet in urban environment, suffering from outliers and cycle slips caused by severe multi-path and nonline- of-sight signals, the navigation and positioning capability remains inadequate. Compared with the extended Kalman filter (EKF), factor graph optimization (FGO) can make full use of historical measurements and restrain the influence of anomalous data through constraints and redundant measurements inside the window. Methods: A GNSS positioning model based on sliding-window FGO is constructed, adopting posteriori residuals test as outlier detection method, and analysis of robust performance between EKF and FGO is presented in terms of minimum detectable bias, correct detection rate and positioning accuracy. Results: The result of urban environment experiment shows that the minimum detectable bias is reduced by [ABSTRACT FROM AUTHOR]

Objectives: Global navigation satellite system (GNSS) satellite chip shape distortions lead to individual biases in the pseudorange measurements. These biases are the results of the correlator's tracking response and thus depend on the correlator and front-end design of the receiver. As a result, different receivers are likely to exhibit inconsistent biases, called receiver-related pseudorange biases. The researches show that the receiver-related pseudorange biases cannot be absorbed by clock offset, which will affect GNSS precision applications. Methods: We estimate the receiver-related pseudorange biases of global positioning system (GPS)/BeiDou satellite navigation system (BDS) /Galileo satellite navigation system (Galileo) from 9 short/zero-baselines stations from the multi-GNSS experiment network using double difference method. The baselines are divided into three groups according to the combinations of two receivers of one baseline. Results: The results indicate that the receiver-related pseudorange biases of receivers with different manufacturers even the same manufacturers with different models cannot be neglected. For base‐lines with receivers from different manufacturers,the inconsistences of pseudorange biases can reach up to 160 cm. Conclusions: Among GPS, Galileo and BDS, Galileo shows the smallest pseudorange biases, while BDS in general is the largest. Furthermore, the stability of pseudorange biases is analyzed, the results suggest that estimated receiver-related pseudorange biases are quite stable with the standard deviation less than 12 cm for the three systems. We investigate the effect of receiver-related pseudorange biases on integer ambiguity resolution and pseudorange relative positioning, with the pseudorange biases correction, the root mean squares of fractional parts of double difference Melbourne ‐Wübbena combination are decreased by 17%, 22%, and 14% for GPS, BDS, Galileo, respectively. An improvement of pseudorange relative positioning accuracy is also confirmed, the 3D positioning errors of GPS, BDS, Galileo are reduced from 84.4, 77.3 and 60.0 to 80.1, 72.1 and 57.0 cm, respectively. [ABSTRACT FROM AUTHOR]

Objectives: In view of the difficulty of quantitative evaluation caused by complex spoofing scenarios and numerous anti-spoofing strategies, it is necessary to form effective indicators and methods for the evaluation of spoofing countermeasures. Methods: The evaluation model and mapping process from spoofing scenes to spoofing countermeasure effect are established. Based on the power space in the spoofing scenes and the spoofing effective probability in the spoofing countermeasure effect, the evaluation method and index based on the power feasible region are designed. Results: Through simulation, the quantitative evaluation results based on power feasible region of two spoofing countermeasure strategies are given, and the simulation results show that the evaluation method can realize the quantitative comparison of the performance of different anti-spoofing strategies. Conclusions: The indicator of the power feasible region can provide a quantitative evaluation method forselection and optimization of the anti-spoofing algorithms in receivers. [ABSTRACT FROM AUTHOR]

