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Deformation monitoring and dynamic characteristic analysis of bridge structures are the vital and basic requirements for the safe operation of bridges. In recent years, Global Navigation Satellite System (GNSS) has become increasingly widely used in bridge structural health monitoring with the development of the GNSS technology, especially the continuous improvement and development of China’s Beidou navigation satellite system (BDS). This article summarizes the application process of GNSS dynamic deformation monitoring and the development of GNSS deformation measurement technology of bridge structural health monitoring, the dynamic characteristic identification method and its application in bridge GNSS monitoring. The positioning solution methods for GNSS monitoring, the high sampling rate GNSS receiver for monitoring, multi-frequency and multi-system GNSS monitoring and the weakening of multipath effect of GNSS monitoring are summarized in detail. Then, the conclusions and prospects are posed for future research and related application.

The detection and repair of cycle slips are key steps in high-accuracy GNSS (Global Navigation Satellite System) data processing using carrier phase observations. BDS (BeiDou Navigation Satellite System) triple-frequency observations provide better combinations for cycle slip detections and repairs compared to dual-frequency observations. Although a number of algorithms have been developed and may correctly detect cycle slips most of the time, the reliability of empirical thresholds methods cannot be guaranteed. In this study, an adaptive threshold is proposed for three sets of triple-frequency Geometry-Free (GF) pseudorange minus phase combinations to improve the cycle slip detection performance and reduce the false alarm rate of the cycle slip detection by combining the predicted epoch-differenced ionospheric delays under active ionospheric conditions. Moreover, in the cycle slip repair, the integral combined cycle slips are determined by directly rounding the estimated float-combined cycle slips, which will lead to a repair error if the between-epoch ionospheric variation is large. In this study, a new rounding method considering the predicted epoch-differenced ionospheric delays is proposed, and it is proven that the new method has a higher success rate for estimating the integer value of a cycle slip than the traditional method. The performance of the newly proposed method is validated by using static BDS triple-frequency observations that contain simulated cycle slips. BDS triple-frequency observations were collected at 30-s sampling intervals under active ionospheric conditions. The results show that this method can successfully detect and repair all slips of more than one cycle. In addition, dynamic BDS data collected with a vehicle-based receiver at a 1-s sampling intervals are processed, and the results show that the proposed method is also effective in the detection and repair of cycle slips in dynamic data.

The integration of the BeiDou Navigation Satellite System(BDS) and the Inertial Navigation System(INS) can provide a more reliable and accurate navigation service than either system alone. The deeply coupled architecture for BDS and INS integration has more superior performance than the loosely coupled or the tightly coupled. Owing to the complicated dynamic scenario and the nonlinear system’s noise uncertainty, the adaptive Kalman filter(AKF) algorithm is often adopted in the deep integration(DI) system. The adaptive Sage window methods including innovation-based adaptive estimation(IAE) and residual-based adaptive estimation(RAE) are widely applied in AKF algorithms, but they have several limitations. We propose an improved adaptive unscented Kalman filter(AUKF) based on forgetting-factor-weight smoothing and multi-factor adaptation to overcome these limitations. Compared with the Sage window methods, the improved AUKF algorithm is immune to the quantity change of the satellites concerning integration and more sensitive to present dynamic. Furthermore, it can reduce the computation and storage burden in implementation. A simulation test based on a software platform and the deeply integrated BDS/INS navigation system is carried out to evaluate the performance of the improved AUKF. Simulation results show that the improved AUKF algorithm outperforms the extended Kalman filter(EKF) and has a similar performance with the RAE-AUKF.

For the purpose of further improving the solution accuracies of the orbital and geodetic parameters for BDS (BeiDou Navigation Satellite System) precise applications, this paper focuses on the enhancements of antenna phase center offsets (PCOs) for BDS-2 IGSOs (Inclined Geosynchronous Satellite Orbits) and MEOs (Medium Earth Orbits) through processing observations from a global tracking network. The daily estimated horizontal and vertical PCO time series of nearly three years from DOY (Day of Year) 001 in 2018 to DOY 180 in 2020 are obtained using the PANDA (Position And Navigation Data Analysis) software. The long-term PCO time series have seasonal variations and systematic effects along with the elevation angle of the Sun with respect to the orbital plane. Then, type-specific x-offsets and y-offsets of IGSOs and MEOs are comprehensively available considering the good consistency for the same satellite type. And a set of satellite-specific vertical offsets are recommended to BDS-2 IGSOs and MEOs since the low coherence of these satellites with the same type. Validation experiments are carried out for comparison between the original MGEX (Multi-GNSS Experiment) PCOs and the newly improved values (iMGEX PCOs for short), including the Precise Orbit Determination (POD) and Precise Point Positioning (PPP). Based on the orbital overlap analysis, the qualities of BDS-2 orbits show great enhancements in the along-track, cross-track and radial components, when the iMGEX PCOs are employed. Results of the independent assessment using SLR (Satellite Laser Ranging) also indicate the improvements on the radial component for C08, C10 and C11 satellites, and most of the orbit RMSs (Root Mean Squares) of iMGEX results decreased by 40.5% on average compared with the MGEX values. Additionally, the experimental station coordinates by static PPP achieve improvements at the rate of 27.1%, 32.6% and 28.4% in the east, north, and up component, respectively, in which more than a half stations realize sub-centimeter positioning accuracy in the north component using the iMGEX PCOs.

