GNSS Positioning Research Articles

Demonstration of a high sensitivity GNSS software receiver for indoor positioning

Advances in signal processing techniques contributed to the significant improvements of GNSS receiver performance in dense multipath environments and created the opportunities for a new category of high-sensitivity GNSS (HS-GNSS) receivers that can provide GNSS location services in indoor environments. The difficulties in improving the availability, reliability, and accuracy of these indoor capable GNSS receivers exceed those of the receivers designed for the most hostile urban canyon environments. The authors of this paper identified the vector tracking schemes, signal propagation statistics, and parallel processing techniques that are critical to a robust HS-GNSS receiver for indoor environments and successfully incorporated them into a fully functional high-sensitivity software receiver. A flexible vector-based receiver architecture is introduced to combine these key indoor signal processing technologies into GSNRx-hs™ – the high sensitivity software navigation receiver developed at the University of Calgary. The resulting receiver can perform multi-mode vector tracking in indoor environment at various levels of location and timing uncertainties. In addition to the obvious improvements in time-to-first-fix (TTFF) and signal sensitivity, the field test results in indoor environments surrounded by wood, glass, and concrete showed that the new techniques effectively improved the performance of indoor GNSS positioning. With fine GNSS timing, the proposed receiver can consistently deliver indoor navigation solution with the horizontal accuracy of 2–15m depending on the satellite geometry and the indoor environments. If only the coarse GNSS timing is available, the horizontal accuracy of the indoor navigation solution from the proposed receiver is around 30m depending on the coarse timing accuracy, the satellite geometry, and the indoor environments. From the preliminary field test results, it has been observed that the signal processing sensitivity is the dominant factor on the availability of the indoor navigation solution, while the GNSS timing accuracy is the dominant factor on the accuracy of the indoor navigation solution.

An Analysis for the Accuracy of Tropospheric Zenith Delay Calculated From ECMWF/NCEP Data Over Asia

AbstractThe tropospheric delay is an important error source for the satellite navigation and positioning, and the numerical forecast data derived from meteorological instruments can be used to calculate the tropospheric delay. In this study, the zenith tropospheric delay (ZTD) observed from 49 Global Positioning System (GPS) sites distributed in Asia was applied to assess the availability and accuracy for ZTD calculated from the European Center for Medium‐Range Weather Forecasts (ECMWF) and the United States National Centers for Environmental Prediction (NCEP). The practicability of ECMWF/NCEP data in Asia and the relation between the resolution and the accuracy for ECMWF were also studied. The results show that relative to GPS observed ZTD (GPS ZTD), the bias and rms for the ECMWF data are –1.0 cm and 2.7 cm, respectively, which are better than that for the NCEP data and can be used for ZTD study and application with high accuracy. The bias and rms for the NCEP forecast data are 2.4 cm and 6.8 cm, which are sufficed for the tropospheric delay correction in the real‐time GNSS navigation and positioning. The bias and rms exhibit a seasonal variation, which are generally of larger values in summer months and smaller values in winter months; however, the relation between the bias and rms and the latitude is not obvious, but the rms is decreasing while the altitude increasing. In addition, the daily bias and rms in eastern Asia in summer and in southern Asia in winter have a relatively larger variation. The resolutions of 2.5 degree and 0.5 degree for the ECMWF data are compared; the rms for the latter is decreased by 1~5 mm than the former. These results provide a reference for the availability and accuracy of the ECMWF/NCEP data used for calculating the tropospheric delay in the space geodesy, InSAR, navigation and positioning.