Objectives: BeiDou-3 satellite navigation system (BDS-3) has been officially completed and provides services, network real time kinematic(RTK) is the main means to improve positioning accuracy, we present a long-distance undifference network RTK method based on BDS multi frequency and multi system observation data. Methods: First, the ambiguity resolution model considering atmospheric error parameters is used to determine the integer ambiguity of multi-frequency carrier phase of long-distance BeiDou satellite navigation system (BDS) reference station.Then,the double difference integer ambiguity between reference stations is converted into non‐difference integer ambiguity of each reference station through linear change,and calculate the undifference observation error of each reference station. Considering the weakening of spatial correlation of observation errors between long-distance stations.According to the difference of error characteristics,obtain the correction number of classified undifference error of reference station network.Finally, by inverse distance weighted interpolation method, we calculate the error correction of mobile station, high precision positioning for users. Results: Select the measured data of long-distance continuous operation reference station network for experiment, the results show that: Compared with BeiDou-2 satellite navigation system (BDS-2), BDS-3 increases by 15.7%, 28% and 11.9% in E, N and U directions respectively. Compared with GPS, the positioning performance is equivalent or even better. The ambiguity fixing success rate and convergence time of multi-frequency data are better than GPS dual-frequency data. Dual system positioning results are better than single system, both of which can achieve centimeter level positioning of mobile stations. Conclusions: This method can avoid the correlation effect of double difference observations, high efficiency of spatial error calculation, mobile station positioning is more flexible, shows the advantages of BDS multi frequency observation data. [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: The performance of the global navigation satellite system (GNSS) spaceborne atomic clock affects the entire navigation system, by having direct impact on GNSS measurement quality, ranging precision, clock prediction and satellite autonomous navigation capabilities. Different time synchronization systems may have different impact on the evaluation of spaceborne atomic clock. Precise clock bias data determined by inter-satellite link (ISL), two-way time transfer (TWTT) and orbit determination and time synchronization (ODTS) system are used to further evaluate the performance of BeiDou-3 on-orbit atomic clock. Methods: Quadratic polynomial model and total Allan / Hadamard variance are used to analyze the performance of the BeiDou-3 navigation satellite system (BDS-3) satellite clock data of the three different time synchronization systems. Results: The result shows that the frequency accuracy and frequency drift of the above three clock bias determination systems are consistent. The frequency accuracy of all satellites is within the range of (− 4~2)×10-11. Frequency accuracy of hydrogen clock is better than that of rubidium clock. The frequency drift based on the ISL system is slightly better than that of the ODTS, the three clock determination systems have their own advantages in evaluating the stability of the atomic clock, respectively. For short-term stability, the ODTS whose 3 000 s stability reaches 3×10−14 is better than that of ISL, and hydrogen clock is better than rubidium clock. For medium-term and long-term stability, when the averaging time is greater than 1×104 s, the result of ISL is closer to the actual condition of the BDS-3 spaceborne clock. For long-term stability more than 7 days, the broadcast ephemeris bias based on TWTT system can be used for rapid evaluation, which result is close to ODTS and ISL. Conclusions: The three clock bias determination systems can be used to evaluate the frequency accuracy and frequency drift of onorbit atomic clocks with basically consistent statistical results. The three clock bias determination systems have their own advantages in assessing the stability of the orbiting satellite clock when selecting different averaging time. [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: With the ability of providing real-time precise point positioning (RTPPP) service for China and surrounding countries, the application of BeiDou-3 navigation satellite system(BDS-3) precise point positioning(PPP)-B2b signals have seen a rapid rise in recent years. And the performance evaluation is a vital issue for large deployment in the future, such as the evaluation about the accuracy of orbit and clock error, as well as the analysis of precision and and convergence time for PPP. Methods: Based on the observations from September 2020 of the China branch stations of international GNSS monitoring and assessment system(iGMAS), the accuracy of orbit and clock error, as well as the positioning accuracy of B1I+B3I and B1c+B2a signal combination are carefully evaluated in this dissertation. Results: The results show that the average accuracy of PPP-B2b orbit products in radial, along-track and cross-track directions are 0.1 m,0.31 m and 0.3 m, respectively. Besides, the root mean square of clock error correction is 2.26 ns. In terms of PPP convergence, the convergence time of the B1I+B3 signal combination using GFZ BeiDou multi-GNSS(GBM) products is the quickest in the north, east and up directions, with the highest final con‐ vergency accuracy. The second is B1I + B3I (PPP-B2b), and B1c + B2a(PPP-B2b) is at the last. Conclusions: Summarizing the above discussions, BDS-3 PPP-B2b signals is capable of providing regional PPP service in China. [ABSTRACT FROM AUTHOR]

Objectives: In recent years, unmanned aerial vehicle has been widely used and their navigation and positioning rely heavily onglobal navigation satellite system(GNSS). In the case of GNSS rejection, visual navigation and positioning technology can compensate for this problem, but the technique will also fail to adapt if the approximate location of unmanned aerial vehicle cannot be estimated. To cope with this problem, we propose a reference satellite image retrieval method that aggregates deep learning features to determine the range of unmanned aerial vehicle image adaption region, which can provide reference for the following unmanned aerial vehicle absolute positioning. Methods: Firstly, the pre-trained deep learning model is used to extract local convolution features of unmanned aerial vehicle images and satellite images. Secondly, the local aggregation descriptor vector is used to generate the global expression of the images. Finally, the global feature of the image is used to perform similarity retrieval and post-processing method of matching precisely and reranking the retrieval results is used, which further improves the retrieval accuracy. A new satellite reference image data set for absolute positioning of unmanned aerial vehicle is designed and tested. Results: When the queried unmanned aerial vehicle image is similar to the satellite image season in the data‐ base, the accuracy of the top 50 candidate images can reach 87.50% using the proposed features for retrieval. Combined with the refined matching re-ranking, the accuracy of the first candidate image can reach up to 76.07%, which satisfies general navigation and positioning applications. Conclusions: Although the global descriptor based on deep feature aggregation can effectively represent the images of texture-obvious regions, it is not strong in representing the images of texture-lacking regions and its overlap range between images is high when retrieval is performed. Therefore, the efficiency problem of retrieval and the image feature representation of texture-sparse regions are the directions that need further research. [ABSTRACT FROM AUTHOR]

