Multi-constellation Global Navigation Satellite Systems Research Articles

A Tightly Coupled Positioning Solution for Land Vehicles in Urban Canyons

The integration between Global Navigation Satellite System (GNSS) and on-board sensors is widely used for vehicle positioning. However, as the main information source in the integration, the positioning performance of single- or multiconstellation GNSSs is severely degraded in urban canyons due to the effects of Non-Line-Of-Sight (NLOS) and multipath propagations. How to mitigate such effects is vital to achieve accurate positioning performance in urban canyons. This paper proposes a tightly coupled positioning solution for land vehicles, fusing dual-constellation GNSSs with other low-cost complementary sensors. First, the nonlinear filter model is established based on a cost-effective reduced inertial sensor system with 3D navigation solution. Then, an adaptive fuzzy unscented Kalman filter (AF-UKF) algorithm is developed to achieve the global fusion. In the implementation of AF-UKF, the fuzzy calibration logic (FCL) is designed and introduced to adaptively adjust the dependence on each received satellite measurement to effectively mitigate the NLOS and multipath interferences in urban areas. Finally, the proposed solution is evaluated through experiments. The results validate the feasibility and effectiveness of the proposed solution.

Real-time Kinematic Positioning of INS Tightly Aided Multi-GNSS Ionospheric Constrained PPP.

Real-time Precise Point Positioning (PPP) technique is being widely applied for providing precise positioning services with the significant improvement on satellite precise products accuracy. With the rapid development of the multi-constellation Global Navigation Satellite Systems (multi-GNSS), currently, about 80 navigation satellites are operational in orbit. Obviously, PPP performance is dramatically improved with all satellites compared to that of GPS-only PPP. However, the performance of PPP could be evidently affected by unexpected and unavoidable severe observing environments, especially in the dynamic applications. Consequently, we apply Inertial Navigation System (INS) to the Ionospheric-Constrained (IC) PPP to overcome such drawbacks. The INS tightly aided multi-GNSS IC-PPP model can make full use of GNSS and INS observations to improve the PPP performance in terms of accuracy, availability, continuity, and convergence speed. Then, a set of airborne data is analyzed to evaluate and validate the improvement of multi-GNSS and INS on the performance of IC-PPP.

Application of Helmert Variance Component Based Adaptive Kalman Filter in Multi-GNSS PPP/INS Tightly Coupled Integration

The integration of the Global Positioning System (GPS) and the Inertial Navigation System (INS) based on Real-time Kinematic (RTK) and Single Point Positioning (SPP) technology have been applied as a powerful approach in kinematic positioning and attitude determination. However, the accuracy of RTK and SPP based GPS/INS integration mode will degrade visibly along with the increasing user-base distance and the quality of pseudo-range. In order to overcome such weaknesses, the tightly coupled integration between GPS Precise Point Positioning (PPP) and INS was proposed recently. Because of the rapid development of the multi-constellation Global Navigation Satellite System (multi-GNSS), we introduce the multi-GNSS into the tightly coupled integration of PPP and INS in this paper. Meanwhile, in order to weaken the impacts of the GNSS observations with low quality and the inaccurate state model on the performance of the multi-GNSS PPP/INS tightly coupled integration, the Helmert variance component estimation based adaptive Kalman filter is employed in the algorithm implementation. Finally, a set of vehicle-borne GPS + BeiDou + GLONASS and Micro-Electro-Mechanical-Systems (MEMS) INS data is analyzed to evaluate the performance of such algorithm. The statistics indicate that the performance of the multi-GNSS PPP/INS tightly coupled integration can be enhanced significantly in terms of both position accuracy and convergence time.

