Journal of Global Positioning Systems

Vol. 2, No. 2, 2003

Journal of Global Positioning Systems
Vol. 2, No. 2, 2003
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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Joel Barnes, Chris Rizos, Jinling Wang, David Small, Gavin Voigt and Nunzio Gambale

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Today, GPS is the most popular and widely used three-dimensional positioning technology in the world. However, in many everyday environments such as indoors or in urban areas, GPS signals are not available for positioning (due to the very weak signals). Even with high sensitivity GPS receivers, positioning for urban and indoor environments cannot be guaranteed in all situations, and accuracies are typically of the order of tens to hundreds of meters at best. Other emerging technologies obtain positions from systems that are not designed for positioning, such as mobile phones or television. As a result, the accuracy, reliability and simplicity of the position solution is typically very poor in comparison to GPS with a clear view of the sky.

Locata is a new positioning technology, developed to address the failure of current technologies for reliable ubiquitous (outdoor and indoor) positioning. In this paper key aspects of the new technology are discussed, with particular emphasis on the positioning network (LocataNet). An innovative characteristic of the LocataNet is its ability to propagate (autonomously) into difficult environments and over wide areas. Through an experimental LocataNet installation, a key mechanism for achieving this is tested, and real-time stand-alone positioning (without a base station and additional data link) with sub-centimetre precision is demonstrated.

Zhengdong Bai and Yanming Feng

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To take advantage of the existing GPS tracking networks established primarily for surveying, geodesy and navigation purpose for meteorology studies, this research effort is made to use hourly surface temperature and pressure (T & P) observations from in Australia for GPS Precipitable Water Vapour (PWV) estimation. After some necessary technical basises are given, the paper presents the experimental results to show: the comparison between the interpolated and observed T and P values, and agreement between the GPS-PWV estimates using the surface meteorological data and the radiosonde PWV results. Data analysis with 36 data points from the Victoria region in Australia has demonstrated that the Ordinary Kriging method with is preferable to pressure interpolation, resulting in an overall standard deviation of 0.40 mbar in pressure or 0.15mm in PWV estimation. We use the interpolated T and P measurements for four Australian IGS GPS sites to estimate GPS-PWV and compare against the radiosonde PWV results for the closely located radiosonde observations. 195 comparisons from all the sites have shown that GPS-PWV estimates agree with the Rad-PWV solutions at an average mean difference of -0.604 mm and RMS of 1.74mm for the tested stations. This agreement level is considered very reasonable. The experimental study shows a possible way to develop GPS meteorology and applications with the existing meteorological data network. This could save significant costs in installation of GPS-Met sensors.

J. A. S. Perez, J. F. G. Monico and J. C. Chaves

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GPS is essential in applications that require high (sub centimeter) positioning precision, such as in the velocity field estimation of tectonic plates. Normally, GPS relative positioning is used for this kind of application. However, GPS Precise Point Positioning (PPP) is a very simple and efficient method, which, as shown in this paper, can also be applied. This paper outlines the use of PPP for processing GPS data. Station coordinates and velocity vectors are inferred, and an estimation of the South American Plate rotation parameters is given. The PPP repeatability of station coordinates is better than 9mm, and comparisons of the final solution with other sources, such as ITRF, NNR-NUVEL 1A and APKIM 2000 generally show good agreement. The formal precision of the station velocity is in the order of 0.6 mm/year, for horizontal and vertical components, which appears to be an optimistic value, and the quality of the estimated rotation parameters is better than those from other sources.

Israel Kashani, Pawel Wielgosz and Dorota Grejner-Brzezinska

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This paper presents the multiple reference station approach to wide area (and regional) instantaneous RTK GPS, implemented in the MPGPS^™ (Multi Purpose GPS Processing Software) software, developed at The Ohio State University. A weighted free-net approach (WFN) was applied in the instantaneous RTK software module, which enabled optimal estimation of the rover coordinates, properly reflecting the accuracies of the observations and the coordinates of the CORS stations. The effect of using the distance-dependent weighting scheme in the WFN approach on the final rover solution is analyzed. The influence of the different weights was studied by introducing the distance-dependent weights as a function of the CORS station separation L to the rover (1/L²). The results show that almost 100% of the differences between the computed horizontal rover coordinates and the known reference coordinates are below a decimeter, and 95% of the differences in the vertical coordinate are below 20 cm, when using the suggested approach. In addition, the accuracy analysis of two other solutions, with different datum definition (minimum constraint and over-constrained), is presented. This analysis verifies the suitability of the stochastic models used in the RTK module and the rigorous approach, taking into account inter-baseline correlation as well as the correlation in-between each baseline components.

J. Wang, H.K. Lee, S. Hewitson and Hyung-Keun Lee

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The integrated GPS/INS system has become an indispensable tool for providing precise and continuous position, velocity, and attitude information for many positioning and navigation applications. Therefore, it is important to gain insights into the characteristics of the integrated GPS/INS system performance, particularly their relationships with key operational factors, such as the trajectory and dynamics. Such knowledge can be used to improve the quality of positioning and navigation results from the integrated GPS/INS systems. In order to analyse the influence of vehicle dynamics and trajectory, simulation and field tests have been carried out in this research. The test results show that the vehicle dynamic changes significantly affect the Kalman filter initialisation time and estimation performance depending on the system operational environments.