Objectives: For the high-reliability and fast cycle slip detection and correction of high-samplingrate global navigation satellite system (GNSS) data, the quality control method for modified geometry-free (MGF) combination is proposed to improve the success rate of phase cycle slip correction. Methods: According to the MGF combination and its error characteristics, the error limits of the MGF method are derived, and the applicability in different navigation satellite systems and the influence of measurement noise on the detecting results are analyzed. Based on the above research, the quality control test of the MGF com ‑ bination is constructed and multi-GNSS experiments are carried out. Results: The experimental results show that the MGF method can be used to detect and correct small cycle slips (such as cycles below 5) rapidly with an accuracy rate of 99.90%. Besides, the quality control test of the MGF combination can identify 100%, 87.57% and 77.42% of the wrong correcting results of GPS, BDS and GLONASS respectively, which reduces the error rate of cycle slip correction to 0.01%.Conclusions: The MGF method is significantly influenced by the noise of phase observation, and the quality control test can identify wrong correcting results effectively, thereby improving the success rate of cycle slip correction. The ability of the quality control test to distinguish wrong detecting results and noise decreases with the increase of noise level. [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]

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: 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: With the completion of BeiDou-3 navigation satellite system (BDS-3) and the development of hardware technology, global navigation satellite system (GNSS) receivers in crustal deformation monitoring network are upgraded towards multi-system and ultra-high rate (≥50 Hz). We aim to study the contribution of BDS-3 and multi-GNSS to coseismic deformation collection and the potential of ultra-high rate GNSS to attenuate the aliasing effect and obtain velocity and acceleration series by using difference methods. Methods: Based on GNSS observation data from the crustal movement observation network of China and continuously operating reference stations in Qinghai Province, we study the 2021 Maduo Mw 7.4 earthquake and obtain coseismic deformation at the nearby stations through kinematic precise point positioning (PPP). Time series before the earthquake are analyzed by time-domain and frequency-domain, namely standard deviation and power spectrum density (PSD).Results: The results show that the accuracy of BDS-3 is about 5 mm horizontally and 18 mm vertically. The accuracy of the multi-GNSS, including GPS, GLONASS, Galileo, BDS-2 and BDS-3, can reach 4 mm horizontally and 12 mm vertically. The results of PSD show that multi-GNSS can contribute to attenuate noise at most frequency and BDS have the lowest noise among all the four systems. Velocity series perform the best at a range of 5—10 Hz, while no acceleration is available at any rate. Conclusions: The accuracy of displacement obtained by BDS including BDS-3 is better than that of GPS in this earthquake. The observation of 50 Hz cannot significantly weaken the aliasing effect which reaches saturation at about 10 Hz. The ultra-high rate velocity and acceleration series obtained by difference method are too noisy for usage. The maximum sampling rate of GNSS seismic monitoring can be set at 5—10 Hz which represents a good trade-off between effect and cost. [ABSTRACT FROM AUTHOR]

Objectives: The ground calibrated antenna phase center offset (PCO) and phase center variation (PCV) for European global navigation satellite Galileo have been released. Compared with in orbit estimated PCO and PCV, the calibrated counterparts are frequency-dependent and satellite-dependent. The characteristics of calibrated satellite antenna parameters are needed to be analyzed. Methods: Firstly, we compare the antenna parameter of different type satellites, in -orbit validation (IOV) satellite and full operational capability (FOC) satellite. Secondly, we use data from 20 MGEX (multi-GNSS experiment) stations for 15 consecutive days, the effects of calibrated antenna parameters on dual -frequency combined/uncombined precise point positioning (PPP) are studied. Results: Experiment results show that the horizontal PCO (h-PCO) are type-dependent, PCV of FOC satellite are more stable than those of IOV satellite and only dependent on the nadir angle. Compared with the positioning results of estimated antenna parameters, the positioning accuracy in horizontal direction is equivalent. The height accuracy based on ionospheric-free combination observation is improved by about 6. 3%, those based on dual -frequency uncombined observation is improved by about 11. 9%. Conclusions: The calibrated antenna parameters based on the individual frequency show better self-consistency in dual-frequency uncombined PPP. [ABSTRACT FROM AUTHOR]