GPS AND GLONASS COMBINED STATIC PRECISE POINT POSITIONING (PPP)

Abstract. With the rapid development of multi-constellation Global Navigation Satellite Systems (GNSSs), satellite navigation is undergoing drastic changes. Presently, more than 70 satellites are already available and nearly 120 more satellites will be available in the coming years after the achievement of complete constellation for all four systems- GPS, GLONASS, Galileo and BeiDou. The significant improvement in terms of satellite visibility, spatial geometry, dilution of precision and accuracy demands the utilization of combining multi-GNSS for Precise Point Positioning (PPP), especially in constrained environments. Currently, PPP is performed based on the processing of only GPS observations. Static and kinematic PPP solutions based on the processing of only GPS observations is limited by the satellite visibility, which is often insufficient for the mountainous and open pit mines areas. One of the easiest options available to enhance the positioning reliability is to integrate GPS and GLONASS observations. This research investigates the efficacy of combining GPS and GLONASS observations for achieving static PPP solution and its sensitivity to different processing methodology. Two static PPP solutions, namely standalone GPS and combined GPS-GLONASS solutions are compared. The datasets are processed using the open source GNSS processing environment gLAB 2.2.7 as well as magicGNSS software package. The results reveal that the addition of GLONASS observations improves the static positioning accuracy in comparison with the standalone GPS point positioning. Further, results show that there is an improvement in the three dimensional positioning accuracy. It is also shown that the addition of GLONASS constellation improves the total number of visible satellites by more than 60% which leads to the improvement of satellite geometry represented by Position Dilution of Precision (PDOP) by more than 30%.

GPS AND GLONASS COMBINED STATIC PRECISE POINT POSITIONING (PPP)

With the rapid development of multi-constellation Global Navigation Satellite Systems (GNSSs), satellite navigation is undergoing drastic changes. Presently, more than 70 satellites are already available and nearly 120 more satellites will be available in the coming years after the achievement of complete constellation for all four systems- GPS, GLONASS, Galileo and BeiDou. The significant improvement in terms of satellite visibility, spatial geometry, dilution of precision and accuracy demands the utilization of combining multi-GNSS for Precise Point Positioning (PPP), especially in constrained environments. Currently, PPP is performed based on the processing of only GPS observations. Static and kinematic PPP solutions based on the processing of only GPS observations is limited by the satellite visibility, which is often insufficient for the mountainous and open pit mines areas. One of the easiest options available to enhance the positioning reliability is to integrate GPS and GLONASS observations. This research investigates the efficacy of combining GPS and GLONASS observations for achieving static PPP solution and its sensitivity to different processing methodology. Two static PPP solutions, namely standalone GPS and combined GPS-GLONASS solutions are compared. The datasets are processed using the open source GNSS processing environment <i>gLAB</i> 2.2.7 as well as <i>magicGNSS</i> software package. The results reveal that the addition of GLONASS observations improves the static positioning accuracy in comparison with the standalone GPS point positioning. Further, results show that there is an improvement in the three dimensional positioning accuracy. It is also shown that the addition of GLONASS constellation improves the total number of visible satellites by more than 60% which leads to the improvement of satellite geometry represented by Position Dilution of Precision (PDOP) by more than 30%.

A new method for satellite selection with controllable weighted PDOP threshold

With the development of multi-constellation Global Navigation Satellite Systems (GNSS), more and more navigation satellites are available for position determination. Although the increased number of visible satellites can improve the positioning performance, the computational load to track and process signals from all visible satellites will increase significantly. In practical applications particularly using mobile devices powered by batteries of limited capacity, it will be desirable to track and process only satellite signals that are necessary to meet given accuracy requirement. This requires the development of appropriate satellite selection algorithms. A new satellite selection algorithm with a controllable Weighted Positioning Dilution Of Precision (WPDOP) threshold is developed in this paper. The new approach can be used to select any number of visible satellites with respect to given positioning accuracy. Numerical computations with multi-constellation GNSS data were conducted to assess the performance of the proposed algorithm.

Fault detection and exclusion using solution separation and chi-squared ARAIM

This paper provides new integrity and continuity risk evaluation methods for fault detection and exclusion (FDE) using receiver autonomous integrity monitoring (RAIM). These methods are developed for both solution separation (SS) and Chi-squared RAIM: they capture the fact that exclusion enables continuity risk reduction in exchange for a higher integrity risk. The two approaches are implemented in an example advanced RAIM (ARAIM) application for worldwide vertical guidance of aircraft using multiconstellation Global Navigation Satellite Systems (GNSS).

