Increasingly, the spatial information sector is turning to the Internet to aid its activities; both public and private sector organisations are using it to develop, promote and deliver their services.
Organisations that use the GPS are no exception. An Internet search using the keyword 'GPS' reveals thousands of GPS-related websites. These sites provide information on applications, hardware, software and services. The field of high-precision geodetic GPS is also well represented; many scientific, private sector and national geodetic agencies maintain useful pages.
The National Mapping Division of Geosciences Australia is one such agency. The division is Australia's national mapping agency, providing fundamental geographic information to support many industries.
The division is also the national body for geodesy in Australia, and as such is responsible for the national geodetic infrastructure. As part of this role, the division maintains a network of permanent GPS receivers throughout both Australia and the Australian Antarctic Territory.
As high-precision global geodetic GPS technology has evolved, processing and analysis software has become more sophisticated and, in general, more automated. One of the more interesting developments has been the implementation of Internet-based geodetic GPS processing services. The first of these was the NASA Jet Propulsion Laboratory's Auto-Gipsy service.
In Australia, these international GPS processing services were being widely used by the geodetic GPS community. However, there was a great deal of confusion between the International Terrestrial Reference Frame (ITRF), as provided by those international GPS processing services, and the Australian national datum, GDA94.
How it worksAUSPOS is based on GPS data received from a network of 15 stations in Australia and some 200 scattered around the world. Data received from these stations is used to build up a model of the instantaneous errors in the GPS signals induced by ionospheric and other effects.Meanwhile, the user in the field records a series of observations from the satellites. In the normal way, these observations will have errors that limit the accuracy of the position to five or ten metres. These observations are recorded digitally either in the receiver or on a separate data logger, in a file in RINEX format.When a user submits a file to the AUSPOS website, the approximate position and exact time of the observations are first deduced, and the error in the userŐs region is then computed from the model.The resultant position, with centimetre accuracy, is then fed back to the user. |
Since GDA94 is based on the ITRF92 at a fixed epoch of 1994, the latest ITRF co-ordinates produced are substantially offset. This difference is due largely to the tectonic motion of the Australian plate, as shown above. Co-ordinate differences between GDA94 and the ITRF are about half a metre at the time of writing (2002).
The division and its predecessor, AUSLIG, offered a GPS processing service to its clients in aviation, defence and other Commonwealth and state government agencies, in the national interest. Data was generally received on digital media such as CD-ROM, floppy disks or by email. Customer service then relied on a hands-on process.
The system did not necessarily meet the needs of clients. Defence clients, for example, increasingly demanded full-time access to a precise GPS processing service.
In this context, in early 2000, a decision was made to develop a web-based GPS processing service that would provide users with access to GDA94-based co-ordinates for Australian users, and ITRF for international users. After several months of development a beta testing program was commenced; several months later version 1 of the processing software was complete. Software improvements continued, and the Parliamentary Secretary to the Minister of Industry, Science and Resources, Warren Entsch, MP, officially released the service on 11 November 2000.
This service, known as the Online GPS Processing Service or AUSPOS, has now been in continuous operation for over 18 months. It is accessible via the division's website at www.auslig.gov.au/geodesy/sgc/wwwgps/
AUSPOS was designed and implemented as a web interface backed up by dual-frequency geodetic GPS data processing capability. Users can access the system by direct upload or by ftp. The service is offered full-time. It will process results in less than 15 minutes per file and return the results by email and ftp server. The system will work anywhere on the Earth; it is GDA94-compliant in Australia and ITRF-compliant elsewhere.
The international component of the AUSPOS system is mediated by the division's membership of the International GPS service. IGS is composed of agencies around the world that provide GPS data and products such as GPS orbits and Earth-orientation parameters. These products are designed to meet the objectives of a wide range of scientific and engineering applications and studies. The division is a Regional Network Associate Analysis Centre of the service. It runs 15 GPS stations on the Australian and Antarctic continents. The IGS consists of some 200 stations.
ITRFThe International Terrestrial Reference Frame (ITRF) produced by the International Earth Rotation Service (IERS) is a realisation of an ideal reference system based on the international terrestrial reference system.The IGS undertakes its own realisation of the ITRF using analysis results from the IGS community. The IGS cumulative solution (IGS-SSC) is one such solution; it is currently aligned to the ITRF2000 reference frame.AUSPOS undertakes all computations using the IGS cumulative solution as its reference frame.GDA94In 1992, GPS observations were completed on eight geologically stable marks at sites across Australia. These marks form the Australian Fiducial Network (AFN). Additional GPS observations were also carried out at a number of existing geodetic survey stations across Australia. In 1993 and 1994 these were supplemented by further observations, resulting in a network of about 70 GPS sites (with a nominal 500 km spacing) across Australia. This is known as the Australian National Network (ANN). GPS observations at both the AFN and ANN sites were combined in a single regional GPS solution in terms of ITRF92. The resulting co-ordinates were mapped to a common epoch of 1994.The positions of the AFN sites were used to define the Geocentric Datum of Australia (GDA). The positions of both the AFN and ANN sites were used to constrain a readjustment of the Australian geodetic networks. GDA94 is officially regarded as the Australian national datum.Transformation between the ITRF and GDA94
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GPS processing is undertaken in accordance with the International Earth Rotation Service computation standards and is summarised in tables 2, 3 and 4. Within the computation module, AUSPOS uses the MicroCosm software suite, which is a full implementation of the IERS96 computations standards. MicroCosm has also been used for geodetic orbit determinations and parameter estimation for Satellite Laser Ranging and DORIS (Doppler Orbitography and Radio Positioning Integrated by Satellite).
The use of AUSPOS has been beyond our expectations. Currently the system is processing an average of 400-500 jobs per month. A large majority of users are Australia-based. However, the service is gaining in popularity around the world.
It is clearly helping the private sector in the growing international spatial information sector by providing access to a sophisticated GPS analysis system free of charge. Feedback shows positive support for the service and Australian companies are competing internationally for contracts using the AUSPOS service as an important selling point. Every small survey organisation that uses GPS effectively now has access to a sophisticated GPS analysis system.
Further development of the AUSPOS system is continuing. Already the users are suggesting that a one metre level accuracy service (possible using less than a few minutes of data) would be useful for a variety of applications, including mapping and GIS control. A service based on single-frequency data may also have a niche.
We continue to seek the advice of GPS users so that development of our GPS/Internet services can best meet user requirements.
The authors are with the National Mapping Division of Geosciences Australia; ph: +61-2-6201-4201. They wish to acknowledge Alan Lun, contract software programmer, who contributed to the design and implementation of the system. They also wish to thank Michael Roff and Nancy Jarrott (Agsystems), Linlin Ge (UNSW), Bob Ross and Neil Brown (NRE, Victoria), Lotti Jivall (LM, Sweden), Chris Hicks (DITM, NSW) and Richard Coleman (UTAS) for their participation in the beta testing phase.
