Showing papers in "Satellite Navigation in 2020"

Basic performance and future developments of BeiDou global navigation satellite system

Abstract: The core performance elements of global navigation satellite system include availability, continuity, integrity and accuracy, all of which are particularly important for the developing BeiDou global navigation satellite system (BDS-3). This paper describes the basic performance of BDS-3 and suggests some methods to improve the positioning, navigation and timing (PNT) service. The precision of the BDS-3 post-processing orbit can reach centimeter level, the average satellite clock offset uncertainty of 18 medium circular orbit satellites is 1.55 ns and the average signal-in-space ranging error is approximately 0.474 m. The future possible improvements for the BeiDou navigation system are also discussed. It is suggested to increase the orbital inclination of the inclined geostationary orbit (IGSO) satellites to improve the PNT service in the Arctic region. The IGSO satellite can perform part of the geostationary orbit (GEO) satellite’s functions to solve the southern occlusion problem of the GEO satellite service in the northern hemisphere (namely the “south wall effect”). The space-borne inertial navigation system could be used to realize continuous orbit determination during satellite maneuver. In addition, high-accuracy space-borne hydrogen clock or cesium clock can be used to maintain the time system in the autonomous navigation mode, and stability of spatial datum. Furthermore, the ionospheric delay correction model of BDS-3 for all signals should be unified to avoid user confusion and improve positioning accuracy. Finally, to overcome the vulnerability of satellite navigation system, the comprehensive and resilient PNT infrastructures are proposed for the future seamless PNT services.

Status, perspectives and trends of satellite navigation

Abstract: This paper reviews the status of satellite navigation (as per 11 May 2020)—without claim for completeness—and discusses the various global navigation satellite systems, regional satellite navigation systems and satellite-based augmentation systems. Problems and challenges for delivering nowadays a safe and reliable navigation are discussed. New opportunities, perspectives and megatrends of satellite navigation are outlined. Some remarks are closing this paper emphasizing the great value of satellite navigation at present and in future.

Inertial sensors technologies for navigation applications: state of the art and future trends

Abstract: Inertial navigation represents a unique method of navigation, in which there is no dependency on external sources of information. As opposed to other position fixing navigation techniques, inertial navigation performs the navigation in a relative sense with respect to the initial navigation state of the moving platform. Hence, inertial navigation systems are not prone to jamming, or spoofing. Inertial navigation systems have developed vastly, from their occurrence in the 1940s up to date. The accuracy of the inertial sensors has improved over time, making inertial sensors sufficient in terms of size, weight, cost, and accuracy for navigation and guidance applications. Within the past few years, inertial sensors have developed from being purely mechanical into incorporating various technologies and taking advantage of numerous physical phenomena, from which the dynamic forces exerted on a moving body could be computed accurately. Besides, the evolution of inertial navigation scheme involved the evolution from stable-platform inertial navigation system, which were mechanically complicated, to computationally demanding strap-down inertial navigation systems. Optical sensory technologies have provided highly accurate inertial sensors, at smaller sizes. Besides, the vibratory inertial navigation technologies enabled the production of Micro-electro-machined inertial sensors that are extremely low-cost, and offer extremely low size, weight and power consumption, making them suitable for a wide range of day-to-day navigation applications. Recently, advanced inertial sensor technologies have been introduced to the industry such as nuclear magnetic resonance technology, cold-atom technology, and the re-introduction of fluid-based inertial sensors. On another note, inertial sensor errors constitute a huge research aspect in which it is intended for inertial sensors to reach level in which they could operate for substantially long operation times in the absence of updates from aiding sensors, which would be a huge leap. Inertial sensors error modeling techniques have been developing rapidly trying to ensure higher levels of navigation accuracy using lower-cost inertial sensors. In this review, the inertial sensor technologies are covered extensively, along the future trends in the inertial sensors’ technologies. Besides, this review covers a brief overview on the inertial error modeling techniques used to enhance the performance of low-cost sensors.

PPP models and performances from single- to quad-frequency BDS observations

Abstract: Nowadays, China BeiDou Navigation Satellite System (BDS) has been developed well and provided global services with highly precise positioning, navigation and timing (PNT) as well as unique short-message communication, particularly global system (BDS-3) with higher precision multi-frequency signals. The precise point positioning (PPP) can provide the precise position, receiver clock, and zenith tropospheric delay (ZTD) with a stand-alone receiver compared to the traditional double differenced relative positioning mode, which has been widely used in PNT, geodesy, meteorology and so on. However, it has a lot of challenges for multi-frequency BDS PPP with different strategies and more unknown parameters. In this paper, the detailed PPP models using the single-, dual-, triple-, and quad-frequency BDS observations are presented and evaluated. Firstly, BDS system and PPP method are introduced. Secondly, the stochastic models of time delay bias in BDS-2/BDS-3 PPP including the neglection, random constant, random walk and white noise are presented. Then, three single-frequency, four dual-frequency, four triple-frequency and four quad-frequency BDS PPP models are provided. Finally, the BDS PPP models progress and performances including theoretical comparison of the models, positioning performances, precise time and frequency transfer, ZTD, inter-frequency bias (IFB) and differential code bias (DCB) are presented and evaluated as well as future challenges. The results show that the multi-frequency BDS observations will greatly improve the PPP performances.