On biases in precise point positioning with multi-constellation and multi-frequency GNSS data

Various types of biases in Global Navigation Satellite System (GNSS) data preclude integer ambiguity fixing and degrade solution accuracy when not being corrected during precise point positioning (PPP). In this contribution, these biases are first reviewed, including satellite and receiver hardware biases, differential code biases, differential phase biases, initial fractional phase biases, inter-system receiver time biases, and system time scale offset. PPP models that take account of these biases are presented for two cases using ionosphere-free observations. The first case is when using primary signals that are used to generate precise orbits and clock corrections. The second case applies when using additional signals to the primary ones. In both cases, measurements from single and multiple constellations are addressed. It is suggested that the satellite-related code biases be handled as calibrated quantities that are obtained from multi-GNSS experiment products and the fractional phase cycle biases obtained from a network to allow for integer ambiguity fixing. Some receiver-related biases are removed using between-satellite single differencing, whereas other receiver biases such as inter-system biases are lumped with differential code and phase biases and need to be estimated. The testing results show that the treatment of biases significantly improves solution convergence in the float ambiguity PPP mode, and leads to ambiguity-fixed PPP within a few minutes with a small improvement in solution precision.

MGR-DCB: A Precise Model for Multi-Constellation GNSS Receiver Differential Code Bias

In this study, we develop a Multi-constellation Global Navigation Satellite System (GNSS) Receiver Differential Code Bias (MGR-DCB) model. The model estimates the receiver DCBs for the Global Positioning System (GPS), BeiDou and Galileo signals from the ionosphere-corrected geometry-free linear combinations of the code observations. In order to account for the ionospheric delay, a Regional Ionospheric Model (RIM) over Europe is developed. GPS observations from 60 International GNSS Servoce (IGS) and EUREF reference stations are processed in the Bernese-5·2 Precise Point Positioning (PPP) module to estimate the Vertical Total Electron Content (VTEC). The RIM has spatial and temporal resolutions of 1° × 1° and 15 minutes, respectively. The receiver DCBs for three stations from the International GNSS Service Multi-GNSS Experiment (IGS-MGEX) are estimated for three different days. The estimated DCBs are compared with the MGEX published values. The results show agreement with the MGEX values with mean difference and Root Mean Square Error (RMSE) values less than 1 ns. In addition, the combined GPS, BeiDou and Galileo VTEC values are evaluated and compared with the IGS Global Ionospheric Maps (IGS-GIM) counterparts. The results show agreement with the GIM values with mean difference and RMSE values less than 1 Total Electron Content Unit (TECU).

First Polarimetric GNSS-R Measurements from a Stratospheric Flight over Boreal Forests