Estimation and exclusion of multipath range error for robust positioning

In integrated systems for accurate positioning, which consist of GNSS, INS, and other sensors, the GNSS positioning accuracy has a decisive influence on the performance of the entire system and thus is very important. However, GNSS usually exhibits poor positioning results in urban canyon environments due to pseudorange measurement errors caused by multipath creation, which leads to performance degradation of the entire positioning system. For this reason, in order to maintain the accuracy of an integrated positioning system, it is necessary to determine when the GNSS positioning is accurate and which satellites can have their pseudorange measured accurately without multipath errors. Thus, the objective of our work is to detect the multipath errors in the satellite signals and exclude these signals to improve the positioning accuracy of GNSS, especially in an urban canyon environment. One of the previous technologies for tackling this problem is RAIM, which checks the residual of the least square and identifies the suspicious satellites. However, it presumes a Gaussian measurement error that is more common in an open-sky environment than in the urban canyon environment. On the other hand, our proposed method can estimate the size of the pseudorange error directly from the information of altitude positioning error, which is available with an altitude map. This method can estimate even the size of non-Gaussian error due to multipath in the urban canyon environment. Then, the estimated pseudorange error is utilized to weight satellite signals and improve the positioning accuracy. The proposed method was tested with a low-cost GNSS receiver mounted on a test vehicle in a test drive in Nagoya, Japan, which is a typical urban canyon environment. The experimental result shows that the estimated pseudorange error is accurate enough to exclude erroneous satellites and improve the GNSS positioning accuracy.

Preface to the special issue on multi-sensor systems in engineering geodesy and navigation

Under the umbrella of the IAG (International Association of Geodesy) the symposium ‘Multi-sensor Systems for Engineering Geodesy’ was held at the XXV General Assembly of the International Union of Geodesy and Geophysics (IUGG) at the end of June, early July 2011 in Melbourne, Australia. The convenors of this symposium were Dorota Grejner-Brzezinska and Hans-Joerg Kutterer. The symposium included two oral sessions with in total ten presentations and one poster session with seven posters. In this issue of the journal four revised papers from the conference are presented, namely the first, fourth, fifth and sixth paper. In addition, two papers were selected which deal with the topic of the use, and their integration, of GNSS and multi-sensors in engineering geodesy and navigation applications. Many applications in engineering geodesy and navigation nowadays rely on GNSS positioning. Due to its limitations the use of multi-sensors as an alternative and/or backup to GNSS positioning has become an increasingly important field of research. In the coming years the IAG Sub-Commission 4.1 ‘Alternatives and Backups to GNSS’ under Commission 4 ‘Positioning and Applications’ will place special emphasis on research and other activities that address multi-sensor system theory and applications. Important topics include integrated guidance, navigation, positioning and orientation of airborne and land-based platforms. The primary sensors of interest will be inertial navigation systems (INS); however, the important role of other emerging techniques, such as pseudolites, vision-based sensors, ultrasonic and magnetic sensors etc., used in navigation, engineering geodesy and environmental monitoring is also recognised. New concepts and innovations in navigation algorithms, sensor calibration, synchronization and inter-calibration, real-time sensor information processing, sensor and data fusion, and automation techniques for information extraction from multi-sensor systems using expert systems continue to be investigated. Furthermore the progress in new applications (not limited to conventional engineering geodesy and navigation) of multi-sensor systems (e.g., in transportation, engineering, car navigation, environmental monitoring personal navigation, indoor navigation) is an important research focus in the years to come.

A smartphone application to extract safety and environmental related information from the OBD-II interface of a car

This paper introduces a methodology for the identification of events or potentially dangerous situations in road traffic by extracting information from the OBD-II interface. The proposed system for identifying hazardous locations includes a GNSS positioning system. As such, with the use of existing navigation systems or with the use of newly installed equipment, the exact geographic location where a given risk occurs will be identified.

Global modeling 2nd-order ionospheric delay and its effects on GNSS precise positioning

Ionospheric delay is one of the major error sources in GNSS navigation and positioning. Nowadays, the dual-frequency technique is the most widely used in ionospheric refraction correction. However, dual-frequency measurements can only eliminate the first-order term of ionospheric delay, while the effect of the second-order term on GNSS observations may be several centimeters. In this paper, two models, the International Reference Ionosphere (IRI) 2007 and International Geomagnetic Reference Field (IGRF) 11 are used to estimate the second-order term through the integral calculation method. Besides, the simplified single layer ionosphere model in a dipole moment approximation for the earth magnetic field is used. Since the traditional integral calculation method requires large calculation load and takes much time, it is not convenient for practical use. Additionally, although the simplified single layer ionosphere model is simple to implement, it results in larger errors. In this study, second-order term ionospheric correction formula proposed by Hoque (2007) is improved for estimating the second-order term at a global scale. Thus, it is more practicable to estimate the second-order term. More importantly, its results have a higher precision of the sub-millimeter level for a global scale in normal conditions. Compared with Hoque’s original regional correction model, which calculates coefficients through polynomial fitting of elevation and latitudes, this study proposes a piece-wise look-up table and interpolation technique to modify Hoque model. Through utilizing a table file, the modified Hoque model can be conveniently implemented in an engineering software package, like as PANDA in this study. Through applying the proposed scheme for the second-order ionospheric correction into GNSS precise positioning in both PPP daily and epoch solutions, the results have shown south-shift characteristics in daily solution at a global scale and periodic change with VTEC daily variation in epoch positioning solution.