Limited by the regional distribution of ground monitoring stations and the geostatic characteristics of geostationary earth orbit (GEO) satellites, the GEO satellites of the BeiDou satellite navigation system (BDS) have poor orbit determination accuracy. The inclusion of spaceborne observation data of low⁃Earth orbit (LEO) satellites by processing combined precise orbit determination (CPOD) can improve the orbit determination accuracy of global navigation satellite systems (GNSS). Methods：This study simulates and analyzes the performance of CPOD with the BDS⁃3 full constellation and LEO satellites in enhancing the orbit determination accuracies of BDS satellites on the basis of a small LEO satellite constellation of 24 LEOs. With the simulated observation data, experiments of POD with ground stations only and CPOD with ground stations and LEOs are conducted. Three types of station networks, i.e. the global network, the Asia⁃Pacific regional network, and the Asia⁃Sea network, are discussed. Results：（1) The three dimension orbit accuracies of GEO satellites are at the decimeter magnitude in the case of POD with ground stations only. When LEO data are added, the corresponding orbit determination accuracy is at the centimeter level, with an increase of over 80%. （2) In the case of the regional station network, the three dimension root mean squares error(RMSE) of navigation satellite orbits are at the decimeter level. When LEO data are added, the orbit determination accuracies of all types of navigation satellites are several centimeters. （3) In the case of the global station network, the orbit determination accuracy of inclined GeoSynchronous orbit (IGSO) and medium Earth orbit (MEO) satellites can still be improved on an millimeter⁃centimeter level. Conclusions：Under the regional monitoring network, the addition of a small number of LEO satellites can significantly improve the orbit accuracy of GNSS satellites. When the global uniform/Asia⁃Sea network is deployed, the orbit accuracy of navigation satellites can still be slightly improved after the addition of LEO satellites. With the more LEO satellites added, the improvement of the orbit determination accuracy of navigation satellites is limited. [ABSTRACT FROM AUTHOR]

Objectives：The BeiDou⁃3 navigation satellite system (BDS⁃3) has designed and implemented a high⁃speed broadband inter⁃satellite link (ISL) network on the basis of the BDS⁃2 to carry out integrated navigation and communication and accumulate valuable measured data for the autonomous orbit determination (AOD) of the satellites. With ISL data, navigation satellite systems can achieve AOD that enables satellites to update their ephemeris on⁃orbit without the support of the operational control segment. However, the key technologies of the AOD system of global positioning system have not been disclosed. In the last few years, some domestic scholars have carried out some useful research and exploration into AOD with simulation data. But the results of applying the ISL to AOD are still relatively few. Methods：The AOD processing model was briefly reviewed on the basis of inter⁃satellite ranging observations, and a distributed AOD model was built with ISL data. Then, the link building efficiency, constellation geometry, and inter⁃satellite measurement noise of the 18 medium Earth orbit (MEO) satellites in the BDS⁃3 constellation were analyzed, and the influences of the initial orbit accuracy and filtering processing interval on the AOD process were also analyzed. Finally, the AOD results obtained via the measured inter⁃satellite ranging data on the 18 MEO satellites were given and compared with the results of the precision orbit for evaluation and analysis. Results：AOD processing and performance analysis are conducted. The analysis results are as follows: (1) The BDS⁃3 ISLs are rather stable, with about 9 inter⁃satellite two⁃way ranging links for each satellite, and the effective ISLs and the constellation geometry could support the AOD of the distributed navigation constellation. (2) The initial orbit accuracy and the filtering interval affect the state evaluation accuracy and convergence velocity. (3) When the initial orbit accuracy is 0.1 m and the filtering interval is 60 s, the user ranging error accuracy of AOD for 30 days is 0.48 m. Conclusions：The results confirm the validity of the proposed distributed AOD algorithm. [ABSTRACT FROM AUTHOR]

Objectives: Flood monitoring not only plays an important role in flood prevention and mitigation, but also provides support for understanding the temporal and spatial evolution of floods and forecasting and warning. The global navigation satellite system-reflectometry (GNSS-R) measurement provides a new solution for flood monitoring. Methods: Firstly, we introduce a coherent signal detection method based on CYGNSS (cyclone global navigation satellite system) raw counts delay-Doppler map (DDM), and then analyze the changes of spatial range and time series before and after flood in South Asia are analyzed, based on the sensitivity of spaceborne GNSS - R signals to floods. Results: Research results show that, compared with the flood detection results of soil moisture active passive (SMAP), the monitoring range of the two methods in the dry season is similar. While in the rainy season, the detection range of the surface reflectivity is larger about 6.69%, and the detection range of the power ratio is smaller about 3.6%.Conclusions: The proposed method of using GNSS-R coherent signals to detect floods is more accurate than the surface reflectivity, and this method does not have to consider the influence of surface characteristics on the incoherent scattering power. It is a simple and effective method for flood detection. [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]