Global capabilities of BeiDou Navigation Satellite System

Abstract: The BeiDou Navigation Satellite System (BDS) has completed the constellation deployment and started to provide global services. After achieving the capabilities of global coverage, global first-class accuracy for Positioning, Navigation and Timing (PNT), global Inter-Satellite Links (ISL) networking, and global featured services, BDS will promote the construction of the comprehensive PNT infrastructure in the new era and play a more active role in international cooperation with other Global Navigation Satellite System (GNSS) providers to better serve humankind and the world.

Advances in BeiDou Navigation Satellite System (BDS) and satellite navigation augmentation technologies

Abstract: Several noteworthy breakthroughs have been made with the BeiDou Navigation Satellite System (BDS) and other global navigation satellite systems as well as the associated augmentation systems, such as the commissioning of the BDS-3 preliminary system and the successful launch of the first BDS-3 GEO satellite which carries the satellite-based augmentation payload. Presently, BDS can provide basic services globally, and its augmentation system is also being tested. This paper gives an overview of BDS and satellite navigation augmentation technologies. This overview is divided into four parts, which include the system segment technologies, satellite segment technologies, propagation segment technologies, and user segment technologies. In each part, these technologies are described from the perspectives of preliminary information, research progress, and summary. Moreover, the significance and progress of the BeiDou Satellite-based Augmentation System (BDSBAS), low earth orbit augmentation, and the national BeiDou ground-based augmentation system are presented, along with the airborne-based augmentation system. Furthermore, the conclusions and discussions covering popular topics for research, frontiers in research and development, achievements, and suggestions are listed for future research.

Machine learning based LOS/NLOS classifier and robust estimator for GNSS shadow matching

Abstract: Global Navigation Satellites Systems (GNSS) is frequently used for positioning services in various applications, e.g., pedestrian and vehicular navigation. However, it is well-known that GNSS positioning performs unreliably in urban environments. GNSS shadow matching is a method of improving accuracy in the cross-street direction. Initial position and classification of observed satellite visibility between line-of-sight (LOS) and non-line-of-sight (NLOS) are essential for its performance. For the conventional LOS/NLOS classification, the classifiers are based on a single feature, extracted from raw GNSS measurements, such as signal noise ratio, pseudorange, elevation angle, etc. Especially in urban canyons, these measurements are unstable and unreliable due to the signal reflection and refraction from the surrounding buildings. Besides, the conventional least square approach for positioning is insufficient to provide accurate initialization for shadow matching in urban areas. In our study, shadow matching is improved using the initial position from robust estimator and the satellite visibility determined by support vector machine (SVM). The robust estimator has an improved positioning accuracy and the classification rate of SVM classification can reach 91.5% in urban scenarios. An important issue is related to satellites with ultra-high or low elevation angles and satellites near the building boundary that are very likely to be misclassified. By solving this problem, the SVM classification shows the potential of about 90% classification accuracy for various urban cases. With the help of these approaches, the shadow matching has a mean error of 10.27 m with 1.44 m in the cross-street direction; these performances are suitable for urban positioning.

Role, path, and vision of “5G + BDS/GNSS”

Abstract: Communication, positioning, navigation, and decision-making abilities have evolved into Positioning, Navigation, and Timing (PNT) intelligence during the long process of human migration and hence promoted human evolution. This article defines intelligence and smartness from the perspective of biological intelligence. New requirements as a result of the development of communication, navigation, time service, and decision making are identified in this study. The article points out that there are many radio PNT service methods, such as 5G, the new-generation high-speed communication networks and the low-latency and ubiquitous mobile communication networks as well as Global Navigation Satellite System (GNSS), but the integrated application is especially important in providing technical support for the adjustment and control of the physical world by intelligent sensing, cognition, decision-making, and precise coordination. The fusion of 5G and GNSS [including BeiDou Navigation Satellite System (BDS)] information with the corresponding equipment can be embedded into a machine to make it intelligent. Furthermore, the fused information of 5G and GNSS together with the environment information may extend human perception and physical world control ability in terms of time and space scale. It will help to develop critical information infrastructure in the age of intelligence, which will also extend the definition of artificial intelligence. Additionally, the “5G + BDS/GNSS” fusion path is analyzed explicitly herein in terms of realization methods, information processing, and new application services. On the whole, the application of “5G + BDS/GNSS + satellite-based communication” as a critical infrastructure for land, sea, air, space and network spatiotemporal control rights is proposed.