Evaluation of the EGNOS troposphere model in precise point positioning applications

One of the systematic error sources in GNSS positioning is the tropospheric delay. The EGNOS augmentation system uses a specific model for taking this effect into account. This model is evaluated in the GNSS precise point positioning technique. Moreover it is compared to other tropospheric models, including the well-known Hopfield and Niell models, as well as a regression model based on Hungarian radiosonde observations. A part of the Hungarian active GNSS network is used for the test, and two study periods have been chosen for the investigations. One of them was a stormy summer period, when the status of the troposphere changes rapidly, while the other one was a calm winter period, when the troposphere contains a low amount of water vapor.

OCEAN TIDE LOADING AND RELATIVE GNSS IN THE BRITISH ISLES

Ocean tide loading (OTL) affects all parts of the British Isles to varying degree, causing peak-to-peak vertical displacements of up to 13 cm in South-West England over semi-diurnal and diurnal timescales. Lateral displacements are typically around one-third of the magnitude of vertical displacements at a point, so are also considerable for carrier phase GNSS surveying. Using a recent numerical ocean tide model, we predict that widespread residual displacements up to ∼1 cm remain in kinematic or short-occupation static relative GNSS positions computed with respect to the nearest continuously operating reference station. Even if OTL is not modelled explicitly, these errors will be mitigated by network GNSS to an extent dependent on the number and location of reference stations used, and the adjustment or error interpolation scheme adopted in the processing. For a selection of error interpolation algorithms, we predict that throughout most inland regions of Great Britain and Ireland, network processing reduces the residual OTL error to within the expected kinematic GNSS system noise. However, pockets of OTL error exceeding 1 cm may remain, especially in coastal locations and in South-West Ireland, South Wales, South-West England, and the islands off the west coast of Scotland. Residual OTL error at semi-diurnal periods dominates that at diurnal periods. We derive a simple heuristic for estimating the magnitude of this error from a sample of observations at a locality, valid within a short window of the spring/neap tidal cycle, and show how the residual error may, if necessary, be mitigated by a suitable averaging scheme.

Satellite Orbit and Clock Error Estimation and Its Applications to GNSS Positioning

Satellite Orbit and Clock Error Estimation and Its Applications to GNSS Positioning

Use of Terrestrial Laser Scanning Technology for Long Term High Precision Deformation Monitoring

The paper presents a new methodology for high precision monitoring of deformations with a long term perspective using terrestrial laser scanning technology. In order to solve the problem of a stable reference system and to assure the high quality of possible position changes of point clouds, scanning is integrated with two complementary surveying techniques, i.e., high quality static GNSS positioning and precise tacheometry. The case study object where the proposed methodology was tested is a high pressure underground pipeline situated in an area which is geologically unstable.

Active RFID Trilateration and Location Fingerprinting Based on RSSI for Pedestrian Navigation