According to the problem that the positioning error increased with the increase of navigation time in the environment of complete lack of satellite signals such as viaduct and tunnel, the precise point positioning (PPP) / inertial navigation system (INS) tight coupled continuous positioning with non-holonomic constraint (NHC) and zero velocity update (ZUPT) in BeiDou navigation satellite system (BDS) was proposed. Firstly, the ionospheric composite model was analyzed from the aspects of observation equation, parameter estimation, fuzziness, degree of freedom and noise. The BDS PPP dynamic positioning model of ionospheric composite was established, and the errors that affect the positioning accuracy and convergence rate were analyzed. According to the motion characteristics of land vehicles, a scheme of velocity NHC was proposed to limit the accumulation of system positioning error with time, and ZUPT was used to increase the navigation accuracy of the integrated system when the vehicle was in the static state at the traffic light. The experimental results showed that the horizontal positioning accuracy of converged BDS PPP was better than 5 cm, the vertical positioning accuracy was better than 10 cm, and the dynamic positioning results are different from the high-precision real-time kinematic (RTK) / INS integrated navigation results by centimeters. The positioning accuracy of the system could be effectively improved by the BDS PPP/INS tight coupling method of NHC+ZUPT. Compared with the PPP/INS tight coupled positioning results without any constraints, the accuracy of the method in the three directions of East, North, and Up was increased by 0.08 m, 0.02 m and 0.01 m respectively.

As one of the commonly used solutions for vehicular dynamic positioning, the stability of Global Navigation Satellite Systems (GNSS)/ Inertial Navigation System (INS) integrated positioning when applied in an urban environment is still struggling. Generally, carrier-phase ambiguity fixing requires continuous several epochs, the stability and continuity of integrated positioning are significantly reduced if the signal is frequently blocked. To reduce the impact of frequent signal blockage, an improved tightly-coupled algorithm is proposed in this paper. Firstly, a step-wise ambiguity processing method is introduced to form instantaneous fixed Wide-Lane (WL) observations for calibrating INS measurement. Secondly, by estimating the differential inter-system bias (DISB) parameter, the pivot satellite can be shared between different constellations, to increase the number of usable satellites under the limited observation conditions and improve the positioning performance. The proposed method is verified with vehicular experiments on semi-simulated and actual urban canyon scenarios. In the artificial GNSS outage experiment, in the situation of 4 satellites that can be observed in BeiDou Navigation Satellite System (BDS) and Global Positioning System (GPS), the positioning accuracy of the proposed algorithm can achieve $4.1cm$ horizontally and $15.2cm$ vertically. An improvement of 26.9% horizontally and 20.4% vertically is gained accordingly compared to the conventional method. In the actual urban environment experiment, in case of insufficient satellite and low signal-to-noise ratio (SNR), when the conventional method can no longer maintain a fixed solution of GNSS/INS integrated positioning, the proposed algorithm can still achieve an accuracy of $49.7cm$ horizontally and $67.0cm$ vertically.