BDS satellite clock offset prediction based on a semiparametric adjustment model considering model errors

Abstract: In view of the influence of model errors in conventional BeiDou prediction models for clock offsets, a semiparametric adjustment model for BeiDou Navigation Satellite System (BDS) clock offset prediction that considers model errors is proposed in this paper. First, the model errors of the conventional BeiDou clock offset prediction model are analyzed. Additionally, the relationship among the polynomial model, polynomial model with additional periodic term correction, and its periodic correction terms is explored in detail. Second, considering the model errors, combined with the physical relationship between phase, frequency, frequency drift, and its period in the clock sequence, the conventional clock offset prediction model is improved. Using kernel estimation and comprehensive least squares, the corresponding parameter solutions of the prediction model and the estimation of its model error are derived, and the dynamic error correction of the clock sequence model is realized. Finally, the BDS satellite precision clock data provided by the IGS Center of Wuhan University with a sampling interval of 5 min are used to compare the proposed prediction method with commonly used methods. Experimental results show that the proposed prediction method can better correct the model errors of BDS satellite clock offsets, and it can effectively overcome the inaccuracies of clock offset correction. The average forecast accuracies of the BeiDou satellites at 6, 12, and 24 h are 27.13%, 37.71%, and 45.08% higher than those of the conventional BeiDou clock offset forecast models; the average model improvement rates are 16.92%, 20.96%, and 28.48%, respectively. In addition, the proposed method enhances the existing BDS satellite prediction method for clock offsets to a certain extent.

Multi-constellation GNSS precise point positioning with multi-frequency raw observations and dual-frequency observations of ionospheric-free linear combination

Abstract: Precise point positioning (PPP) is famous for its capability of high-precision positioning with just one station as long as the receiver can receive global navigation satellite system (GNSS) signals. With the rapid development of BDS and Galileo, the number of available satellites for positioning has increased significantly. In addition, GPS III, GLONASS-K, BDS, and Galileo satellites can transmit triple-frequency signals. The potentials of multi-constellation GNSS PPP requires further analysis on a global scale. Therefore, we selected 96 multi-GNSS experiment (MGEX) stations with a global distribution and used 1 week’s data to assess the PPP performance. The results show that the PPP based on multi-frequency raw observations with spatial and temporal constraints has better performance than PPP using dual-frequency ionospheric-free observations. The main contribution of multi-constellation GNSS PPP is to shorten the convergence time. The convergence time for GPS PPP is approximately 40 min, which can be shortened to less than 20 min in multi-GNSS PPP. After convergence, the positioning accuracy of multi-GNSS PPP is improved by 0.5 to 1.0 cm compared with GPS or GLONASS PPP. The positioning accuracy of multi-GNSS could be further improved in the future with the BDS and Galileo precise products of orbits, clock and phase center offset/variation.

Precise time scales and navigation systems: mutual benefits of timekeeping and positioning

Abstract: The relationship and the mutual benefits of timekeeping and Global Navigation Satellite Systems (GNSS) are reviewed, showing how each field has been enriched and will continue to progress, based on the progress of the other field. The role of GNSSs in the calculation of Coordinated Universal Time (UTC), as well as the capacity of GNSSs to provide UTC time dissemination services are described, leading now to a time transfer accuracy of the order of 1–2 ns. In addition, the fundamental role of atomic clocks in the GNSS positioning is illustrated. The paper presents a review of the current use of GNSS in the international timekeeping system, as well as illustrating the role of GNSS in disseminating time, and use the time and frequency metrology as fundamentals in the navigation service.

Orbit determination and time synchronization for BDS-3 satellites with raw inter-satellite link ranging observations

