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Journal of Global Positioning Systems Vol. 16, No. 1, 2018 |
Cover |
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Journal of Global Positioning Systems
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JGPS Team Structure, Copyright and Table of Contents |
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JGPS Team Structure, Copyright
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1. Review of triple-frequency GNSS: ambiguity resolution, benefits and challenges |
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Bofeng Li
Triple-frequency GNSS has been intensively studying in the past decades, especially with the open service of China’s BeiDou system. In this review, we will address the ambiguity resolution, benefits gained from additional frequency signals compared to the dual-frequency GNSS signals, as well as analyse the challenges of triple-frequency GNSS for future development. We first review and compare the three carrier ambiguity resolution models of geometry-based, geometry-free, geometry-ionosphere-free (GIF). The benefits gained from triple-frequency GNSS are then comprehensively examined with respect to dual-frequency case, including the improved ambiguity resolution, extra-widelane based RTK, the augmented RTK service, the shortened PPP convergence, the improved availability and reliability. In addition, some challenges are discussed from both theoretical and practical aspects to open eyes for future research.
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2. BDS code bias periodical mitigation by low-pass filtering and its applications in precise positioning |
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Xin Li, Keke Zhang, Yongqiang Yuan, Xiaohong Zhang and Xingxing Li
The code-phase divergences, which are minimal for GPS, GLONASS, and Galileo satellites, are commonly found in BeiDou Navigation Satellite System (BDS) Geostationary Orbit (GEO), Inclined GeoSynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites. Several precise positioning applications which use code observations are severely affected by these code biases. We present an analysis of code bias based on multipath (MP) combination observations. To mitigate the effect of BDS code bias on precise positioning, we proposed a periodical correction method using a low-pass filter for BDS GEO, IGSO and MEO satellites. The auto-correlation of MP series over long periods is analyzed to obtain the periods of the dominant repeating components for three types of BDS satellites. The periods of the dominant daily repeating components are close to 86,160 s for BDS GEO and IGSO satellites while 603,120 s for MEO satellites. The zero phase-shift low-pass filter was used to extract the low-frequency components of MP series and then low-frequency components are applied to mitigate the code bias periodically. The developed correction methods can make a more remarkable improvement for the accuracy of MP series, compared to the current elevation-dependent correction models. Data sets collected at 50 Global Navigation Satellite System (GNSS) ground stations including 15 of the International GNSS Monitoring and Assessment System (iGMAS) and 35 of the Multi-GNSS Experiment (MGEX) stations are employed for this study. To analyze the influence of code bias on precise positioning and validate the effectiveness of the correction methods, some applications such as single point positioning (SPP), wide-lane (WL) ambiguity analysis and Uncalibrated Phase Delays (UPDs) estimation are conducted. After applying the proposal correction method to the code observations, SPP solutions outperform the uncorrected ones in term of positioning accuracy. The positioning accuracy decreased by 0.28 and 0.1 m in the north and east components and the improvements are more significant for the U components decreased by 0.42 m. In addition, the systematic variations of Melbourne-WÜbbena (MW) combination are greatly removed and the convergence time of the MW series are decreased. Moreover, significant improvement is also achieved in terms of the usage rate and residuals of UPDs estimation.
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3. GPS + Galileo tightly combined RTK positioning for medium-to-long baselines based on partial ambiguity resolution |
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Guangcai Li, Jianghui Geng, Jiang Guo, Sen Zhou and Shuang Lin
With the modernization of the GNSS, the techniques of multi-GNSS navigation and positioning are becoming increasingly important. For multi-GNSS double-difference data processing, a tight combination (TC) strategy can provide more observations and higher reliability, which emploies a single reference satellite for all observations from different GNSS. However, multi-GNSS will bring some challenges to the high-dimension ambiguity resolution (AR). In this contribution, a GPS + Galileo tightly combined real-time kinematic (RTK) positioning strategy is proposed, which introduces the partial ambiguity resolution (PAR) method. A set of baselines ranging from about 22 to 110 km are used to test the positioning performance of this strategy. Experimental results demonstrate that the TC strategy can improve the success rate, but it can't increase the ambiguity ratio values. Using the PAR method can reduce convergence times and improve the ambiguity fixing rate. Combining the TC strategy with the PAR method can provide better positioning performance, especially for long baselines.