Objectives：Global navigation satellite system (GNSS) tomography, characterized by reconstructing the three⁃dimensional distribution of the atmospheric water vapor, has proved its capacity for studying the extreme weather events. The ill⁃conditioned problem resulting from the GNSS acquisition geometry is the critical issue of the GNSS tomography system. Algebraic reconstruction techniques, with the advantages of simple iteration and fast convergence, has been widely applied to the tropospheric tomography. However, the error allocation principle based on the intersection length of GNSS rays with each ray⁃voxel affects the accuracy of tomographic results. An improved adaptive algebraic reconstruction tech⁃ niques are proposed to address this problem.Methods：：In view of the unreasonable error allocation in the traditional algorithm, we suggest a new error allocation principle based on the variation of water vapor den⁃ sity (WVD) in voxels for the adaptive algorithms, in which the product of the intersection length and WVD is considered as a new principle to redistribute the difference between the GNSS slant water vapor (SWV) and the reconstructed SWV. Besides, the weight matrix model of elevation angles is introduced to the im⁃ proved algorithm to optimize the tomographic results.Results：：The new algorithms are tested by measured data from Xuzhou continuously operating reference stations (CORS) network and radiosonde during July 2016. Experimental results reveal that the WVD derived from the adaptive algorithms performs better than that of the general algorithms in root mean squared error (RMSE), standard deviation (STD) and mean ab⁃ solute error (MAE), and the RMSE is decreased by 25.91%, 15.81%, and 24.64% for adaptive algebraic reconstruction technique (AART), adaptive multiplicative algebraic reconstruction technique (AMART) and adaptive simultaneous iterative reconstruction technique (ASIRT) respectively. Under the conditions of light rain, moderate rain and heavy rain, the adaptive algorithms retrieve better water vapor profile than the traditional ones.Conclusions: Overall, the water vapor profile derived from the adaptive algorithms agrees with the radiosonde distribution better than the traditional algorithms, which reveals the advantage of the proposed method on optimizing the tomography solutions. [ABSTRACT FROM AUTHOR]

Objectives：Prediction of navigation satellite clock bias (SCB) plays an important role in maintaining system time synchronization and optimizing SCB parameters in navigation messages. SCB is a non‐linear and non‐stationary complex random sequence, which is difficult to make accurate prediction with a single mathematical model. Besides, the traditional prediction models of SCB lack of the in‐depth research of data structure characteristics. Methods: With the single difference of SCB as the research object, we first analyzed the graphical distribution pattern of the single difference structure. Then, by extracting the trend and cyclic of the single difference, a SCB prediction model including trend part, cyclic part and random part was proposed. Results: We used International GNSS Service (IGS) precision SCB as reference for prediction experiments. Based on the root mean squared error between the prediction result and the IGS SCB, we analyzed the accuracy and stability of the SCB prediction between different methods. The mean error was taken as the index to evaluate the accuracy of the SCB prediction from the perspective of satellite. The confidence interval of the prediction result distribution was used to evaluate the stability of the SCB prediction. Compared to quadratic polynomial model (QP), grey model(1,1) (GM (1,1)), and autoregressive integrated moving average model (ARIMA), the prediction quality of the method was superior to that of the other three methods. Conclusions: The proposed method avoids the disadvantage of the traditional model in prediction quality and algorithm complexity by constantly optimizing the parameters, and effectively improves the accuracy and stability of the SCB prediction. [ABSTRACT FROM AUTHOR]

Objectives: The BeiDou global navigation satellite system (BDS‐3), which provides high‐accuracy positioning, navigation and timing (PNT) services to global users, has completed the construction in 2020. Methods:To analyze the performance of BDS‐3,the total number of visible satellites of BDS‐3 preliminary system and BDS‐2 over the ‘Belt and Road’ area is calculated by using a method based on satellite distribution probability, then the satellite visibility of the full constellation BDS ‐ 3 over the global areas is also estimated with the same method. In addition, the dilution of precision (DOP) and their differences between BDS ‐3 and BDS ‐2 for stations at different geographical locations are investigated.Results: Results indicate that, over the ‘Belt and Road’ area, the total number of visible BDS‐3 preliminary system and BDS‐2 satellites is 8‐20, and the geometrical DOP (GDOP) is less than 1.85.In China and its surrounding areas, the visible number is more than 16, the GDOP is between 1.3 and 1.4. For the full constellation BDS‐3‐only, the number of visible satellite is 7‐15 over the global areas. The total number of visible full constellation BDS ‐ 3 geostationary earth orbit satellites and inclined geosynchronous orbit satellites is less than that of BDS‐2, but for medium earth orbit satellites, the number of visible satellites of the former system is more than that of the latter system by 6‐8. With a better satellite visibility, the average BDS‐3 position DOP, horizontal DOP, vertical DOP and time DOP are 1.356， 0.759， 1.123 and 0.682 over China and its surrounding areas, respectively, which are 22.6%, 28.8%, 19.2% and 37.0% lower than those of BDS ‐2, and have 4% ‐13% improvement compared to the global DOP averages.Conclusions: Compare to the BDS‐2, BDS‐3 satellite visibility has been improved significantly, while it will provide higher accuracy and more reliable PNT services to global users for its better satellite constellation. [ABSTRACT FROM AUTHOR]