Abstract: To provide competitive global positioning and timing services under the condition that monitoring stations are confined to Chinese territory, inter-satellite link (ISL) technology is used by the third-generation BeiDou Navigation Satellite System (BDS-3). The ISL, together with the dual one-way links between satellites and anchor stations, may enable autonomous navigation for BDS-3. In this paper, we propose a general observation model for orbit determination (OD) and time synchronization (TS) directly using non-simultaneous observations, such as raw ISL pseudoranges. With the proposed model, satellite orbits, clocks, and hardware delay biases of ISL equipment can be determined simultaneously by jointly processing inter-satellite one-way pseudorange data and observation data from ground monitoring stations. Moreover, autonomous OD and TS are also achievable with one-way pseudorange data from anchor stations and satellites. Data from eight BDS-3 satellites, two anchor stations, and seven monitoring stations located in China were collected to validate the proposed method. It is shown that by jointly processing data from the ISL and seven monitoring stations, the RMS of overlap orbit differences in radial direction is 0.019 m, the overlap clock difference (95%) is 0.185 ns, and the stability of the estimated hardware delay biases for each satellite is greater than 0.5 ns. Compared with the results obtained with the seven stations, the improvements of orbits in radial direction and clocks are 95.7% and 90.5%, respectively. When the hardware delay biases are fixed to predetermined values, the accuracies of orbits and clocks are further improved. By jointly processing pseudoranges from the satellites and the two anchor stations, the RMS of overlap orbit differences is 0.017 m in the radial direction, and the overlap clock difference (95%) is 0.037 ns. It has also been demonstrated that under the condition of one-way ranging links, the accuracies of orbits and clocks obtained by the above two modes are still significantly better than those obtained by using the data from the monitoring stations alone.

Improved precise positioning with BDS-3 quad-frequency signals

Abstract: The establishment of the BeiDou global navigation satellite system (BDS-3) has been completed, and the current constellation can independently provide positioning service globally. BDS-3 satellites provide quad-frequency signals, which can benefit the ambiguity resolution (AR) and high-precision positioning. This paper discusses the benefits of quad-frequency observations, including the precision gain of multi-frequency high-precision positioning and the sophisticated choice of extra-wide-lane (EWL) or wide-lane (WL) combinations for instantaneous EWL/WL AR. Additionally, the performance of EWL real-time kinematic (ERTK) positioning that only uses EWL/WL combinations is investigated. The results indicate that the horizontal positioning errors of ERTK positioning using ionosphere-free (IF) EWL observations are approximately 0.5 m for the baseline of 27 km and 1 m for the baseline of 300 km. Furthermore, the positioning errors are reduced to the centimetre level if the IF EWL observations are smoothed by narrow-lane observations for a short period.

Initial analysis for characterizing and mitigating the pseudorange biases of BeiDou navigation satellite system

Abstract: Pseudorange bias has become a practical obstacle in the field of high-precision global navigation satellite system (GNSS) applications, which greatly restricts the further development of high-precision applications. Unfortunately, no studies have been conducted on the pseudorange biases of the BeiDou navigation satellite system (BDS). To mitigate the effects of pseudorange biases on the BDS performance to the greatest extent possible, the origin of such BDS pseudorange biases are first thoroughly illustrated, based upon which the dependency of the biases on the receiver configurations are studied in detail. Owing to the limitations regarding the parameter re-settings for hardware receivers, software receiver technology was used to achieve the ergodicity of the receiver parameters, such as the correlator spacing and front-end bandwidth, using high-fidelity signal observations collected by a 40-m-high gain dish antenna at Haoping Observatory. Based on this, the pseudorange biases of the BDS B1I and B3I signals and their dependency on different correlator spacings and front-end bandwidths were adequately provided. Finally, herein, the suggested settings of the correlator spacing and front-end bandwidth for BDS receivers are in detail proposed for the first time. As a result, the pseudorange biases of the BDS signals will be less than 20 cm, reaching even under 10 cm, under this condition. This study will provide special attention to GNSS pseudorange biases, and will significantly promote a clear definition of the appropriate receiver parameter settings in the interface control documents of BDS and other individual satellite systems.

Initial assessment of single- and dual-frequency BDS-3 RTK positioning

Abstract: The BeiDou navigation satellite system with global coverage (BDS-3) has been fully operational since July 2020 and provides comprehensive services to global users. BDS-3 transmits several new navigational signals based on the signals inherited from the BeiDou navigation satellite (regional) system (BDS-2). Previous studies focused on the positioning performance of BDS-2 plus BDS-3 and that of combining BDS-3 and other Global Navigation Satellite Systems (GNSSs), but there was no in-depth discussion on the positioning performance of the BDS-3-only. In this contribution, the BDS-3-only Real-Time Kinematic (RTK) positioning is analysed using the data collected in zero and short baselines in Wuhan, China. The RTK model based on Single-Differenced is first presented, and the BDS-3-only RTK positioning in cases of single and dual-frequencies is evaluated with the model in terms of the empirical integer ambiguity resolution success rates and positioning accuracy. Our numerical tests suggest two major findings. First, the positioning performance for the B1I and B3I retained from BDS-2 and the new frequency B1C is comparable, while that for the new frequency B2a is poorer. Second, the positioning performance of the new frequency combination of the B1C + B2a is not as good as that of the B1C only, owing to the unrealistic stochastic model used.

