The Effect of Signal Multipath on the Global Positioning SystemNicholas Talbot and Kefei Zhang
Department of Geospatial Science, RMIT University
The Global Positioning System (GPS) is widely used for navigation and timing applications throughout the world. The satellite-based system was developed by the United States Department of Defense for military applications, however non-military use now dominates. The surveying and mapping profession has been quick to adopt GPS technology. GPS techniques are often faster and more cost effective than conventional techniques and in many cases more accurate.
GPS theory and techniques are currently delivered within the Department of Land Information, RMIT, for undergraduate and postgraduate programs. Traditional lectures covering theory and practical exercises are run throughout the year and at an intensive field camp to reinforce GPS concepts and techniques.
Students and professionals using GPS techniques need to have an understanding of the capabilities and limitations of the system. There are a variety of factors that influence the accuracy attainable with GPS. One of the most significant and difficult error sources to model is termed 'signal multipath'. Multipath is caused by satellite signal reflection and diffraction from nearby and overhead objects. Another important consideration is satellite geometry, that is, the relative location of satellites overhead. Multipath and satellite geometry varies with both time and location and therefore it is difficult to provide students with strict rules on how both factors will affect estimated position. A practical exercise, where the students acquire actual satellite data and analyse a limited data set has been introduced to aid this understanding. However, due to expensive equipment, software costs, large student groups, limited time and staff resources, it is not feasible to conduct an experiment that reveals the necessary information.
A virtual package
To address these issues, a web-based package, which incorporates a predict-observe-explain paradigm, was developed to enable students to discover the effects of multipath and satellite geometry on GPS in Australian environments (see Figure 1). The package focuses on a GPS project, where the user of the package is asked to provide guidance on the positioning aspects of a freeway construction.
Figure 1. Entry point for the web-based package
GPS is used at four of the sites on the project:
Each site presents different satellite tracking scenarios and multipath environments. The entry point into each site provides some background material and a panoramic view of the surroundings (Figure 2). Experienced GPS users will normally take a look around a site before they start observing to assess the likely quality of the signals. The panoramas replace the need for physically visiting each site. All panoramas were constructed from a collage of digital photographs using MacroMedia Director software.
Figure 2. A moving panorama for the built-up site
The flow through the package was carefully considered during the design phase. Access points for more detailed analysis tools appear in sequence, rather than all at once. This provides uninitiated users with a progression in their understanding of the concepts and minimises the need for external and internal help text. The challenge was to orchestrate flow and progression without making the package cumbersome for experienced users.
The quality of a GPS exercise is normally measured in terms of the accuracy of the results and the efficiency with which the data was obtained. Positioning accuracy is a temporal quantity with GPS. The 24 GPS satellites (currently 27 including three spares) orbit the earth every 12 hours and therefore the prevailing satellite geometry can change from being very good to very poor. The Position Dilution Of Precision (PDOP) is commonly used to assess satellite geometry. Generally the more satellites that are tracked at once, the stronger the geometry and the lower the PDOP. High PDOP (>7) indicates weak geometry and normally occurs when six or less satellites are in view.
In open areas, such as the Clear Site in the package, satellite geometry is the dominant factor in positioning accuracy. In Built-Up and Bush sites, signal multipath and attenuation degrade results.
Skyplots graph satellite location overhead and can also be used to show obstructions (see Figure 3). Graphs of east, north and height errors over time were generated from field data collected at each site. The combination of the skyplots and position errors are the focus of the analysis component of the package.
Figure 3. Analysis view of the built-up site
Users are asked specific questions about the relative quality of each site and the optimal time to observe at each site. The analysis view can be scrolled in time to help make the necessary judgements.
The on-line report at the end of the package endeavours to stimulate the user to question the reasons for variation in accuracy at each site over time. This is designed to consolidate students' understanding of multipath effects with an emphasis on typical Australian environments. Answers and explanations are provided once the report is completed. The package has been aimed at both self-paced learning as well as laboratory-based practicals.
A preliminary student evaluation of the package was conducted and the results are encouraging. After using the package for one hour, all students reported a deeper understanding of multipath, importance of site selection prior to GPS survey and the impact of satellite geometry on GPS positioning. A more detailed evaluation will be run over the second semester and will be reported late in 2000.
The Global Positioning System is an important tool for surveying, mapping and engineering professionals. Effective use of GPS for positioning applications only comes from a comprehensive understanding of the underlying error sources. Of particular importance is the effect of signal multipath and satellite geometry. The web-based package developed, takes users through a virtual tour of a variety of tracking environments. Site view, skyplot, position error and geometry graphs can be played and viewed over time to analyse the causes of errors. The interactive nature of the package is aimed at stimulating users to predict-observe and then explain the relationship between site environments, multipath and position accuracy. The package forms an important component of enhancing the professional judgement of surveying graduates on the use of GPS.
UniServe Science News Volume 16 July 2000
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