A new model is presented to resolve cycle slips detection for triple-frequency observations of BeiDou navigation satellite system (BDS) in this article when pseudorange observations are missing or insufficiently accurate under harsh or special situations. Based on the first-order time-difference geometry-free (GF) pseudorange-phase combination model, the new cycle slips detection and correction method based on the triple-frequency carrier phase and Doppler observations is proposed. With analyses on the two common sampling intervals (30 s and 1 s), it can be concluded that the optimal combination coefficients of the proposed model relate to sampling intervals. Combinations [4, -2, -3], [-1, -5,6], and [-3,6, -2] are selected to detect and correct cycle slips for 30 s sampling interval, while combinations [0, -1,1], [1,0, -1], and [-3,2,2] are selected for 1 s sampling interval. The validity of the phase-Doppler combination model under the static condition and the steady ionosphere with 30 s sampling interval and 1 s sampling interval is verified by two static experiments. Results show that the phase-Doppler combination model can achieve the same performance as the pseudorange-phase combination model. All the small, insensitive, and large cycle slips added to the three types of BDS satellites which separately belong to Geostationary Earth Orbit (GEO), Inclined Geosynchronous Orbit (IGSO), and Medium Earth Orbit (MEO) are detected and corrected successfully by the proposed model.

The integrity and accuracy of positioning are crucial factors in vehicle safety. This article proposes an enhanced real-time array calibration (ERAC) technique based on real-time array error calibration with the Beidou Navigation Satellite System (BDS). Through fixed-point positioning tests and road tests, it is found that the ERAC method is superior to the traditional methods in terms of accuracy, stability, confidence, and the time required to reconstruct reliable points. The performance of the tested receiver is better than that of a commercial receiver with standard single-point positioning (SSP) in the context of multipath interference. Furthermore, the ERAC method meets the requirements for autonomous vehicle (AV) monitoring and public safety, especially in terms of the lane-level accuracy indicator, and its integrity indicators are greater than 95% to 97%. Additionally, the ERAC method has the potential to become an alternative solution for automatic vehicle safety monitoring in certain scenarios. This technique also has the ability to meet the wide positioning requirements of various vehicles and scenarios by adjusting its technical details.

Induced polarization (IP) is the primary method used for the exploration of various metal minerals and for groundwater prospecting. The function and performance of IP instruments have significantly improved over the years; however, most instruments still have some shortcomings, such as limited data acquisition accuracy, large size, separate transmitter and receiver, complex human-computer interaction, difficult networking and remote monitoring. In this study, a novel multifunctional IP instrument system is developed based on remote wireless communication technology, which has high data acquisition accuracy, small size, lightweight, flexible and diverse functions (single instrument can realize transmission and reception), remote wireless real-time monitoring, and convenient networking of multiple instruments. We independently designed a high-precision data acquisition circuit with a noise floor of 0.29 μV and a 5-fold increase in acquisition accuracy. With an intelligent power module as its core, the designed transmitter has a maximum output power of 6 kW and can transmit stepped waves and rectangular waves of various frequencies and duty cycles. We also developed a host computer program on an Android mobile terminal based on Java, which allows users to wirelessly control the instrument through mobile phones over Wi-Fi networks. In addition, we introduced the Internet of Things technology in geophysical instruments and designed a real-time remote monitoring system for data quality and multiple instruments networking scheme based on the short message service of the BeiDou navigation satellite system and cloud service platform. Multiple tests revealed that the proposed instrument meets the requirements of field exploration.

From November 2017 to December 2018, Beidou navigation satellite system (BDS) completed the basic system construction of Beidou-3 system (BDS-3) and opened the initial service, BDS-3 satellites are equipped with Ka-band inter-satellite link (ISL) payloads, satellite autonomous orbit determination (OD) and time synchronization (TS) can be realized through ISL technology. At present, 18 BDS-3 medium earth orbit (MEO) satellites have been formally networked, and a series of ground-satellite link (GSL) test and connection test of Ka-band ISL signals are carried out, through the test, the working state and functional performance of the ISL payloads can be evaluated. This article focuses on the GSL measurements and performance evaluation of the ISL signals of BDS-3, the basic architecture of the GSL test platform is given, the principle of Ka-band GSL measurement and basic data processing methods and performance evaluation methods are introduced. Obtained the GSL observation data of 18 BDS-3 MEO satellites using the Ka-band ground station of the National Time Service Center (NTSC), and evaluate the performance of the GSL ranging and time synchronization. The result shows that GSL ranging accuracy of 18 MEO satellites is better than 0.02m and GSL time synchronization accuracy is better than 0.2ns. Thus, this meets the requirements of ISL system. The assessment methods and analysis results presented in this article can provide the reference of the function and performance verification for inter-satellite link system, and provide technical and data accumulation for the time synchronization extension application based on ISL signals.