In the work package ‘Integrated Positioning’ of the Ubiquitous Cartography for Pedestrian Navigation project (UCPNAVI) alternative location methods using active Radio Frequency Identification (RFID) are investigated for positioning of pedestrians in areas where no GNSS position determination is possible due to obstruction of the satellite signals. In most common RFID applications, positioning is performed using cell-based positioning. RFID tags can be installed at active landmarks (i.e., known locations) in the surroundings and a user equipped with an RFID reader can be positioned using Cell of Origin (CoO). The positioning accuracy, however, depends on the size of the cell defined by the maximum range of the signal. Using long range RFID for positioning the cell size can be quite large, i.e., around 20 m. Therefore, the paper proposes two new methods for positioning, i.e., trilateration and location fingerprinting based on received signal strength indication (RSSI) if more than one RFID tag is visible. The trilateration approach is based on the deduction of ranges to the RFID tags from RSSI. An iterative approach to model the signal propagation will be introduced, i.e., the International Telecommunication Union (ITU) indoor location model that can be simplified to a logarithmic model, and a simple polynomial model is employed for the signal strength to range conversion. In a second attempt, location fingerprinting based on RSSI is investigated. In this case, RSSI is measured in a training phase at known locations inside the building and stored in a database. In the positioning phase these measurements are used together with the current measurements to obtain the current location of the user. For the estimation of the current location different approaches are employed and tested, i.e., a direction-based approach, a tag-based approach, a direction-tag-based approach and a heading-based approach. Using trilateration or fingerprinting positioning accuracies on the one to a few metres level can usually be achieved. The concept and the iterative approach of the different methods and test results are discussed in this paper.

A behavioral approach to GNSS positioning and DOP determination

In this paper, a behavioral framework is proposed to solve dynamic GNSS positioning problems, which in the mean time may also provide a way to determine the DOP (dilution of precision) of a moving ...

Estimation of Ionospheric Delays in Dual Frequency GNSS Positioning

The correct estimation of ionospheric delays is very important in precise satellite-based kinematic positioning, especially over long baselines. In the case of triple frequency system, the ionospheric delays can be estimated from the measurements. However, in the case of dual frequency system, the situation becomes more complicated. The precision of those ionosphere models supplied by the external information source such as JPL IONEX Data is not sufficient, as the high frequency component is neglected. The precision of the low frequency component is not sufficient for the use in long baseline kinematic positioning. On the other hand, the high frequency component can be estimated from the phase range measurements. If the low frequency components are estimated by using an external information source or pseudo-range measurements, more reasonable estimates of ionospheric delays may be possible. In this paper, the estimation using the pseudo-range measurements is discussed. The accuracy of the estimates for ionospheric delays is not sufficient at present because of the bias errors in the pseudo-range measurements. However, if the accuracy of the pseudo-range measurements is improved in future by GPS Modernization and GALILEO, the method would become very promising and convenient, since any external informations are needed and realtime estimation of ionospheric delays becomes possible.

A KNOWLEDGE-BASED KALMAN FILTER FOR AN INTELLIGENT PEDESTRIAN NAVIGATION SYSTEM

Continuous and reliable position determination is very important in any navigation application. Therefore a combination and integration of different location techniques and positioning sensors is required. In most navigation applications this integration is performed using a Kalman filter approach. In this paper a new approach which makes use of knowledge-based systems for preprocessing the sensor observations is presented. In the preprocessing step the quality and reliability of the sensor observations is tested and gross errors and outliers are detected and eliminated. Furthermore the preprocessing step is used to determine the weightings of the sensor observations in the stochastic model of the following central Kalman filter. The weightings of the sensor observations can then be adjusted in the filter depending on their availability and quality. This approach is developed in a research project at our University for a pedestrian navigation and guidance service. In this project different location techniques such as GNSS and indoor positioning are combined with dead reckoning sensors (e.g. digital compass for heading determination, accelerometers for measurement of travelled distance, barometric pressure sensor for altitude determination) for continuous position determination of a pedestrian user. The project takes a user case into account, i.e., the navigation and guidance of visitors of our university to certain offices and persons. Selected results of field tests using different sensors are also presented in the paper. From the tests it could be seen that such a service can achieve a high accuracy and reliability for continuous position determination of a pedestrian user. It can also be expected that the performance of the system can be increased using the new intelligent knowledge-based Kalman filter approach for the integration of all available sensor observations.