Objectives: BeiDou navigation satellite system (BDS) is the only satellite navigation system that provides triple ‐ frequency signals as a whole constellation of global navigation satellite system (GNSS). There are three kinds of differential code biases (DCBs): DCBC2I-C7I、DCBC2I-C6I、DCBC7I-C6I. Theoretically, there are zero algebraic relationship among the three DCBs.Methods: Based on the zero algebraic relationship, BDS triple‐frequency data of 2018 DOY 001−030 from multi‐GNSS experiment (MGEX) stations are selected, and medium earth orbit (MEO) and inclined geosynchronous satellite orbit (IGSO) satellites are used as reference satellites to estimate three DCBs.Results: Taking the DCB products provided by Chinese Academy of Sciences (CAS) and Deutsches Zentrum für Luft und Raumfahrt (DLR) as reference values, we analyze the accuracy and stability of the estimated DCB and the DCB time series of some typical satellites. It suggests that results derived from the proposed method are in good agreement with the reference values. And the experiment of standard point positioning (SPP) in BDS single frequency verifies the effect of DCB on accuracy of SPP.Conclusions: All the results demonstrate that the proposed method is applicable and scientific to the estimation of BDS DCBs. [ABSTRACT FROM AUTHOR]

High ‐ precision ionospheric model is of great significance for improving the positioning accuracy of navigation satellite system. With the rapid development of low earth orbit satellites, the establishment of a high ‐ precision ionospheric model has provided new opportunities. Based on the simulation data, this pa‐ per obtains LEO (low earth orbit) and GNSS (global navigation satellite system) satellites observation data of January 1 to 30, 2017 by means of simulation. The constellation types include 60, 96, 192 and 288 satel‐ lites respectively. Based on these data and taking African region as an example, the coverage of GNSS and LEO satellites’ ionospheric pierce points and the joint modeling accuracy are studied. The results show that, after LEO satellites are added, the distribution of ionospheric pierce points is significantly improved, which leads to a noticeable rise of their density. The range of ionospheric pierce points of single low ‐ orbit satellite is larger than that of GNSS satellite, and the altitude angle and azimuth angle of LEO satellite change remarkably. With the increase of the number of low ‐orbit satellites, the accuracy of joint modeling also rises up. Within different latitudes of 30° E at UTC 12: 00, the difference between GNSS ‐ only and GNSS+288 LEO ionospheric modeling results is the largest, reaching -1.6 TECU. With the increase of modeling time, the difference between the joint modeling results and GNSS ‐ only results gradually de‐ creases. [ABSTRACT FROM AUTHOR]

The positioning performance of BeiDou Navigation Satellite System （BDS） /Quasi⁃Zenith Satel⁃ lite System （QZSS） and its combined systems in China, Japan and their peripheral areas is evaluated and analyzed by using the observation data of the multi⁃GNSS experiment (MGEX) from the number of visible satellites, dilution of precision, the multipath effect, the signal⁃to⁃noise ratio (SNR), the static precise point positioning （PPP） and kinematic PPP . The results show that the combination of QZSS and BDS system can increase the number of satellites and provide better spatial geometry distribution, thus ensuring the accu⁃ racy, reliability and availability of positioning. Because the constellation structure of QZSS satellite is similar to that of BDS satellite, its multipath effect is consistent with that of BDS satellite. The SNR of MEO(medi⁃ um earth orbit) satellite is 2⁃3 dBHz higher than that of inclined geosynchronous orbit(IGSO) satellite at the same altitude angle. For static and kinematic PPP, the improvement of BDS+QZSS combination is more obvious than that of BDS single system at low altitude angle when the altitude angle is larger (40°). The im⁃ provement rate of E, N and U directions static PPP can reach more than 20%， where kinematic PPP can reach more than 30%. QZSS plays a complementary and enhanced role in BDS positioning in Japan, China and surrounding areas. In the complex environment (such as city, building intensive area, mountain area, et al.) where the signal is easy to block, it will have better application value. [ABSTRACT FROM AUTHOR]

The BeiDou Navigation Satellite System (BDS) provides wide area differential service (WADS) for users in China territory by broadcasting equivalent satellite clock, orbit correction, ionospheric grid information and zone correction through Geostationary Earth Orbit (GEO) satellites. By using WADS corrections correctly, users with code observations can get meter-level positioning accuracy while carrier phase users can get decimeter-level result. However, the imperfection and discontinuity of zone correction bring a negative influence upon positioning. Aiming at solving the precision deterioration and positioning reconvergence caused by the discontinuity of zone correction, a fitting algorithm is proposed for zone correction generation and user’s positioning. The fitting algorithm is established on the basis of the stability of double-differenced zone corrections. By fixing the zone correction of one reference satellite and the dou⁃ ble-differenced bias between multiple zones and satellites, the correction gap can be filled up successfully. Then, the validity of the algorithm and the performance of fitting zone correction was evaluated by differential positioning experiments using observations from six Multi-GNSS Experiment (MGEX) stations within zone correction service range under kinematic PPP mode. Experiment results show that although the positioning precision of fitting zone corrections, which is 0.20 m and 0.44 m for horizontal and vertical components respectively, is slightly inferior to the result derived by using the original corrections. In the discontinuous periods of zone correction, the positioning results are much more stable and continuous, and without posi⁃ tioning re-convergence. With no additional information used in correction fitting, the algorithm can be used for the zone correction generation period and position, which will improve the service integrity and positioning reliability. [ABSTRACT FROM AUTHOR]