Analysis of Galileo signal-in-space range error and positioning performance during 2015–2018

Abstract: A long-term analysis of signal-in-space range error (SISRE) is presented for all healthy Galileo satellites, and the first pair of full operational capability satellites in wrong elliptical orbits. Both orbit and clock errors for Galileo show an obvious convergence trend over time. The annual statistical analyses show that the average root mean squares (RMSs) of SISRE for the Galileo constellation are 0.58 m (2015), 0.29 m (2016), 0.23 m (2017), and 0.22 m (2018). Currently, the accuracy of the Galileo signal-in-space is superior to that of the global positioning system (GPS) Block IIF (0.35 m). In addition, the orbit error accounts for the majority of Galileo SISRE, while the clock error accounts for approximately one-third of SISRE due to the high stability of the onboard atomic clock. Single point positioning results show that Galileo achieves an accuracy of 2–3 m, which is comparable to that of GPS despite the smaller number of satellites and worse geometry. Interestingly, the vertical accuracy of Galileo, which uses the NeQuick ionospheric model, is higher than that of GPS. Positioning with single frequency E1 and E5 show a higher precision than E5a and E5b signals. Regarding precise point positioning (PPP), the results indicate that a comparable positioning accuracy can be achieved among different stations with the current Galileo constellation. For static PPP, the RMS values of Galileo-only solutions are within 1 cm horizontally, and the vertical RMSs are mostly within 2 cm horizontally. For kinematic PPP, the RMSs of Galileo-only solutions are mostly within 4 cm horizontally and 6 cm vertically.

On inertial navigation and attitude initialization in polar areas

Abstract: Inertial navigation and attitude initialization in polar areas become a hot topic in recent years in the navigation community, as the widely-used navigation mechanization of the local level frame encounters the inherent singularity when the latitude approaches 90°. Great endeavors have been devoted to devising novel navigation mechanizations such as the grid or transversal frames. This paper highlights the fact that the common Earth-frame mechanization is sufficiently good to handle the singularity problem in polar areas. Simulation results are reported to demonstrate the singularity problem and the effectiveness of the Earth-frame mechanization.

Inversion of surface vegetation water content based on GNSS-IR and MODIS data fusion

Abstract: Obtaining high-precision, long-term sequences of vegetation water content (VWC) is of great significance for assessing surface vegetation growth, soil moisture, and fire risk. In recent years, the global navigation satellite system-interferometric reflection (GNSS-IR) has become a new type of remote sensing technology with low cost, all-weather capability, and a high temporal resolution. It has been widely used in the fields of snow depth, sea level, soil moisture content, and vegetation water content. The normalized microwave reflectance index (NMRI) based on GNSS-IR technology has been proven to be effective in monitoring changes in VWC. This paper considers the advantages and disadvantages of remote sensing technology and GNSS-IR technology in estimating VWC. A point-surface fusion method of GNSS-IR and MODIS data based on the GA–BP neural network is proposed to improve the accuracy of VWC estimation. The vegetation index products (NDVI, GPP, LAI) and the NMRI that unified the temporal and spatial resolution were used as the input and output data of the training model, and the GA–BP neural network was used for training and modeling. Finally, a spatially continuous NMRI product was generated. Taking a particular area of the United States as a research object, experiments show that (1) a neural network can realize the effective fusion of GNSS-IR and MODIS products. By comparing the GA–BP neural network, BP neural network, and multiple linear regression (MLR), the three models fusion effect. The results show that the GA–BP neural network has the best modeling effect, and the r and RMSE between the model estimation result and the reference value are 0.778 and 0.0332, respectively; this network is followed by the BP neural network, in which the r and RMSE are 0.746 and 0.0465, respectively. MLR has the poorest effect, with r and RMSE values of 0.500 and 0.0516, respectively. (2) The spatiotemporal variation in the 16 days/500 m resolution NMRI product obtained by GA–BP neural network fusion is consistent with that in the experimental area. Through the testing of GNSS stations that did not participate in the modeling, the r between the estimated value of the NMRI and the reference value is greater than 0.87, and the RMSE is less than 0.049. Therefore, the method proposed in this paper is optional and effective. The spatially continuous NMRI products obtained by fusion can reflect the changes in VWC in the experimental area more intuitively.