The first-ever dual-frequency multi-constellation Global Navigation Satellite Systems Reflectometry (GNSS-R) polarimetric measurements over boreal forests and lakes from the stratosphere are presented. Data were collected during the European Space Agency (ESA) sponsored Balloon Experiments for University Students (BEXUS) 19 stratospheric balloon experiment using the P(Y) and C/A Reflect Ometer (PYCARO) instrument operated in closed-loop mode. Maps of the polarimetric ratio for L1 and L2 Global Positioning System (GPS) and GLObal Navigation Satellite System (GLONASS), and for E1 Galileo signals are derived from the float phase at 27,000 m height, and the specular points are geolocalized on the Earth’s surface. Polarimetric ratio ( ) maps over boreal forests are shown to be in the range 2–16 dB for the different GNSS codes. This result suggests that the scattering is taking place not only over the soil, but over the different forests elements as well. Additionally to the interpretation of the experimental results a theoretical investigation of the different contributions to the total reflectivity over boreal forests is performed using a bistatic scattering model. The simulated cross- (reflected Left Hand Circular Polarization LHCP) and co-polar (reflected Right Hand Circular Polarization RHCP) reflectivities are evaluated for the soil, the canopy, and the canopy–soil interactions for three different biomass densities: 725 trees/ha, 150 trees/ha and 72 trees/ha. For elevation angles larger than the Brewster angle, it is found that the cross-polar signal is dominant when just single reflections over the forests are evaluated, while in the case of multiple reflections the co-polar signal becomes the largest one. The first-ever dual-frequency multi-constellation Global Navigation Satellite Systems Reflectometry (GNSS-R) polarimetric measurements over boreal forests and lakes from the stratosphere are presented. Data were collected during the European Space Agency (ESA) sponsored Balloon Experiments for University Students (BEXUS) 19 stratospheric balloon experiment using the P(Y) and C/A Reflect Ometer (PYCARO) instrument operated in closed-loop mode. Maps of the polarimetric ratio for L1 and L2 Global Positioning System (GPS) and GLObal Navigation Satellite System (GLONASS), and for E1 Galileo signals are derived from the float phase at 27,000 m height, and the specular points are geolocalized on the Earth’s surface. Polarimetric ratio ( ) maps over boreal forests are shown to be in the range 2–16 dB for the different GNSS codes. This result suggests that the scattering is taking place not only over the soil, but over the different forests elements as well. Additionally to the interpretation of the experimental results a theoretical investigation of the different contributions to the total reflectivity over boreal forests is performed using a bistatic scattering model. The simulated cross- (reflected Left Hand Circular Polarization LHCP) and co-polar (reflected Right Hand Circular Polarization RHCP) reflectivities are evaluated for the soil, the canopy, and the canopy–soil interactions for three different biomass densities: 725 trees/ha, 150 trees/ha and 72 trees/ha. For elevation angles larger than the Brewster angle, it is found that the cross-polar signal is dominant when just single reflections over the forests are evaluated, while in the case of multiple reflections the co-polar signal becomes the largest one.

GNSS reliability and positioning accuracy enhancement based on fast satellite selection algorithm and RAIM in multiconstellation

In order to improve the performance of positioning reliability, accuracy, and robustness in Global Navigation Satellite System (GNSS), several special actions, including receiver autonomous integrity monitoring (RAIM), weighted matrix estimation in weighted least squares (WLS), geometric dilution of precision (GDOP) in satellite geometry distribution, need to be taken into consideration in multiconstellation. With the extensive application and popularization of GPS, it makes remarkable contributions to scientific applications and engineering services. Currently, two emerging constellations (BDS, Galileo) as well as the recovery of GLONASS, the multiconstellation GNSS is undergoing dramatic development with better performance. Once the four systems are fully deployed, about 120 satellites will be available for GNSS users [1]. If all the visible satellites are used for position resolving, it will burden the receiver processor significantly. Meanwhile, there may be larger pseudorange measurement errors when the satellites have low elevation for the transmission of ionospheric and tropospheric delay. In addition, we cannot neglect simultaneous multiple outliers in multiconstellation GNSS compared with conventional RAIM. With respect to the weighted matrix factor in WLS, i.e., the uncertainty of the pseudorange measurements, estimation error is often used for weighted matrix estimation, without concerning real-time transmission error and signal quality, which somewhat enlarges the positioning accuracy. Thus, there is a need to deal with the above concerns.

Retrieving of atmospheric parameters from multi‐GNSS in real time: Validation with water vapor radiometer and numerical weather model

AbstractThe multiconstellation Global Navigation Satellite Systems (GNSS) (e.g., GPS, GLObal NAvigation Satellite System (GLONASS), Galileo, and BeiDou) offers great opportunities for real‐time retrieval of atmospheric parameters for supporting numerical weather prediction nowcasting or severe weather event monitoring. In this study, the observations from different GNSS are combined to retrieve atmospheric parameters based on the real‐time precise point positioning technique. The atmospheric parameters, retrieved from multi‐GNSS observations of a 180 day period from about 100 globally distributed stations, including zenith total delay, integrated water vapor, horizontal gradient, and slant total delay (STD), are analyzed and evaluated. The water vapor radiometer data and a numerical weather model, the operational analysis of the European Centre for Medium‐Range Weather Forecasts (ECMWF), are used to independently validate the performance of individual GNSS and also demonstrate the benefits of multiconstellation GNSS for real‐time atmospheric monitoring. Our results show that the GLONASS and BeiDou have the potential capability for real‐time atmospheric parameter retrieval for time‐critical meteorological applications as GPS does, and the combination of multi‐GNSS observations can improve the performance of a single‐system solution in meteorological applications with higher accuracy and robustness. The multi‐GNSS processing greatly increases the number of STDs. The mean and standard deviation of STDs between each GNSS and ECMWF exhibit a good stability as function of the elevation angle, the azimuth angle, and time, in general. An obvious latitude dependence is confirmed by a map of station specific mean and standard deviations. Such real‐time atmospheric products, provided by multi‐GNSS processing with higher accuracy, stronger reliability, and better distribution, might be highly valuable for atmospheric sounding systems, especially for nowcasting of extreme weather.