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4. Dynamic characteristic of the forth road bridge estimated with GeoSHM |
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Xiaolin Meng, Ruijie Xi and Yilin Xie
The importance of bridge health monitoring and management has been recognized by authorities of long-span bridges throughout the world in recent years. The GeoSHM consortium, led by the University of Nottingham, was awarded a Feasibility Study (FS) grant in 2013 by the European Space Agency (ESA) to investigate how to use integrated GNSS and Earth Observation technologies for the structural health monitoring of large bridges. During the GeoSHM FS period a small monitoring system was installed on the Forth Road Bridge in Scotland and the consortium have gathered huge data sets and rich experience regarding the design and implementation of GeoSHM according to essential user needs. This paper, based on the data from GNSS receivers installed on the two middle span sites and top of the southern tower of the Forth Road Bridge, intends to reveal the dynamic characteristics of the bridge. By using a moving average filter, Fast Fourier Transformation (FFT) and the peak-picking approach, the three-dimensional (3D) displacement time series under ambient excitation were decomposed into long-period movement and dynamic vibration response. The results demonstrate that the movement of the Forth Road Bridge in lateral direction is mainly caused by wind loading, and the correlation is about 0.7. In vertical direction, the displacements of middle span sites under the normal traffic loadings can reach 0.3 m and because of the main cable linking the middle span and top of the tower, the longitudinal movement of the southern tower top site has a high correlation with the vertical displacements of middle span sites. It has been found that due to the stiffness of the tower the trend terms inside lateral and vertical time series mainly consist of multipath effect and quasi-static displacement. The dynamic vibration frequencies and corresponding motion amplitudes were also extracted. It is found that the first natural frequencies of the middle span of the Forth Road Bridge are 0.065 Hz, 0.15 Hz and 0.104 Hz for lateral, longitudinal and vertical directions, respectively. For the south tower, vibration frequencies of 0.18 Hz can be seen in all three directions, but 0.104 Hz is only visible in longitudinal component because of the cables linking the tower and middle span. It demonstrates that with a proper data mining approach both the low frequency responses and dynamic vibration characteristics of a large bridge under ambient loadings can be extracted from GNSS data sets. Thus, GeoSHM can be used by bridge owners as an effective tool to assess the operational conditions of the bridge.
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5. A new approach for cycle slip detection and fix using single GPS receiver's single satellite dual frequency data containing arbitrarily large pseudorange errors |
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Zhizhao Liu
This paper presents a new cycle slip detection and repair method using Total Electron Content Rate (TECR) information derived from individual satellite dual-frequency data of a single Global Positioning System (GPS) receiver while pseudorange measurements are subject to arbitrarily large range errors. Sudden Increase of Pseudorange Error (SIPE), similar to cycle slips in nature, is quite common in various data acquisition scenarios. The basic principle of this method is to take advantage of the fact that the ionospheric TECR does not exceed certain threshold, which is set as 0.35 TECU/s in this study. Analytic expressions to evaluate the effect of SIPE on cycle slip detections have been developed. The search spaces for cycle slip candidate pairs are defined, given a predefined (sufficiently large) SIPE value. Two cycle slip validation rules are proposed to validate the cycle slip candidates. Over 99.9% of candidates can be rejected with the application of two validation rules. The theoretically maximal number of remaining cycle slip candidate pairs (NRCP) can be exactly calculated based on the magnitude of SIPE, TECR threshold, and the data sampling interval. After applying validation rules, the correct cycle slip pairs can be identified using a modified low-order polynomial fitting method. This method is tested on 13 high rate (1-Hz) dual-frequency datasets recorded by both ground-based static and satellite-borne high dynamic GPS receivers under various levels of ionospheric activities. Simulated cycle slips in 12 different possible cases and varying SIPE magnitudes are introduced into the data sets. In each test scheme, averagely 600-750 pairs of cycle slips are simulated. The SIPE magnitudes are set to vary from 50.0 m to 1000.0 m. Test results show that all the cycle slips in all the test schemes and all the datasets have been successfully detected and fixed even with a maximum SIPE of 1000.0 m in pseudoranges. A distinct advantage of this method is that it works in real-time with individual satellite's data from a single dual-frequency receiver, even if the carrier phases have virtually any size of cycle slips and the pseudoranges have virtually arbitrarily large errors.
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6. A hybrid error modeling for MEMS IMU in integrated GPS/INS navigation system |
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Maged Ismail and Ezzeldin Abdelkawy
Continuity of accurate navigational data for intelligent transportation applications has been widely provided by utilizing low-cost navigation systems through integrating GPS with micro-electro-mechanical-system (MEMS) inertial sensors. To achieve the required accuracy, augmentation of Kalman filter (KF) with nonlinear error modeling techniques such as fast orthogonal search (FOS) was introduced to enhance the navigational solution by estimating and eliminating a great part of both linear and nonlinear errors of azimuth angle sensed by MEMS gyro. Although this augmented approach enhanced the overall navigational accuracy to some extent, it still suffers from some drawbacks that diverge the system accuracy during GPS long outage periods. These drawbacks stem from the wide-variational behavior and high nonlinearities of the errors in MEMS gyros which make it difficult to depend on the non-adaptive linear error model provided by KF to model the two types of MEMS azimuth errors. In this paper we tried to minimize the effect of uncertainties associated with the KF azimuth prediction during the absence of GPS by introducing a hybrid error model which employs support vector machine (SVM) to model the KF output and FOS, based on autoregressive (AR) concept, to model the nonlinear azimuth errors. The performance of the proposed hybrid SVM-FOS approach is evaluated for GPS/ RISS (Reduced inertial sensor system integrated system) and the results were compared with the conventional KF and augmented KF-FOS approaches.