Sandra Verhagen

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The probability of correct integer estimation, the success rate, is an important measure in the case of fast and high precision positioning with a Global Navigation Satellite System. Integer ambiguity estimation is the process of mapping the least-squares ambiguity estimates, referred to as the float ambiguities, to an integer value. It is namely known that the carrier phase ambiguities are integer-valued, and it is only after resolution of these parameters that the carrier phase observations start to behave as very precise pseudorange measurements.

The success rate equals the integral of the probability density function of the float ambiguities over the pull-in region centered at the true integer, which is the region in which all real values are mapped to this integer. The success rate can thus be computed without actual data and is very valuable as an a priori decision parameter whether successful mbiguity resolution is feasible or not.

The pull-in region is determined by the integer estimator that is used and therefore the success rate also depends on the choice of the integer estimator. It is known that the integer least-squares estimator results in the maximum success rate. Unfortunately, it is very complex to evaluate the integral in the case of integer least-squares. Therefore, approximations have to be used. In practice, for example, the success rate of integer bootstrapping is often used as a lower bound. But more approximations have been proposed which are known to be either a lower or upper bound of the actual integer least-squares success rate.

In this contribution an overview of the most important lower and upper bounds will be given. These bounds are compared theoretically as well as based on their performance. The performance is evaluated using simulations, since it is then possible to compute the ’actual’ success rate. Simulations are carried out for the two-dimensional case, since its simplicity makes evaluation easy, but also for the higher-dimensional geometry-based case, since this gives an insight to the performance that can be expected in practice.

Experts Forum

"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 an 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, we have three different topics covered by the invited experts.

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).

Charles S. Dixon

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Dr. Charles Dixon is a Specialist Navigation Systems Engineer with some 15 years experience in various aspects of GNSS and communications technology. He is a member of the technical team at EADS Astrium Ltd, Portsmouth UK developing Europe's Galileo Satellite Navigation System. Chaz holds PhD and MSc degrees in Radionavigation from Leeds University, UK, and a BSc degree in Physics from the University of Newcastle Upon Tyne, UK. He has authored a number of GNSS-related papers, is a Member of the US-based ION and an Associate Fellow of UK's RIN.

Werner Enderle

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Dr. Werner Enderlg holds a Master and Ph.D. in Aerospace Engineering, both received from the Technical University of Berlin. He worked for the German Aerospace Centre/ German Space Operations Centre (DLR/GSOC) in the space flight dynamics division, where he specialised in space borne GPS applications (Orbit/Attitude Determination). He also was responsible for a DLR GPS receiver development for space applications. Between 2000 and 2001 he joined the Galileo Support Team (GAST), which was supporting the European Commission in the context of the design studies for the European Global Navigation Satellite System (Galileo). Since 2001 he is an Associate Professor for Aerospace Avionics at the Queensland University of Technology in Brisbane, Australia. He is also leader of the Global Navigation Satellite System - Space Application group. This group is considered the premier academic GNSS R&D group for space applications in Australia.

Dr Werner Enderle of Queensland University of Technology will review the future impact of Galileo on spacecraft navigation systems. The article outlines the Galileo benefits for spacecraft navigation systems, including the number of available frequencies and services, up to 3 separated frequencies for one service; application of the Three Carrier Ambiguity Resolution (TCAR) technique for substantial improved spacecraft attitude determination algorithms; high data rates for improved signal acquisition - Time To First Fix (TTFF) and reacquisition receiver behavior, and so on. It is the author’s belief that the real impact from Galileo for new spacecraft navigation systems is driven by the interoperability between Galileo and GPS and subsequently the dual use of both systems. This feature will generate new ideas and concepts, which will lead to advanced spacecraft navigation systems with a maximum degree of on-board autonomy.

M.O. Kechine, C.C.J.M. Tiberius and H. van der Marel

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Maxim Kechine graduated from the Saint-Petersburg State University in 1992 and obtained his PhD-degree in astrometry and celestial mechanics in 1998 at the Institute of Applied Astronomy in St. Petersburg, Russia. Currently he is, as a Postdoc, with the Mathematical Geodesy and Positioning section at the Delft University of Technology.

Christian Tiberius is an assistant professor with the Mathematical Geodesy and Positioning section. He is involved in several areas of GNSS positioning research as carrier phase ambiguity resolution, data quality control, analysis of geodetic-grade equipment pseudorange and carrier phase measurement noise, and evaluation of various modes of positioning and approaches for data processing.

Hans van der Marel is an assistant professor with the Mathematical Geodesy and Positioning section. He was responsible for the design and set up of the permanent GPS reference station system in the Netherlands (AGRS.NL). He is actively involved in national geometric infrastructure and in European reference systems (EUREF/EPN). At present he is involved in several projects on GPS meteorology, also in cooperation with the Royal Dutch Meteorological Institute (KNMI).

Dr M.O. Kechine, C.C.J.M. Tiberius and H. van der Marel of Delft University will examine their recent kinematic positioning results with the NASA's global internet-based differential GPS system and Real Time Gipsy (RTG) core user software. It is observed from this independent study that the well-designed JPL's RTG can achieve decimeter accuracy with the IGDG corrections in real time worldwide. The problem is still the long initialization time at the order of tens of minutes. It is interesting to notice their further research and tests to address this problem. They are seeking "fast and smooth convergence of the filtered position estimates during the first seconds" and establishing "whether the constrained troposphere errors (taken from a-priori models) are capable of decreasing the filter convergence time". The authors pointed out that the problem of single-receiver carrier phase ambiguity resolution nowadays becomes one of the most important and interesting challenges to be investigated in the future.

Instructions to Authors

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CPGPS Management Team (2003) Structure

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