Journal of Global Positioning Systems

Vol. 1, No. 2, 2002

Journal of Global Positioning Systems
Vol. 1, No. 2, 2002
ISSN 1446-3156 (Print Version)
ISSN 1446-3164 (CD Version)

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JGPS Team Structure, Copyright
Table of Contents

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Guenter W. Hein and Thomas Pany

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This paper starts with a brief discussion of the Galileo project status and with a description of the present Galileo architecture (space segment, ground segment, user segment). It focuses on explaining special features compared to the American GPS system. The presentation of the user segment comprises a discussion of the actual Galileo signal structure. The Galileo carrier frequency, modulation scheme and data rate of all 10 navigation signals are described as well as parameters of the search and rescue service. The navigation signals are used to realize three types of open services, the safety of life service, two types of commercial services and the public regulated service. The signal performance in terms of the pseudorange code error due to thermal noise and multipath is discussed as well as interference to and from other radionavigation services broadcasting in the E5 and E6 frequency band. The interoperability and compatibility of Galileo and GPS is realized by a properly chosen signal structures in E5a/L5 and E2-L1-E1 and compatible geodetic and time reference frames. Some new results on reciprocal GPS/Galileo signal degradation due to signal overlay are presented as well as basic requirements on the Galileo code sequences.

Chaochao Wang and Gérard Lachapelle

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This paper describes different methods to improve reliability of attitude estimation using low cost GPS receivers. Previous work has shown that low cost receiver attitude determination systems are more susceptible to measurement errors, such as multipath, phase center offsets, and cycle slips. In some cases, these error sources lead to severely erroneous attitude estimates and/or to a lower availability. The reliability control in the attitude determination becomes imperative to users, as most attitude applications require a high level of reliability.

The three methods tested herein to improve reliability are the use of a high data rate, fixed angular constraints, and a quality control algorithm implemented with a Kalman filter. The use of high rate measurements improves error detection as well as ambiguity fixing time. Fixed angular constraints in a multi-antenna attitude system is effective to reject incorrect solutions during the ambiguity resolution phase of the process. Utilizing a Kalman filter with a high data rate, e.g. 10 Hz, not only increases reliability through an increase of information, but also can improve accuracy and availability. The simultaneous utilization of the above methods significantly improves reliability, as demonstrated through a series of hardware simulations and field tests. The low cost receiver type selected is the CMC Allstar receiver equipped with a commercially available low cost antenna.

Finally, the use of statistically reliability measures, namely internal and external reliability measures, shows the inherent limitations of a low cost system and the need to either use better antennas and/or external aiding in the form of low cost sensors.

Hansjörg Kutterer

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In the geodetic applications of the Global Positioning System (GPS) various types of data uncertainty are relevant. The most prominent ones are random variability (stochasticity) and imprecision. Stochasticity is caused by uncontrollable effects during the observation process. Imprecision is due to remaining systematic deviations between data and model due to imperfect knowledge or just for practical reasons. Depending on the particular application either stochasticity or imprecision may dominate the uncertainty budget. For the joint treatment of stochasticity and imprecision two main problems have to be solved. First, the imprecision of the original data has to be modelled in an adequate way. Then this imprecision has to be transferred to the quantities of interest. Fuzzy data analysis offers a proper mathematical theory to handle both problems. The main outcome is confidence regions for estimated parameters which are superposed by the effects of data imprecision. In the paper two applications are considered in a general way: the resolution of the phase ambiguity parameters and the estimation of point positions. The paper concludes with numerical examples for ambiguity resolution.

Ning Zhou and Yanming Feng

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In dynamic orbit determination, the problem is that a batch estimator assumes use of sophisticated models for both force and observation models, dealing with large amounts of observations. As a result, the computational workload may not be acceptable for onboard orbit determination. In this paper, the short-arc batch estimation is experimentally studied in order to address both estimation robustness and computational problems in GPS-based onboard orbit determination. The technical basis for the batch estimation will be outlined. The experimental results from three 96-hour data sets collected from Topex/Poseidon (T/P), SAC-C and CHAMP missions are presented. These results have demonstrated that use of shorter data arcs allow for simplifications of both orbit physical and observational models, while achieving a 3D RMS orbit accuracy of meter level consistently.

G.R. Hu, V. H.S. Khoo, P. C. Goh and C. L. Law

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Multiple reference station networks have been established for high precision applications in many countries worldwide. However, real-time application is still a difficult task in practice. Virtual reference station (VRS) concept is an efficient method of transmitting corrections to the network users for RTK positioning. Today's challenge for VRS RTK positioning lies in adapting advance wireless communication technologies for real time corrections. With the availability of GPRS technology, an Internet-based VRS RTK positioning infrastructure via GPRS has been developed and tested. This paper discusses the VRS data delivery mechanism, and gives an overview on VRS data generation for RTK positioning. Field test results are presented to evaluate the performance of the proposed system. The results demonstrate that Internet-based VRS RTK positioning can be achieved to better than 4 centimeters accuracy in horizontal position. Height accuracy is in the range of 1 to 6 centimeters.