A method of improving ambiguity fixing rate for post-processing kinematic GNSS data

Abstract: Global Navigation Satellite System precise positioning using carrier phase measurements requires reliable ambiguity resolution. It is challenging to obtain continuous precise positions with a high ambiguity fixing rate under a wide range of dynamic scenes with a single base station, thus the positioning accuracy will be degraded seriously. The Forward–Backward Combination (FBC), a common post-processing smoothing method, is simply the weighted average of the positions of forward and backward filtering. When the ambiguity fixing rate of the one-way (forward or backward) filter is low, the FBC method usually cannot provide accurate and reliable positioning results. Consequently, this paper proposed a method to improve the accuracy of positions by integrating forward and backward AR, which combines the forward and backward ambiguities instead of positions—referred to as ambiguity domain-based integration (ADBI). The purpose of ADBI is to find a reliable correct integer ambiguities by making full use of the integer nature of ambiguities and integrating the ambiguities from the forward and backward filters. Once the integer ambiguities are determined correctly and reliably with ADBI, then the positions are updated with the fixing ambiguities constrained, in which more accurate positions with high confidence can be achieved. The effectiveness of the proposed approach is validated with airborne and car-borne dynamic experiments. The experimental results demonstrated that much better accuracy of position and higher ambiguity-fixed success rate can be achieved than the traditional post-processing method.

Uncombined precise orbit and clock determination of GPS and BDS-3

Abstract: There is increasing concern about the uncombined (UC) observation model in the field of global navigation satellite system (GNSS). Based on the global positioning system (GPS) and the third-generation BeiDou navigation satellite system (BDS-3), this study processed the UC precision orbit determination (POD) for single and dual systems. First, a UC observation model suitable for multi-GNSS POD was derived, and the ionospheric-free (IF) combination observation model was presented. Although the ambiguity parameters of UC and IF strategies were different after reparameterization, the difference could be removed when processing ambiguity resolution, and the equivalence was proved theoretically. To demonstrate the accuracy of BDS-3 orbits fully, the observation data of approximately 1 month were selected for determining the precise orbit for global positioning system (GPS) only, BDS-3 only, and GPS/BDS-3 systems based on the UC and IF models. The orbit precision of BDS-3 satellites was validated by using metrics, including comparison with precision products released by Wuhan University, orbit boundary discontinuity, and satellite laser ranging (SLR) residuals. The results show that the orbit accuracies of the IF and UC models are almost the same, the difference in orbits is approximately several millimeters, and the clock difference is within 0.01 ns. The GPS/BDS-3 combined solution shows better accuracy compared to other solutions. The average accuracies in the R and 3D directions are approximately 4 and 15 cm, and the clock standard deviation is approximately 0.2 ns compared to external orbit product. The root mean square of SLR residuals is approximately 4 cm.

BDS B1I multipath channel statistical model comparison between static and dynamic scenarios in dense urban canyon environment

Abstract: Global Navigation Satellite System (GNSS) multipath channel models are fundamental and critical for signal simulation and receiver performance evaluation. They also aid the designing of suitable multipath error mitigation algorithms when the properties of multipath channel are available. However, there is insufficient existing research on BeiDou Navigation Satellite System (BDS) signal multipath channel models. In this study, multipath channel statistical models are established on the basis of extensive datasets of the BDS B1I signal. A multipath parameter estimation algorithm is designed to extract information of multipath rays from the intermediate frequency data. The delay, power loss, Doppler fading frequency, and lifetime distribution models for static and dynamic vehicle platforms are established and compared, and the effects of the satellite orbit type and platform speed on the models are analyzed. The results reveal the detailed distribution and variation characteristics of the multipath parameters and are valuable for the development of accurate urban navigation systems.

Estimation method of SBAS dual-frequency range error integrity parameter

Abstract: The development of a dual-frequency multi-constellation satellite-based augmentation system (DFMC SBAS) is in progress worldwide. The broadcasted dual-frequency range error (DFRE) integrity parameter reflects the effects of satellite ephemeris and clock corrections. A user uses the DFRE to calculate the protection level and then determines whether the DFMC SBAS service satisfies the requirements of the current flight phase. However, the calculation of the DFRE has not been reported. Herein, a DFRE estimation method is proposed based on the projection method. Using the ephemeris–clock covariance matrix of each satellite, the maximal projection direction was solved, and the projection of the covariance matrix on this direction was used as the DFRE to form an envelope for the maximal corrected error. Results show that the DFRE can form an envelope of the maximal corrected error with a set probability, and the integrity performance in the user segment satisfies the Category I precision approach requirement.

Research on BeiDou anti-spoofing technology based on comprehensive radio determination satellite service

Abstract: The BeiDou system (BDS) plays a significant role in people’s lives, but its security is easily affected by spoofing attacks. The radio determination satellite service (RDSS) is a special service of BDS that provides two-way communication, positioning, and timing services independently of the traditional radio navigation satellite service (RNSS). It can additionally be combined with RNSS to provide a comprehensive RDSS (CRDSS) service. In RDSS, after receiving a signal from the master station, the user needs to send a response signal back to the master station through a satellite. Therefore, the RDSS signal is difficult to spoof. In this study, based on the security feature of RDSS signals, an anti-spoofing method based on CRDSS is proposed to detect and mitigate spoofing attacks, verifying the advantages of the BeiDou system over other satellite navigation systems.

