School of Surveying and Spatial Information Systems

The University of New South Wales


A Study of Web-Based GPS Processing Engines

by Limin Chua

Supervised by Prof. Chris Rizos

Edited by J. M. Rüeger

October 2002


Aim

The objective of the thesis was to test the available online GPS processing engines thoroughly and to develop guidelines for surveyors using GPS in survey applications. The thesis describes and discusses the use of each engine, paying attention to the methods employed by the engines for the uploading of data, the reference frames used in the analysis, the presentation of results and the factors affecting the precision.

Overview

The Global Positioning System (GPS) is a satellite-based navigation system that allows any user with a GPS receiver to obtain an ‘accurate’ position using measurements of the transmitted satellite signals. The accuracy depends on a number of factors, but primarily the type of receiver used and the positioning technique that is employed. Over the last decade or so, the surveying profession increasingly used the Global Positioning System (GPS) for a number of surveying applications.

In the last few years, several organisations have set up web sites where GPS data can be up-loaded to and where data can be processed free of charge, using additional data from stations of the International GPS Service (IGS) permanent receiver network. These organisations include: National Mapping Division of GeoScience Australia (formerly AUSLIG), Jet Propulsion Laboratory (JPL, NASA, USA), and the Scripps Orbit and Permanent Array Centre (SOPAC) at the University of California, San Diego, USA.

Another web-based processing engine is provided by the United States National Geodetic Survey (NGS). It is known as the Online Positioning User Service (OPUS). This engine was not included in this investigation, as it only accepts and processes GPS data gathered in the United States of America.

Extensive testing of the above-mentioned engines was carried out to determine the level of performance (and the level of service provided). The 24 hour data sets, that were used for these tests, were obtained from the Australian Regional Network Data (ARGN) through GeoScience Australia (http://www.auslig.gov.au/gpsdata/). These data were segmented into 3 h, 6 h and 12 h blocks (as well as the original 24 h data files) to test each of the processing engines, and to investigate any differences or anomalies that were found. Guidelines on how these services could be used by practising GPS surveyors were developed.

Brief Guidelines

To take advantage of these online engines, the user needs two things: access to the internet and an email account. The speed of the internet connection is not of major importance. It is, however, essential that the next two ‘terms’ be met. Firstly, the GPS data must be observed in the dual frequency mode. Secondly, the file has to be stored in the RINEX format.

AUSPOS of the National Mapping Division of GeoScience Australia

At the web-interface, the user can choose to upload directly or have AUSPOS access the RINEX file via anonymous ftp. The user can process one to seven files at the same time. AUSPOS will choose the default antenna type and height as provided in the submitted RINEX file. However, there is an option for entering the antenna height and antenna type, if it is necessary to make a modification. A valid email address has to be entered so that the finished report can be sent to the user. The time taken to process the data and send a report is 20 ­ 25 minutes.

SCOUT of SOPAC

SCOUT requires that the GPS data is stored in RINEX format and is observed in a dual frequency mode. Also, the data must be stored on an anonymous ftp server for easy retrieval by SCOUT for processing. To use this ‘engine’, the user first needs to enter a valid email address followed by the URL of the anonymous ftp server storing the relevant data. To form a network solution, SCOUT processes the user's data together with the data from the three (reference) stations that are closest to the user’s site. The user can override the automatic selection of the reference stations. SCOUT provides a box that allows the user to choose these stations. 

Auto-Gipsy of JPL

Unlike the two other engines investigated, Auto-Gipsy has no web interface to interact with the user. It is a mail-in service where a user has to have an email account and access to an anonymous ftp server on which to store the GPS data. The user simply needs to send an email to ag@cobra.jpl.nasa.gov, titled “Static” and, in the body of the email, the ftp address to the server hosting the relevant data. A reply email with the title “Output” will be sent to the user within 5 ­ 10 minutes, depending on the number of jobs in the queue. The body of the email will contain the ftp addresses of the results file. The users are warned that the results are only available for a few days. Hence, it is highly recommended that the user download the result files immediately after the reply email is received.

Links to the Engines

AUSPOS can be found at http://www.auslig.gov.au/cgi-bin/gps.cgi

SCOUT can be found at http://sopac.ucsd.edu/cgi-bin/SCOUT.cgi

 

Suitable Applications

 

Service

Applications

AUSPOS

DGPS reference station positioning

 

Remote GPS station positioning

 

Ultra-long GPS baseline positioning

 

GPS connections to IGS and ARGN stations

 

High accuracy vertical GPS positioning

   

Auto-Gipsy

Static positioning

 

Rapid static positioning

 

Kinematic tracking of receivers

 

Further Information

For more information, please contact:

Miss Limin Chua
Email: l.chua@student.unsw.edu.au

or

Prof. Chris Rizos
Email: c.rizos@unsw.edu.au

Mail:
School of Surveying and Spatial Information Systems
University of New South Wales
UNSW SYDNEY NSW 2052
Australia

Phone: +61-2-9385-4205

Fax: +61-2-9313-7493

WWW: http://www.gmat.unsw.edu.au