Phasor measurement unit (PMU), which could provide high-frequency phasors including the amplitude and phase angle synchronized with Global Positioning System (GPS)/BeiDou Navigation Satellite System (BDS) signal, plays a vital role in dynamic monitoring of power systems. However, if the GPS/BDS signal could not be received, the time of PMU, which depends on the crystal oscillator (CO), would cause measurement deviation due to frequency shifts of CO, and then the PMU data would engage deviations and deteriorate the relevant applications. In recognizing the above problems, this article proposes a method to detect the frequency deviation of the crystal oscillator (FDCO) based on the correlation coefficient, according to its relation with the characteristic of the voltage phase angle difference (VPAD) between the two ends of the line. In detail, firstly, the quality of the VPAD of PMU between the two ends of the line is analyzed and the related problem is stated. Secondly, the characteristics of PMU data, when FDCO exists, are derived and analyzed, especially the VPAD between the line, and the suitability of the data for detecting the FDCO is discussed. Thirdly, the ideas and specific methods of detecting the FDCO in case of malfunction of GPS/BDS signal are proposed. Furthermore, the validity of the proposed method with simulation data and modified measured data is presented. Finally, the conjectures on the reasons for the approximately constant VPAD deviation in the previous section are discussed.

The latest generation of Global Navigation Satellite System (GNSS) satellites are transmitting signals on three or more frequencies, it brings new opportunities and challenges for data integration in Multi-GNSS Experiment (MGEX). To reduce the convergence time, less computationally intensive method is three-carrier ambiguity resolution (TCAR). But the three combinaitons can not be found in precise point positioning (PPP) yet, especially for the third generation BeiDou Navigation Satellite System (BDS-3). This contribution concentrates on the multi-frequency carrier-phase integer combinations for BDS-3 in PPP. More specifically, the triple-frequency plane is degraded to two-dimensional plane for BDS-3 under the assumption of a low observation noise. And then third frequency coefficient should be limited as a constraint, considering that inter frequency bias (IFB) is dynamic quantity. Thereafter, a new searching algorithm based on the Satisfiability Modulo Theories (SMT) is presented to search the optimal integer combinations fast. Meanwhile, ionosphere-free combinations are exhibited and are not suitable for TCAR. Futhermore, the availability of the SMT-based searching algorithm (SMTSA) is verified. Finally, the most interesting carrier-phase combinations that can be used for PPP ambiguity resolution are screened out for later research. Meanwhile, in order to give a referenced comprehensive assessment of the integer combination, some valuable combinations for BDS-3 are displayed. It provides scientific support for the triple-frequency AR which has less convergence time than dual-frequency AR. Moreover, it also gives the important and valuable reference for future research on multi-frequency ambiguity-enabled PPP.

Although precise point positioning (PPP) is a well-established and promising technique with the use of precise satellite orbit and clock products, it requires a long convergence time to reach a centimeter-level positioning accuracy. The availability of triple-frequency observations from a modernized global navigation satellite system (GNSS) constellations makes it possible to improve performance by formulating new observation models. The contribution of this paper is to propose two new observation models using triple-frequency data. The first model (UofC3) is the triple-frequency extension of dual-frequency University of Calgary model. The second model (UofB) is a combining observation model with five-dimensional observation equations. Thereafter, by theoretically analyzing the dimension of the triple-frequency observation models, all feasible triple-frequency observation models are systematically found out. Finally, the positioning experiments with 3-h observation period using real data at four BeiDou Navigation Satellite System (BDS) Asia-Pacific distributed reference stations on the day of the year (DOY) 42-48, 2018 are conducted to compare the performance of observation models. The results show that the triple-frequency models have less convergence time than the dual-frequency models. Meanwhile, most of the triple-frequency models have approximate convergence time in an experiment. Furthermore, UofB, which is one of the most stabilized observation models as well as uncombined observation model with the triple-frequency data (UC3), has less processing time than UC3. Compared with UofB, UofC3 has less processing time with sacrificing stability. These results show the significance of the UofB and UofC3 for future PPP applications in modernized GNSS.

Power inversion (PI) arrays have been employed to suppress interference in satellite navigation systems. When the incident direction of a satellite signal is near that of the interference, there will be a significant degradation in the signal to noise ratio (SNR). To reduce this degradation, a PI antenna array composed of a right-hand circular polarized (RHCP) antenna and a spatially-spread electromagnetic vector sensor (SS-EMVS) subarray is proposed, which can suppress interference and acquire satellite signals in both the polarized and spatial domains. Even when the satellite signal is incident from the same direction as the interference, the SNR degradation can be reduced by exploiting the difference in polarization states. It is shown that the filtering is improved in the polarization domain compared with an RHCP PI array, and the SNR loss is lower than with an SS-EMVS PI array while the computational complexity is not increased. Furthermore, the proposed PI array is effective for all Beidou satellite system public service signals.