The successful launch of Chang'E ⁃4 relay satellite in June 2018 has laid a good foundation for the mission of landing on the back of moon at the end of the year. Very long baseline interferometry (VLBI) system plays an important role in satellite positioning and orbital transfer. In this paper, the data accuracy of delay, delay rate and positioning of VLBI observations during the real⁃time phase of the Chang'E⁃4 relay satellite mission are calculated, and the application of positioning reduction in the real⁃time track monitoring of the orbit control arcs of the Chang'E⁃4 relay satellite is presented. In the process of the relay satellite en⁃ tering Halo orbit, we show the real⁃time change of the orbit elements converted by our positioning method and compare the real⁃time positioning results with the post⁃time orbit determination results to further verify the validity and accuracy of our real⁃time positioning results. We provide a new Halo real⁃time track moni⁃ toring method. [ABSTRACT FROM AUTHOR]

The experimental satellites of BeiDou global navigation satellite system (BDS‐3) broadcast new B1C / B2a / B2b signals that overlap with GPS L1/L5 signals and European Galileo E1/E5a/E5b signals, which provides the opportunity of tightly combined relative positioning for short baselines with observations from GPS, Galileo, and BDS ‐ 3 experimental satellites. In this contribution， we first evaluate the value and time‐domain stability of differential inter‐system bias (DISB) between overlapping frequencies of GPS, Galileo, and BDS ‐ 3. The results show that the DISB of the same type of receiver is close to 0, and its influence can be ignored in the tightly combined relative positioning. Then we preliminarily evaluated performance of single‐epoch tightly combined relative positioning for short baselines with observations from GPS， Galileo， and BDS‐3 experimental satellites. The results show that, compared with the traditional loosely combined model, the tightly combined model can significantly improve the success rate and reliability of ambiguity resolution. The improvement is especially significant (approximately 25% ‐45%) under environments where the observed satellites for each system are limited and only single‐frequency observations are available. [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]

Aiming at the characteristics of quantity and fluctuation range on the gross error from global navi- gation satellite system (GNSS) deformation monitoring time series, we propose an improved 3σ method of gross error detection based on wavelet analysis. We use simulated data and actual engineering data, and compare the detection results with the traditional 3σ method and inter-quartile range (IQR) method respec- tively. The experimental results show that compared with the traditional 3σ method and IQR method, the improved 3σ method not only has better effect on the overall detection rate, but also has more obvious ad- vantages in detecting 3σ to 5σ gross error. Thus, it is more suitable for the actual requirement of GNSS de- formation monitoring project. [ABSTRACT FROM AUTHOR]

The influence of the number and distribution of ground tracking stations on the precision orbit determination of navigation satellites is analyzed? and a stochastic optimization method for selecting the ground tracking stations for GNSS satellite precise orbit determination is proposed. The proposed method takes the information such as site location and quality into account, by assigning a certain probability to the global station. An algorithm is designed to optimally select a set of IGS ground tracking stations to achieve GPS satellite precise orbit determination of high precision. The results show that, based on the proposed method, by using 30 stations, the optimally-selected GPS precision orbit accuracy can reach 2. 15 cm, while by using 60 stations the accuracy is better than 1. 26 cm, the number of stations reaches 90, the precision of orbit determination can be increased to 1 cm. [ABSTRACT FROM AUTHOR]

The IGSO satellites in China's navigation system have two attitude control modes: Yaw-steering mode and orbit-normal mode. The orbit-normal mode will be adopted when the acute angle between solar vector and orbit plane is minor. The satellite attitude control modes switching results in significant decrease of the satellite orbit determination accuracy. Switching causes in variations of solar radiation pressure, and therefore the two statuses are incompatible with one group of solar radiation pressure parameters while processing data arc segments containing both yaw-steering and orbit-normal modes. Hence, this article presents the orbit determination strategy using a piecewise linear model to describe the influences of change in solar radiation pressure, which can enhance 75% orbit overlap accuracy. This change is demonstrated with a resolved solar radiation pressure parameters ex-periement and is an effective method for orbit determination during the attitude switching. [ABSTRACT FROM AUTHOR]