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7. Accuracy analysis of GPS/BDS relative positioning using zero-baseline measurements |
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Gethin Wyn Roberts, Xu Tang and Xiufeng He
This paper focuses on assessing the precision of carrier phase relative positioning using GPS-only, BDS-only and GPS/BDS measurements. A zero baseline is used in order to achieve this. Software for GPS and BDS processing has been developed, allowing static and kinematic data processing, as well as the combined GPS and BDS processing. Ionospheric and tropospheric delays are significantly reduced by double differencing between satellites and receivers, but the Multipath signals are still a major source of error for the various general GNSS baseline applications. In this paper, two Multi-GNSS receivers are connected to one antenna by an antenna splitter. This strategy results in all the delays or errors being mitigated, leaving only the random measurement noises resulted from the double difference processing. The time series of the final baseline error reveal that both GPS and BDS can achieve a precision of millimetres, but GPS performs better than BDS. Results from the combined processing of GPS and BDS demonstrate that the integration of GPS and BDS can significantly improve the precision, compared with the GPS-only and BDS-only results.
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8. Spatiotemporal variability of water vapor over Turkey from GNSS observations during 2009-2017 and predictability of ERA-Interim and ARMA model |
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Kutubuddin Ansari, Ozsen Corumluoglu, Sampad Kumar Panda and Payal Verma
The present paper investigates precipitable water vapor (PWV) variations over the lower middle latitude Turkish region from the International GNSS Services (IGS) and Turkish Permanent GNSS Network (TPGN) observations. The diurnal, seasonal, annual, and rainfall time behavior of PWV and their relative deviations are presented covering the period from 2009 to 2017. Additionally, the predictions from Auto Regressive Moving Average (ARMA) model and ERA-Interim reanalysis datasets are analyzed to understand their effectiveness over the region. In the first observation, diurnal profile indicates maximum value of PWV over the ocean climate regions whereas minimum value is noticed over the semi-arid continental climate areas of Turkey. The seasonal maximum value of PWV is observed in June solstice followed by September equinox and the lowest value is seen in December solstice followed March equinox. The studies also cover annual variation, grand-mean of tropospheric PWV and PWV intensity from the TPGN confirming the range of PWV between 0 to 45 mm. The PWV time series during 2009 to 2017 exhibit a strong annual variation at all sites, with distinctive peaks and dips occurring approximately in summer and winter, respectively. The precipitation plots displayed a clear increasing pattern in summer but the values are less in winter. However, the annual relative deviation of PWV lies in the range of - 0.5 to 1.5 units for all stations. The highest grand-mean of PWV is registered in 2010 (~ 22 mm) whereas the lowest value is seen in 2011 (~ 11 mm). The spatial variation of PWV shows that the northern boundary of the Turkey, western part of the country and Northern Cyprus have higher magnitude of PWV while the other part of the country like Central and Eastern Turkey have the lower magnitudes of PWV. PWV analysis during the precipitation period reconfirms that the rainfall pattern is not necessary to follow the PWV time series due to interlinked atmospheric processes. However, we found the PWV predictability of ARMA model is relatively better than the ERA-Interim model. Comprehensive analysis of TPGN data over the region may complement towards a better understanding of the tropospheric dynamics, their effects and the future refinements of atmospheric models over the lower middle latitude Turkish region.
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9. Development of a GPS spoofing apparatus to attack a DJI Matrice 100 Quadcopter |
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Eric Horton and Prakash Ranganathan
Global Positioning System (GPS) Spoofing attacks threaten technologies that our modern society depends on. To successfully develop defensive mechanisms against these attacks, methods to model the attacks and subsequently distinguish them from normal GPS operation must be developed. This paper primarily details the step-by-step implementation of a low-cost GPS spoofing and high-level spoofing data collection apparatus to model a simplistic spoofing attack that could be implemented with limited resources. The spoofing apparatus developed has been used to successfully attack a DJI Matrice 100 quadcopter and a portion of the collected spoofing data is presented.
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10. A MIMU/GPS/magnetometer integrated alignment method based on sequential robust estimation |
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Shida Wang, Jian Wang, Hang Yu and Haobo Li
For a micro-electro-mechanical system inertial measurement unit (MEMS-IMU), Global Positioning System (GPS) and magnetometer integrated navigation system, the procedure of initial alignment is a necessity. However, both magnetometer and GPS receiver are susceptible to external complex environment. Outliers in the observations of the two devices are inevitable. In order to reduce the influence of the outliers and improve the initial alignment accuracy, a sequential robust Kalman filter (SRKF) algorithm based on the square dimension of the innovation vector is proposed. The proposed SRKF algorithm can reasonably decrease the contribution of outlier affected elements of the observation vector in an element-wise way. Robust factor, used to suppress the influence of outliers, is constructed according to the Mahalanobis distance and the Chi-square distribution significance level. The results of different simulations and field tests all indicate the feasibility and the effectiveness of the proposed method.
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Back Cover |
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Journal of Global Positioning Systems
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