Influence of the GEO satellite orbit error fluctuation correction on the BDS WADS zone correction

Abstract: Decimeter-level service is provided by the BeiDou satellite navigation system wide area differential service (BDS WADS) for users who collect carrier phase measurements. However, the fluctuations in Geostationary Earth Orbit (GEO) satellite orbit errors reduce the spatial correlation of orbit errors. These fluctuations not only decrease the accuracy and stability of zone correction service provided by BDS WADS, but also shorten its effective range. In this paper, we proposed an algorithm to weaken the influence of GEO satellite orbit error fluctuations and verified the method using data from eight sparsely distributed zones. The results show that orbit errors can be stabilized using orbit fluctuation corrections, and the positioning precision and stability of the BDS WADS can be improved simultaneously. Under normal circumstances, the horizontal and vertical positioning accuracy of users within 1000 km from the center of the zone can reach 0.19 m and 0.34 m. Furthermore, the effective range is increased. The positioning performance within 1800 km could reach 0.24 m and 0.38 m for the horizontal and vertical components, respectively.

A practical method utilizing multi-spectral LiDAR to aid points cloud matching in SLAM

Abstract: Light Detection and Ranging (LiDAR) sensors are popular in Simultaneous Localization and Mapping (SLAM) owing to their capability of obtaining ranging information actively. Researchers have attempted to use the intensity information that accompanies each range measurement to enhance LiDAR SLAM positioning accuracy. However, before employing LiDAR intensities in SLAM, a calibration operation is usually carried out so that the intensity is independent of the incident angle and range. The range is determined from the laser beam transmitting time. Therefore, the key to using LiDAR intensities in SLAM is to obtain the incident angle between the laser beam and target surface. In a complex environment, it is difficult to obtain the incident angle robustly. This procedure also complicates the data processing in SLAM and as a result, further application of the LiDAR intensity in SLAM is hampered. Motivated by this problem, in the present study, we propose a Hyperspectral LiDAR (HSL)-based-intensity calibration-free method to aid point cloud matching in SLAM. HSL employed in this study can obtain an eight-channel range accompanied by corresponding intensity measurements. Owing to the design of the laser, the eight-channel range and intensity were collected with the same incident angle and range. According to the laser beam radiation model, the ratio values between two randomly selected channels’ intensities at an identical target are independent of the range information and incident angle. To test the proposed method, the HSL was employed to scan a wall with different coloured papers pasted on it (white, red, yellow, pink, and green) at four distinct positions along a corridor (with an interval of 60 cm in between two consecutive positions). Then, a ratio value vector was constructed for each scan. The ratio value vectors between consecutive laser scans were employed to match the point cloud. A classic Iterative Closest Point (ICP) algorithm was employed to estimate the HSL motion using the range information from the matched point clouds. According to the test results, we found that pink and green papers were distinctive at 650, 690, and 720 nm. A ratio value vector was constructed using 650-nm spectral information against the reference channel. Furthermore, compared with the classic ICP using range information only, the proposed method that matched ratio value vectors presented an improved performance in heading angle estimation. For the best case in the field test, the proposed method enhanced the heading angle estimation by 72%, and showed an average 25.5% improvement in a featureless spatial testing environment. The results of the primary test indicated that the proposed method has the potential to aid point cloud matching in typical SLAM of real scenarios.

Real-time integrity monitoring for a wide area precise positioning system

Abstract: The wide area precise positioning system (WAPPS) is a high-precision positioning system based on a global navigation satellite system. Using a GEO satellite or a communication network, it provides users, in its service area, with real-time satellite orbit, clock, and other corrections. Users can achieve centimeter-level static positioning or decimeter-level kinematic positioning by precise point positioning. With the demands for applications of both high-precision and safety of life in real time, WAPPS is facing urgent needs to improve its service integrity. This study presents a real-time integrity monitoring approach for WAPPS. Using dual-frequency ionosphere-free corrections of GPS and BDS, along with monitor station data, related error models are established and the integrity monitoring is achieved, based on the analysis of satellite corrected residuals. In addition, satellite faults are simulated for performance verification. The results show that the algorithm can monitor both step and drift faults effectively and alert users in time.