Precise positioning with current multi-constellation Global Navigation Satellite Systems: GPS, GLONASS, Galileo and BeiDou.

The world of satellite navigation is undergoing dramatic changes with the rapid development of multi-constellation Global Navigation Satellite Systems (GNSSs). At the moment more than 70 satellites are already in view, and about 120 satellites will be available once all four systems (BeiDou + Galileo + GLONASS + GPS) are fully deployed in the next few years. This will bring great opportunities and challenges for both scientific and engineering applications. In this paper we develop a four-system positioning model to make full use of all available observations from different GNSSs. The significant improvement of satellite visibility, spatial geometry, dilution of precision, convergence, accuracy, continuity and reliability that a combining utilization of multi-GNSS brings to precise positioning are carefully analyzed and evaluated, especially in constrained environments.

Fast satellite selection method for multi‐constellation Global Navigation Satellite System under obstacle environments

Satellite selection plays an important role in a multi-constellation Global Navigation Satellite System (GNSS) receiver. A well selected satellite combination can provide better positioning accuracy and lighter computational load. In this study, typical obstacle environments in urban areas and their effects on satellite selection are discussed. A distribution-free satellite selecting method is proposed. The main idea of this proposed method is to expand the selected satellite subset sequentially. The performance of the proposed methods is validated and compared with four other methods by simulation in typical scenarios. Simulation results show that the proposed method is suitable for urban GNSS applications.

Solution Separation Versus Residual-Based RAIM

Future multi-constellation Global Navigation Satellite Systems (GNSS) will offer the possibility to fulfill stringent navigation integrity requirements using Receiver Autonomous Integrity Monitoring (RAIM). This paper presents a detailed comparison of residual-based (RB) and solution separation (SS) RAIM. A complete step-by-step derivation of the two methods, which is currently missing in the literature, is given for the detection of single and multi-measurement faults, starting from common definitions of the integrity and continuity risks. In addition, a parity-space representation of SS RAIM is developed. It is shown that the integrity monitoring performance of SS RAIM can be superior to RB RAIM because SS test statistics are tailored to the fault hypotheses, and to the state of interest. However, for practical reasons of computational efficiency in conventional SS implementations, which are fully detailed in the paper, RB RAIM ultimately provides tighter bounds on integrity risk. Copyright © 2014 Institute of Navigation.

A Chip Scale Atomic Clock Driven Receiver for Multi-Constellation GNSS

This paper presents the design and implementation of a Chip Scale Atomic Clock (CSAC) driven dual-channel Digitally Configurable Receiver (DCR) for Global Navigation Satellite Systems (GNSS). The receiver is intended to be used for research applications such as; multipath mitigation, scintillation assessment, advanced satellite clock and spatial frame transformation modelling, Precise Point Positioning (PPP) as well as rapid development and assessment of novel circuits and systems for GNSS receivers. A novel sub-Nyquist sampling (subsampling) receiver architecture incorporating dual-band microstrip RF filters is employed in order to minimize the complexity of the multi-frequency Radio Frequency (RF) front-end. Moreover, the digital receiver incorporates a novel and complexity-reduced Fast Fourier Transform (FFT) core for signal acquisition as well as COordinate Rotation DIgital Computer (CORDIC) cores for the code/carrier discriminators in order to minimize the resource allocation on the FPGA. The receiver also provides easy access to enable adjustment of its internal parameters such as; RF gain, position update rate, tracking channel correlator spacing and code/carrier loop noise bandwidth. Correlator outputs, code/carrier error, Carrier-to-Noise Ratio (C/N0), navigation and RINEX data are provided to the end-user in real-time. This paper collectively highlights and reports on the implementation, test and validation of the novel techniques, elements and approaches in both the RF and digital part of the DCR that comprise the multi-constellation receiver.