A Difference Test Method for Early Detection of Slowly Growing Errors in GNSS Positioning

Satellite based navigation system integrity monitoring is essential both for liability and safety critical services. The threats to system integrity are the potential failure modes that could occur at system, operational and user sensor levels. Research on identification, characterisation and modelling of such failure modes, suggests that failures that grow slowly over time (e.g. ramp error less than 2 m/s), also referred to as slowly growing errors (SGE) are the most difficult to detect early. Conventional snapshot algorithms detect SGEs when the test statistic crosses the threshold usually after relatively long periods of time. A recent concept based on the average of residuals over time has been found to have a significant weakness. This paper proposes a “difference test” algorithm capable of detecting SGEs early. In this method, the test statistic is the difference between the norm of current residuals and the norm of the residuals at a previous epoch. The distribution of the test statistic is over-bounded by a normal distribution whose parameters are derived from two Chi-distributions. Results show that the new algorithm results in a significant reduction in detection time compared to conventional methods.

GNSS Receiver Autonomous Integrity Monitoring with a Dynamic Model

Traditionally, GNSS receiver autonomous integrity monitoring (RAIM) has been based upon single epoch solutions. RAIM can be improved considerably when available dynamic information is fused together with the GNSS range measurements in a Kalman filter. However, while the Kalman filtering technique is widely accepted to provide optimal estimates for the navigation parameters of a dynamic platform, assuming the state and observation models are correct, it is still susceptible to unmodelled errors. Furthermore, significant deviations from the assumed models for dynamic systems may also occur. It is therefore necessary that the state estimation procedure is complemented with effective and reliable integrity measures capable of identifying both measurement and modelling errors. Within this paper, fundamental equations required for the effective detection and identification of outliers in a kinematic GNSS positioning and navigation system are described together with the reliability and separability measures. These quality measures are implemented using a Kalman filtering procedure formulated with Gauss-Markov models where the state estimates are derived from least squares principles. Detailed simulations and analyses have been performed to assess the impact of the dynamic information on GNSS RAIM with respect to outlier detection and identification, reliability and separability. The ability of the RAIM algorithms to detect and identify dynamic modelling errors is also investigated.

Sensitivity of Adop to Changes in the Single-Baseline GNSS Model

Sensitivity of Adop to Changes in the Single-Baseline GNSS Model The ADOP (Ambiguity Dilution Of Precision) is a measure for the precision of the carrier phase ambiguities involved in precise relative GNSS positioning. By computing the ADOP one may get knowledge in whether ambiguity resolution can be expected successful or not, already in a stage before the GNSS data are collected. In Odijk and Teunissen (2008) compact closed-form expressions have been derived for the ADOP of single-baseline GNSS models. In this paper these expressions are used to study the impact of certain changes in these models, as there are the observation time span, the weighting of the ionospheric delays, the number of frequencies, the weights of the phase and code data, the number of satellites, elevation-dependent observation weights and taking linear combinations of data.

차분위성항법 위상평활화코드 필터의 성능 해석

This paper proposes a theoretically rigorous analysis procedure that compares the position domain and range domain carrier-smoothed-code filters for differential GNSS positioning. Utilizing consistent error covariance formulation, it is shown that filtering in the position domain is, in theory, more advantageous than range domain carrier-smoothed-code filtering. It is also shown that if the visible satellite set does not change during a sufficiently long time interval the performances of position and range domain filters are similar.

GNSS CODE AND CARRIER TRACKING IN THE PRESENCE OF MULTIPATH

It is well known that multipath represents a major error source in differential GNSS positioning. Errors produced are unique to each antenna position and so are uncorrelated between the reference station and the mobile receiver. Reduction techniques, such as narrow correlator spacing1 and the multipath estimating delay lock loop (MEDLL)2 have improved the situation. However, reflected signals emanating close to the antenna are not reduced by either technique. Carrier multipath is a particular problem because short delay multipath signals cause maximum carrier errors. This study describes the analysis that has been carried out into code and carrier tracking in the presence of multipath. Code tracking in this situation produces a non-zero mean error,3 and the source of this bias has been investigated. Code and carrier multipath errors of the global orbiting navigation satellite system (GLONASS) have been compared with those of the global positioning system (GPS). The effects of pre-correlation filtering on code and carrier multipath errors have been determined using data from a measured correlation function. The multipath fading bandwidth is also discussed, and fast and slow fading effects are presented. A comparison has been made of the performance of typical coherent and non-coherent code discriminators in the presence of multipath. The performance of the GPS and GLONASS P code signals have been determined with respect to code and carrier multipath errors. © 1997 by John Wiley & Sons, Ltd.