An alternative integer recovery clock method for precise point positioning with ambiguity resolution

Abstract: When using Global Navigation Satellite System (GNSS) measurements, Precise Point Positioning with Ambiguity Resolution (PPP-AR) has been a popular substitute for relative positioning in geoscience applications. Compared with the Fractional Cycle Biases (FCB) method, the processing of Integer Recovery Clocks (IRC) products estimate, especially for ambiguity datum fixing, is so complex that its application has been greatly limited. Based on the concept of “carrier range”, we introduce an efficient way to implement the IRC method, termed as the alternative IRC method in this paper. In this method, the fixed ambiguities derived from PPP-AR using the FCB method, and not a fixed-ambiguity datum, are fixed in the IRC products estimate. This greatly reduces the complexity of implementing the IRC method and does not influence the accuracy of positioning. The alternative IRC method outperforms the FCB method by corroborating the consistency of daily positions in nature with international GNSS service weekly solution. To confirm this improvement, global positioning system measurements acquired over a year (2016) from approximately 500 globally distributed stations were processed. The accuracy of IRC products is approximately 20 ps and is highly stable for this year. Moreover, comparing the positioning accuracy of the FCB method to the alternative IRC method, we find that the mean root mean square over the year falls evidently from 2.03 to 1.65 mm at the east component. Therefore, we suggest that the alternative IRC method should be implemented when estimate IRC products.

Impact of China’s high speed train window glass on GNSS signals and positioning performance

Abstract: High speed train (HST) is an excellent platform to perform ultra-high spatial and temporal resolution observations of atmosphere using global navigation satellite systems (GNSS). However, we find that signal attenuation caused by HST window glass is a major barrier for HST-based GNSS applications inside HST chambers. A field experiment is conducted to analyze the effect of HST glass on GNSS signal propagation. In the experiment, GNSS observations are collected and analyzed from a receiver covered with an HST window glass and one with an open-sky view. The size of the HST window glass is 670 mm × 720 mm, with a thickness of 34 mm. The window glass is a double-glazing glass in which each layer has an actual thickness of 6 mm, and the two layers are separated by an air gap of 22 mm. The experiment results indicate that HST window glass can cause significant degradation to GNSS signals and even loss of tracking of the signal. Based on statistical results, HST window glass causes 39%, 56%, 49%, and 59% loss in GPS, GLONASS, Galileo, and BDS signals, respectively. Additionally, up to 20 dB-Hz of carrier-to-noise ratio (C/N0) degradation is also observed in the remaining observations. The significant signal attenuation and loss further lead to the decrease in the number of tracked satellites and occurrence of more cycle slips. The results of the study indicate that 44–230 cycle slips are detected for the HST glass-covered receiver whereas the receiver without glass does not exhibit more than 16 cycle slips. Additionally, the number of GNSS satellites tracked by the HST glass-covered receiver is reduced by 65% owing to the loss of signal. Furthermore, GNSS positioning performances from two receivers are also tested. With respect to GPS + GLONASS static precise point positioning (PPP), HST glass causes a degradation of 1.516 m and 1.159 m in the single-frequency and dual-frequency three-dimensional positioning accuracy, respectively. With respect to the GPS + GLONASS kinematic PPP, the accuracy degradations for single-frequency and dual-frequency kinematic PPP are 2.670 m and 4.821 m, respectively.

Efforts towards a low-temperature-sensitive physics package for vapor cell atomic clocks

Abstract: Strong environmental dependence is an intractable problem for vapor cell clocks, for which the high-temperature sensitivity of the physics package is considered one of the dominant reasons. In this paper, we report the design and realization of a low-temperature-sensitive physics package for vapor cell clocks. The physics package comprises three layers of magnetic shields, three layers of heating ovens, and the cavity-cell assembly. The cavity-cell assembly employs a compact magnetron-type cavity and a Rb vapor cell sealed with N2-Ar mixed buffer gas. The dependence of the clock frequency on temperature fluctuation is evaluated to be 2 × 10−11/°C. In pursuit of the stable temperature, a three-stage temperature regulator is implemented on the physics package. It adopts a combination of open and closed-loop control to address the problem of significant thermal coupling between the heating ovens. Under a laboratory environment, the measured Hadamard deviation of the temperature variation is 4 × 10−5 °C in 1 day of averaging.

Correction to: Advances in BeiDou Navigation Satellite System (BDS) and satellite navigation augmentation technologies

Abstract: First, in the first paragraph of this article, the sentence “In recent years, China has been actively promoting the construction and development of the BeiDou Navigation Satellite System (BDS), and by the end of the year 2000 the construction of BDS-1 was complete and BDS-1 began to provide GPS services for China.” should be changed into “In recent years, China has been actively promoting the construction and development of the BeiDou Navigation Satellite System (BDS), and by the end of the year 2000 the construction of BDS-1 was completed and BDS-1 began to provide services for China.” The word “GPS” should be removed.