Based on MSIS90 atmospheric and 3D NeUoG ionospheric models, three types of ionospheric disturbances, i. e. ionospheric storm, traveling ionospheric disturbance (TID) and ionospheric trough were simulated, and their effects on bending angle residual ionospheric errors (RIEs) and temperature accuracy were analyzed. The results show that under high solar activity level, the bending angle RIE and its standard deviation can reach 1.5 μrad and 0.9 μrad, in the impact height range of 35~50 km; 4 μrad and 1.4 μrad, respectively, in the impact height range of 15~35 km. The ionopsheric storm and trough can enlarge the bending angle RIE standard deviations by more than 20 times, and TID can enlarge the bending angle RIE standard deviations by several times. Under the influence of ionospheric disturbances, temperature errors can reach 8 K and can pollute the GPS RO daily-mean and month-mean observations. Hence, it is desirable to develop ionospheric correction algorithms; the temperature outliers caused by ionospheric disturbances should be removed according to ionospheric monitoring results. [ABSTRACT FROM AUTHOR]

For the three types of GNSS system time offset monitoring approach, we analyze the basic principles and methods and made a comparison of their accuracy, reliability and implementation, and finding that the PPS-based method is more suitable for time offset monitoring. We present a detailed discussion and description, including hardware configuration, data collection and post processing content for the PPS-based method. In an experiment we use both GPS and GLONASS dual-mode receivers to generate the 1 PPS of GPS and GLONASS second pulse signals, and used the Stanford SR620 time interval counter for PPS precision measurement. We thereby obtained GPS and GLONASS system time offset measurement values, and adopted a FIR filter with a Hanning window to get the GPS and GLONASS system time offset average 317.1 ns, with a standard deviation of 5.01 ns. [ABSTRACT FROM AUTHOR]

Dilutions of precision (DOP) is a key criterion to measure the geometric strength of single-point-positioning configuration. Revealing the configuration set composed of configurations with minimal DOPs has practical significance in geodetic positioning design and application. A kind of nested cones composed of many single cones was provided and the precondition of minimizing the PDOP (position dilution of precision) was deduced based on the cone configuration. A general method of structuring configurations with minimal GDOP (geometric dilution of precision) is given. The nested cones with minimal GDOP structured by the nested cones with minimal PDOP are discussed. At last, five regular polyhedrons are deduced by the nested cones with minimal GDOP. It was shown that any regular polyhedron has minimal GDOP. Moreover, the configurations with minimal GDOP are more numerous than the known polyhedrons. [ABSTRACT FROM AUTHOR]

The conventional tracking loops in GPS receivers usually lose locked high dynamic signals. Thus, to achieve better FLL performance in dynamic situations and higher PLL measurement accuracy, we implemented a second order FLL in an assisted third order PLL (FPLL) based carrier tracking loop. These methods were employed to analyze the measurement accuracy FPLL of code and carrier phases, and implemented on our software receiver platform with Matlab. FPLL was tested using the data collected through Spirent GSS7700 simulator. The results show that the average measurement accuracy for code the phase and carrier phase was 1.1 m (1σ) and 4.24 X 10-3 m/s (1σ), respectively, as the signals are around 40 dB/Hz of SNR (signal to noise ratio), with 26 g of acceleration and 9 g/s of jerk. [ABSTRACT FROM AUTHOR]

Kalman filter has been widely used in GNSS/INS integrated navigation system, and many adaptive Kalman algorithms have been developed to avoid divergence caused by various exceptions, such as abnormal observations and dynamic model disturbance etc. In the GNSS/INS loosely integrated navigation system, the adaptive filter is generally constructed based on the predicted residual because the observations are deficient compared to the state parameter number. Thus, the GNSS/INS loosely integrated navigation system is highly dependent on observations. The system performance and stability will degrade when the impact of abnormal observations cannot be decreased effectively. To improve the filter robustness in the GNSS/INS loosely integrated navigation system, the neural network algorithm is employed to monitor the abnormal observations, and combined with the traditional adaptive algorithm to form enhanced adaptive filter. Based on the proposed method, four adap-tively robust Kalman filtering algorithms are developed, which is tested on the raw GNSS/INS data. Results show that, the new algorithms can not only improve the filter estimation accuracy, but also reduce the impacts of abnormal observations and dynamical model disturbance. [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]

In GPS/INS tightly-coupled navigation, low precision of GPS pseudo-range observation reduces the navigation accuracy. In order to resolve the issue, a GPS/INS tightly-coupled navigation algorithm based on Doppler smoothing pseudo-range is proposed. First the dynamical model and observation model of GPS/INS tightly-coupled are introduced. Based on phase smoothing pseudo-range, the algorithm of Doppler smoothing pseudo-range and design formulas of variance-covariance are given. At the same time, genetic algorithm (GA) is introduced to estimate the smooth factor. A vehicle experiment was made to test the validity of new algorithm. Experimental results indicate that smooth algorithm is able to improve the precision of pseudo-range observation, which increases the position accuracy of integrated navigation. Compared with phase smoothing pseudo-range, Doppler smoothing pseudo-range has litter better performance as integrated observation. The Doppler smoothing pseudo-range eliminates the influence of cycle slip and increases computational efficiency. At the same time, the accuracy of pseudo-range observation is improved to the accuracy of pseudo-range rate, which enhances the stability of filter system. [ABSTRACT FROM AUTHOR]