12.2.3 The Global GPS Infrastructure

THE IGS ORGANISATION

The I GS is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS), and operates in close cooperation with the International Earth Rotation Services (IERS). The primary objective of the IGS is to provide a service to support, through GPS data products, geodetic and geophysical research activities. The secondary objective is to support a range of activities performed by users, such as those developing and maintaining national geodetic datums. Hence, the IGS also develops the necessary standards and specifications (not too dissimilar to those developed for standard GPS surveys), and encourages international adherence to these standards and its conventions.

The IGS collects, archives, and distributes GPS observation datasets of sufficient accuracy to satisfy the requirements of a wide range of applications. These datasets are used by the IGS to generate the following data products:

• High accuracy GPS satellite ephemerides (a variety of "post-processed" orbits, such sa "rapid" and "final" orbits, as well as a new "predicted" product orbit ).
• Earth rotation parameters (a by-product of the orbit determination process).
• Coordinates and velocities of the IGS tracking stations.
• GPS satellite and tracking station clock information.
• Ionospheric and tropospheric information.

The accuracies of these products are continually being improved in order to support current and planned scientific objectives such as:

• Realisation and improvement of the International Terrestrial Reference Frames (ITRFxx).
• Monitoring deformations of the solid earth crust (mainly the horizontal motion attributable to plate tectonics, as well as local and regional fault motion).
• Monitoring earth rotation.
• Monitoring variations in the liquid earth, particularly through monitoring the vertical component of the tracking stations (sea level, ice sheets, etc.).
• Scientific satellite orbit determination.
• Ionosphere and troposphere monitoring.

The IGS accomplishes its mission through the following organisation :

• Networks of tracking stations
• Data Centres
• Analysis and Associated Analysis Centres
• Analysis Centre Coordinator
• The Central Bureau
• The Governing Board

The GPS networks consists of approximately 80 "core" tracking stations (Figure 1 in section 12.2.4), and a large number (150-200) "fiducial" tracking stations (Figure 2 in section 12.2.4). The core stations provide continuous GPS tracking for the primary purposes of computing satellite ephemerides, monitoring the ITRS and determining the earth rotation parameters. The fiducial stations may be occupied continuously or intermittently (and then repeatedly at regular intervals) for the purpose of extending the ITRS to all parts of the globe. Three stations of the Australian Regional Network are part of the core GPS network, while the remainder can be considered to belong to the fiducial GPS network.

The data centres fall into three categories: operational, regional and global. The operational data centres are in direct contact with the tracking stations, downloading the data on a timely basis (generally daily), reformatting the data files to the RINEX format, maintenance of a local data archive, and the electronic transmission of the data files to a regional or global data centre. The regional data centres collect tracking data from several operational data centres, maintain a local data archive, and transmit these data to the global data centres. The Australian Survey and Land Information Group (AUSLIG), in Canberra, is one of the regional data centres. The global data centres are the main interfaces to the analysis centres and to the outside user community, maintaining archives of the raw tracking data as well as the IGS data products. The three global data centres are:

• Crustal Dynamics Data Information System, NASA Goddard Space Flight Center, Greenbelt, USA.
• Institut Geographique National (IGN), Paris, France.
• Scripps Institution of Oceanography, University of California, San Diego, USA.

The analysis centres receive and process tracking data to produce the IGS products. The analysis centres are committed to generate products, without interruption, and to deliver them to the global data centres, to the Analysis Centre Coordinator, to the IERS, and to other designated bodies. The main products are the satellite ephemerides (on a variety of time scales), and station coordinates (on a quarterly basis). There are also several Associate Analysis Centres. The seven analysis centres are:

• CODE, Astronomical Institute, University of Bern, Switzerland.
• European Space Operations Center, European Space Agency, Germany.
• FLINN Analysis Center, Jet Propulsion Laboratory, USA.
• GeoForschungsZentrum (GFZ), Potsdam, Germany.
• Geosciences Research Lab, National Oceanic and Atmospheric Administration (NOAA), USA.
• Natural Resources Canada.
• Scripps Institution of Oceanography, University of California, San Diego, USA.

The Analysis Centre Coordinator monitors the analysis centres' activities to ensure that the IGS objectives are carried out. Specific tasks include quality control and performance evaluation through the inter-comparison of the products of the different analysis centres. The Analysis Centre Coordinator is also responsible for the combination of the analysis centre products into a single IGS product. The current IGS Analysis Centre Coordinator is Tim Springer, Astronomical Institute, University of Bern, Switzerland.

The Central Bureau is responsible for the general coordination and management of the IGS. The primary functions of the Central Bureau are: (a) to facilitate communications (through the maintenance and operation of the Central Bureau Information System -- section 3.4.2); (b) coordinate day-to-day IGS activities; (c) coordinate the establishment of IGS standards and promote compliance with the standards; (d) monitor quality assurance of the data and products; and (e) maintain documentation, organise reports, meetings, and workshops. The IGS Central Bureau is located at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. The IGSCB director is Ruth Neilan.

The IGS Governing Board is the international body which exercises oversight and control over the activities of the service. There are 15 members of the Governing Board.

Up to date details can be obtained from the IGS documentation such as the Annual Reports, and the conference/workshop proceedings regularly published.

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© Chris Rizos, SNAP-UNSW, 1999