General purpose graphics processing unit (GPGPU) has a great advantage in parallel computation, which is appropriate in the development of signal simulators for global navigation satellite system (GNSS) signals. Real-time software signal simulators for BeiDou navigation satellite system (BDS) signals, including navigation signals and radio determination satellite service (RDSS) signals, are developed in this paper. The characteristics of the continuous signal simulation and the burst signal simulation are considered in the development of GPU algorithms. The GPU algorithm optimization considers memory usage, signal combination method, and GPU block design under the goal of the least time consumption. To get the best GPU block design, a generalized block design model is built in this paper. The optimized GPU block parameters are got for the BDS B1 signal simulation and the RDSS signal simulation, separately. For the BDS B1 simulation, when the parameters are not optimized, the time consumption is more than three times of the optimized result. For the RDSS signal simulation, this value is 4.5 times. The correctness of the signals is verified in many aspects, including the power spectral density (PSD) and the pseudo range precision.

GNSS is now being speedily expanded to our daily life, but the positioning precision still can hardly meet the demands of many high-precision applications, such as approaching landing system on airports. Due to the development of GNSS, quadruple-frequency signals are now available in China's BeiDou Navigation Satellite System (BDS) and the European Galileo system, which can contribute to positioning precision. Positioning precision can not be improved obviously by quadruple-frequency carrier phases until cycle slips are detected and repaired. A method using four linear combinations to detect and repair quadruple-frequency cycle slips is proposed in the paper. The choices of the four linear combinations are conducted in cascaded steps in accordance to the cycle slip fixing probability. When the four detection combinations are determined, cycle slips on original carrier phase observations can be uniquely determined. The proposed algorithm has been tested on real 30-second quadruple-frequency static observations of BDS and Galileo and on real 0.05-second quadruple-frequency kinematic observations of BDS and Galileo. Simulated and real cycle slips are tested. The results show that the proposed algorithm can detect and repair cycle slips even for one cycle effectively.

The present research is carried out on the issue that the BDS (BeiDou Navigation Satellite System) receiver exerts some vulnerability to CW (continuous wave) interference. The analysis on the interference mechanism of navigation signal is implemented based on the model of direct sequence spread spectrum communication system while the method of parallel code phase search is employed to simulate the capture capability of the receiver under CW interference. In the CST-MWS, the simulation was made for the coupling path and electromagnetic energy into the navigation receiver, concluding that the antenna functions as the major obtained coupling path. On that basis, the RF radiation interference experiment for the antenna of the BDS receiver was conducted in the semi-anechoic chamber. In addition, the RF injection interference experiment was made on the basis of the power coupler. The sensitive frequency band of the receiver under the interference was obtained through the experiment. Furthermore, findings are made that the ratio of the output power of the RF signal generator under the RF irradiation method to the output power with the RF injection method takes no variation with change of the interference frequency. Therefore, The RF injection method can be used instead of RF irradiation method under CW interference in order to gain simplified experimental equipment, which provides a more professional theoretical basis for anti-interference research.

The acquisition performance of weak global navigation satellite system signals, especially those with secondary codes, is susceptible to sign transitions. To better understand and overcome the negative impact of sign transitions, this paper derived a new receiver auto-correlation function. The straightforward formula shows that sign transitions can reduce the accuracy of the Doppler estimate. To meet the initialization requirements of the tracking block, a two-stage high-sensitivity acquisition frame is proposed based on sign combinations search using matrix notation. The first stage adopts the conventional non-coherent combination (NC) algorithm to obtain the coarse code phase. And the second stage achieves high-resolution frequency estimation through fast Fourier transform (FFT). Besides, a novel grouping FFT algorithm is proposed to reduce the complexity of the second stage. The Beidou navigation satellite system (BDS) B1I signal is used as an example to verify the performances. Compared with conventional NC algorithm, the statistical performance of the new algorithm, in terms of the detection probabilities and frequency errors, can be greatly improved. To be specific, the proposed algorithm can actualize the 90% detection possibility of BDS B1I signal at a carrier-to-noise ratio (C/N0) of 23.9 dB-Hz within 200 ms with a Doppler accuracy of about 31 Hz. And the computing load of the second stage is reduced by about 11.6% compared with the conventional FFT approach. Actual data test further verifies the superiority of the new method. Inevitably, the total processing time increases by about 30%.