Analysis of the PLL phase error in presence of simulated ionospheric scintillation events

The functioning of standard phase locked loops (PLL), including those used to track radio signals from Global Navigation Satellite Systems (GNSS), is based on a linear approximation which holds in presence of small phase errors. Such an approximation represents a reasonable assumption in most of the propagation channels. However, in presence of a fading channel the phase error may become large, making the linear approximation no longer valid. The PLL is then expected to operate in a non‐linear regime. As PLLs are generally designed and expected to operate in their linear regime, whenever the non‐linear regime comes into play, they will experience a serious limitation in their capability to track the corresponding signals. The phase error and the performance of a typical PLL embedded into a commercial multiconstellation GNSS receiver were analyzed in presence of simulated ionospheric scintillation. Large phase errors occurred during scintillation‐induced signal fluctuations although cycle slips only occurred during the signal re‐acquisition after a loss of lock. Losses of lock occurred whenever the signal faded below the minimumC/N0threshold allowed for tracking. The simulations were performed for different signals (GPS L1C/A, GPS L2C, GPS L5 and Galileo L1). L5 and L2C proved to be weaker than L1. It appeared evident that the conditions driving the PLL phase error in the specific case of GPS receivers in presence of scintillation‐induced signal perturbations need to be evaluated in terms of the combination of the minimumC/N0 tracking threshold, lock detector thresholds, possible cycle slips in the tracking PLL and accuracy of the observables (i.e. the error propagation onto the observables stage).

Tackling ionospheric scintillation threat to GNSS in Latin America

Scintillations are rapid fluctuations in the phase and amplitude of transionospheric radio signals which are caused by small-scale plasma density irregularities in the ionosphere. In the case of the Global Navigation Satellite System (GNSS) receivers, scintillation can cause cycle slips, degrade the positioning accuracy and, when severe enough, can even lead to a complete loss of signal lock. Thus, the required levels of availability, accuracy, integrity and reliability for the GNSS applications may not be met during scintillation occurrence; this poses a major threat to a large number of modern-day GNSS-based applications. The whole of Latin America, Brazil in particular, is located in one of the regions most affected by scintillations. These effects will be exacerbated during solar maxima, the next predicted for 2013. This paper presents initial results from a research work aimed to tackle ionospheric scintillation effects for GNSS users in Latin America. This research is a part of the CIGALA (Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America) project, co-funded by the EC Seventh Framework Program and supervised by the GNSS Supervisory Authority (GSA), which aims to develop and test ionospheric scintillation countermeasures to be implemented in multi-frequency, multi-constellation GNSS receivers.

On exact solutions of the multi-constellation GNSS navigation problem

In Global Navigation Satellite System (GNSS) positioning, the receiver measures the pseudorange with respect to each observable navigation satellite and determines the position and clock bias. In addition to the GPS, several other navigation satellite constellations including Glonass, Galileo and Compass can/will also be used to provide positioning, navigation, and timing information. The paper is concerned with the solvability of the navigation problem when the receiver attempts to process measurements from different constellations. As two different constellations may not be time-synchronized, the navigation problem involves the determination of position of the receiver and clock bias with respect to each constellation. The paper describes an analytic approach to account for the two-constellation navigation problem with three measurements from one constellation and two measurements from another constellation. It is shown that the two-constellation GNSS navigation problem becomes the solving of a set of two simultaneous quadratic equations or, equivalently, a quartic equation. Furthermore, the zero-crossover of the leading coefficient and the sign of the discriminant of the quartic equation are shown to play a significant role in governing the solvability, i.e., the existence and uniqueness of the navigation solutions.