Journal of Global Positioning Systems

Vol. 7, No. 1, 2008

Journal of Global Positioning Systems
Vol. 7, No. 1, 2008
ISSN 1446-3156 (Print Version)
ISSN 1446-3164 (CD Version)

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JGPS Team Structure, Copyright

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Table of Contents

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Global Navigation Satellite Systems (GNSS) provide various types of positioning state solutions, such as single point positioning (SPP), precise point positioning (PPP), differential GPS (DGPS) and real time kinematic (RTK) solutions. These solutions are obtained involving different data types, receivers, samples, serving different classes of users. Previous studies on performance characteristics have mainly focused on SPP solutions for safety-of-life navigation applications. This paper defines various useful performance characteristics for carrier phase Ambiguity Resolution (AR) and Position Estimation (PE) solutions in the RTK context. These parameters, including base-rover distance, time-to-first fix (TTFF), AR reliability, RTK accuracy, availability and integrity, etc, effectively represent the performance of a commercial RTK system and can be used to evaluate RTK systems and algorithms, and processing strategies through extensive experimental results. Statistical results from extensive field experiments were obtained using a commercial RTK system, demonstrating convincing overall system performance in different perspectives. Experimental results from three baselines were also analysed using a version of research-oriented RTK software, showing that AR performance improvement of using Wide-lane (WL) and Narrow-lane (NL) signals with respect to the original L1 and L2 signals when the baselines exceed 20 kilometres.

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GPS alone will be unable to meet emerging performance requirements for maritime applications with respect to service robustness, accuracy, integrity and availability. Even when Galileo (or indeed other nascent GNSS) eventually become operational there will be performance gaps. In particular, identified applications in port areas (for example automated docking) and in inland waterways, have very stringent performance requirements.

EADS Astrium Ltd has developed a Test Environment for a Maritime Navigation System in order to perform demonstrations for the maritime community. This includes a number of Transmit Stations mounted on existing structures and terrain at accurately known locations overlooking a navigation zone (such as a maritime harbour). These continuously transmit replica GNSS signals. Another key component of the Navigation System is the single Monitoring and Control Station. This monitors each transmitter's signal, calculates clock corrections and provides each Transmit Station with the contents of its navigation messages, emulating the function of the Galileo central control facility. It also controls and monitors any demonstration signal scenarios.

This paper presents details of Astrium’s Pseudolite-based Navigation System that is the foundation of a Maritime Test Environment. As well as design details, the paper presents outcomes from a public demonstration undertaken in Oban, on the West Coast of Scotland in March 2008.

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Though GPS is the most popular positioning system at present it does not perform well in indoor environments and metropolitan city areas. Wi-Fi positioning has received much attention due to its advantages with respect to indoor positioning and the wide spread of the Wi-Fi access points (APs). Its performance in an outdoor environment is also of interest as a Wi-Fi based positioning system can overcome the shortcomings of GPS. In this paper, the Wi-Fi positioning technologies which can be used in outdoor environments - trilateration and fingerprinting, are discussed. An experiment based on fingerprinting has been carried out in the Sydney CBD area where Wi-Fi APs are densely deployed. The test results show that the Wi-Fi positioning system based on fingerprinting works well for outdoor localization, especially when directional information is utilized.

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Most people are not familiar with the indoor environment because most interior spaces are similar, and as such do not arouse the interest of most visitors. Although the GPS combined with the GIS has been broadly applied to many navigation applications, it might be still insufficient in an indoor environment where GPS signals are often severely obstructed. To meet the indoor requirements, the technique of RFID (Radio Frequency IDentification) was utilized to play an important locating role in the implementation of an indoor guidance system. The RFID tags, containing 1 KB capacity divided by 64 blocks, were adopted to accommodate the spatial-related information for working with the shortest routing for the system. By selecting the start and end points in the operation, the guidance system can suggest a direct, shortest distance, path. The locations of the passing points were identified and obtained by retrieving the spatial-related data from the tags next to the moving RFID reader. The guidance messages, including suggested path, modified path and moving directions, can be immediately presented to the users and arrive at their destination via the system's graphic and voice interfaces. This guidance system has been comprehensively tested for its operation functions and was evaluated by a group of users, indicating that the average time for way finding in an indoor guidance trial can be efficiently reduced by 50%. This prototype for an indoor guidance system is expected to be capable of working on a portable device, such as a PDA or mobile phone, thereby extending its practical application.

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The success of high-precision Global Navigation Satellite Systems (GNSS) kinematic positioning depends partly on the ability to resolve the integer phase ambiguities. In this paper, we propose a new algorithm for instantaneous kinematic ambiguity resolution, for present and modernised GPS and for Galileo. This approach - the GEneral Criterion Cascading Ambiguity Resolution (GECCAR) - selects the integer set of ambiguities using the General Ambiguity Search Criterion (GASC). Simulation runs have shown that single-epoch L1/E1 frequency ambiguity resolution was possible 99% of the considered epochs, when the three frequencies from both systems were used together. This new approach shows an improvement in the selection of the correct set of ambiguities when compared with the selection made by the Integer Least Squares Criterion (ILSC). We conclude that the GECCAR approach is a very promising algorithm for instantaneous ambiguity resolution.