Discussion Forum

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The Discussion Forum offers the opportunities for our readers and authors to express and exchange their views on technical issues which may be somewhat controversial in the field. Regular articles, short technical notes, comments on the papers published in this journal and the follow-up rejoinders from the authors are welcome.

Guochang Xu

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A general criterion for integer ambiguity searching is derived in this paper. The criterion takes into account not only the residuals caused by ambiguity parameter changing, but also the residuals caused by coordinates changing through ambiguity fixing. The search can be carried out in a coordinate domain, in an ambiguity domain or in both domains. The three searching scenarios are theoretically equivalent. The optimality and uniqueness properties of the proposed criterion are also discussed. A numerical explanation of the general criterion is outlined. The theoretical relationship between the general criterion and the commonly used least squares ambiguity search (LSAS) criterion is derived in an equivalent case in detail. It shows that the LSAS criterion is just one of the terms of the equivalent criterion. Numerical examples are given to illustrate the behaviour of the two components of the equivalent criterion.

Experts Forum

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"Experts Forum" is a regular column in this Journal featuring discussions on recent advances in global satellite positioning systems and their applications. Experts in various fields are welcome to contribute a short article to briefly describe their research directions and current activities, present recent results or identify remaining problems, freely expressing ideas and visions for future development. In this issue, Professor Gerard Lachapalle of Calgary University, Professor Chris Rizos of the University of New South Wales, Dr. Herbert Landau of Trimble Terrasat, Dr. Todd Walter of Stanford University, along with their associates, will review regional/local network based GPS technology for precise real time positioning services. The topics include Multiple Reference Station Approach, Network RTK research and implementation, Virtual Reference Station Systems and Wide Area Augmentation Systems.

The column is coordinated by Dr Yanming Feng of Queensland University of Technology, who appreciates your contribution to this column, along with your comments or ideas for topics for future issues (y.feng@qut.edu.au).

G. Lachapelle And P. Alves

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Dr. Gérard Lachapelle, holds a CRC/iCORE Chair in Wireless Location in the Department of Geomatics Engineering. He has been involved with GPS developments and applications since 1980 and has authored/co-authored numerous related publications and software. More information is available at www.geomatics. ucalgary.ca/faculty/lachap/lachap.html.

Paul Alves, is a Ph.D. candidate in the Department of Geomatics Engineering of the University of Calgary. He has been involved in ambiguity resolution and Network RTK research within the field of positioning and navigation for the past two years.

Herbert Landau, Ulrich Vollath and Xiaoming Chen

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Dr. Herbert Landau, is Managing Director of Trimble Terrasat and Director of GPS Algorithms and Infrastructure software in Trimble's G&E Division. He received his Ph.D. in Geodesy from the University FAF Munich, Germany in 1988. He has many years of experience in GPS and has been involved in a large variety of GPS and GLONASS developments for high precision positioning systems and applications.

Dr. Ulrich Vollath, received a Ph.D. in Computer Science from the Munich University of Technology (TUM) in 1993. He is Senior Manager of the department of kinematic positioning and real-time systems of Trimble Terrasat.

Dr. Xiaoming Chen, is a software engineer at Trimble Terrasat. He holds a PhD in Geodesy from Wuhan Technical University of Surveying and Mapping.

C. Rizos

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Prof. Chris Rizos, is a Professor at the School of Surveying & SIS, UNSW, Australia, and leader of the Satellite Navigation and Positioning (SNAP) Group (http://www.gmat.unsw.edu.au/snap/). He has been engaged in GPS research since the mid-1980s, which was at first focused on geodetic applications. More recently Chris has broadened the SNAP group's research across a wide range of positioning applications that can be addressed by GNSS and various ground-based wireless location technologies. He is currently secretary of Section 1 "Positioning" of the International Association of Geodesy (IAG).

Todd Walter

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Dr. Todd Walter received his B. S. in physics from Rensselaer Polytechnic Institute and his Ph.D. in 1993 from Stanford University. He is currently a Senior Research Engineer at Stanford University. He is a member of the WAAS Integrity Performance Panel (WIPP) focused on the implementation of WAAS and the development of its later stages. Key contributions include: early prototype development proving the feasibility of WAAS, significant contribution to MOPS design and validation, co-editing of the Institute of Navigation's book of papers about WAAS and its European and Japanese counterparts, and design of ionospheric algorithms for WAAS. He was the co-recipient of the 2001 ION early achievement award.

Instructions to Authors

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CPGPS Management Team (2002) Structure

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