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Global Positioning System (GPS) currently fulfills the positioning requirements of many applications under Line-Of-Sight (LOS) environments. However, many Location-Based Services (LBS) and navigation applications such as vehicular navigation and personal location require positioning capabilities in environments where LOS is not readily available, e.g., urban areas, indoors and dense forests. Such environments either block the signals completely or attenuate them to a power level that is 10-30 dB lower than the nominal signal power. This renders it impractical for a standard GPS receiver to acquire and maintain signal tracking, which causes discontinuous positioning in such environments.

In order to address the issue of GPS tracking and positioning in degraded signal environments, a novel architecture for ultra-tight integration of a High Sensitivity GPS (HSGPS) receiver with an inertial navigation system (INS) is proposed herein. By enhancing receiver signal tracking loops through the use of optimal estimators and with external aiding, the capabilities of the receiver can be substantially improved. The proposed approach is distinct from the commonly used ultra-tightly coupled GPS/INS approaches and makes use of different tracking enhancement technologies used in typical HSGPS receivers, multi-channel cooperated receivers and the current ultra-tightly coupled GPS/INS methods. Furthermore, the effects of inertial measurement unit (IMU) quality, receiver oscillator noise and coherent integration time on weak signal tracking are also analyzed.

Simulated test results in both static and dynamic testes show that, the designed INS-aided GPS receiver can track the incoming weak GPS signals down to 15 dB-Hz without carrier phase locked, or 25 dB-Hz with carrier phase locked. When there are multiple strong GPS signals in view, the other weak signals can be tracked down to 15 dB-Hz with carrier phase locked.

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Precise Point Positioning (PPP) using dual frequency GPS receivers is capable of providing centimetre level point positioning accuracy anywhere around the world, without the need for a base station. However, when using single frequency GPS receivers, the accuracy of the positioning decreases, particularly in the height component. One main factor for this degradation in accuracy is the unmodeled ionospheric error.

This paper investigates the performance of three different ionospheric error mitigation methods used in single frequency PPP in the Australian Region. They are the GRAPHIC (GRoup And PHase Ionospheric Correction) algorithm, the Global Ionospheric Maps (GIMs) and the Klobuchar model. Numerical results show that the GRAPHIC and GIMs methods are able to provide point positioning accuracy better than 1m for session duration less than an hour using geodetic quality single frequency receivers. For 12 to 24 hours data sets, the positioning accuracy can be as good as < 0.1m.

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Future Satellite Based Navigation Systems based on GPS, like US Wide Area Augmentation Sysytem (WAAS), are of global interest to the scientific community. Precise estimation of ionospheric delay is most crucial for successful implementation of the systems. Due to the complex ionospheric structure and large variation of Total Electron Content (TEC) in low latitudes, it is necessary to compare and validate the efficiency of the existing algorithms in this region. In this study, performances of the various interpolation algorithms for TEC calculation at ionospheric grid point (IGP) and user position for grid-based Single Shell Model have been tested for 72 test days of 2005. Based on the results obtained from this analysis, it has been found that, for the Indian region, it would be more suitable to use Ordinary Kriging in place of Planar Fit to estimate delay at an IGP (as used by US WAAS). It has also been found that Ordinary Kriging performs better than the Bilinear Interpolation technique at the user end.

Technical Note

Technical Notes is a new column in this Journal, featuring reviews of technical or theoretical tools for topics of positioning systems and their applications. Specialists in various fields are welcome to contribute a normal article to outline the issues of interest as systematic possible. In general, the manuscripts may aim to fill the gaps between the textbooks and scientific papers published for the specific topic. In this issue, Dr Jianguo Wang, York University, will review testing statistics in the context of Kalman filter, providing useful tools for design and use of a Kalman filter in navigation and positioning applications.

The column of this issue is coordinated by Dr Jianguo Wang, who appreciates your contribution to this column, along with your comments or ideas for topics for future issues (jgwang@yorku.ca).

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Many estimation problems can be modeled using a Kalman filter. One of the key requirements for Kalman filtering is to characterize various error sources, essentially for the quality assurance and quality control of a system. This characterization can be evaluated by applying the principle of multivariate statistics to the system innovations and the measurement residuals. This manuscript will systematically examine the test statistics in Kalman filter on the ground of the normal, -, t- and F- distributions, and the strategies for global, regional and local statistical tests as well. It is hoped that these test statistics can generally help better understand and perform the statistical analysis in specific applications using a Kalman filter.

Corporate Members of CPGPS

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Instructions to Authors

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CPGPS Management Team (2008) Structure

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