International GNSS Service

A pilot becomes a service

By the early 1990s many universities and agencies tracked GPS satellites for geodesy, each in its own way. The International Association of Geodesy organised a test campaign and pilot service in 1992, and the official service began on 1 January 1994 as the International GPS Service for Geodynamics. It was renamed the International GPS Service in 1999, and in 2005 the International GNSS Service when it began to include products for the Russian GLONASS satellites. Gerhard Beutler, of the University of Bern, was its first chair of the Governing Board, and Ruth Neilan led the Central Bureau at the Jet Propulsion Laboratory.

The Central Bureau coordinates, but the work is done by hundreds of contributing agencies: station operators, data centres and analysis centres.

What the federation produces

Stations around the world record signals from the navigation satellites continuously, and send the data to data centres. Analysis centres in several countries compute the satellite orbits and clocks, the motion of the Earth's pole and the positions of the stations, and the IGS combines their results into official products, whose agreement with one another is itself a check. The precise orbits are accurate to a few centimetres, far better than the broadcast ones, and are used for geodesy, for surveying and for precise positioning of every kind.

The station positions contribute to the International Terrestrial Reference Frame, the global coordinate system in which the motion of continents is measured.

Terms of use

IGS data and products are made available to participants and users under the IGS data and product terms, which are open but do not state an unambiguous licence for commercial use. For that reason, Nexus Observatory does not republish IGS data; this page describes the organisation, and uses public NOAA and other open sources for station data instead.

Why products, not data, are the output

Most users of the IGS never see a raw observation: they use the precise orbits and clocks, which are products of the analysis. This is the point of a federation: the individual contributions are combined and checked, and the output is of higher quality than any one contribution. The reference frame that results is used to measure sea-level change, tectonic motion and the shape of the Earth, and so underlies much of the rest of this atlas. A product is a form of data that has been made useful to others, and its provenance is part of its value.

Instruments and technologies

• Geodetic GNSS receivers Receivers with choke-ring antennas on monuments tied to bedrock, recording the phase of the carrier signals from every visible satellite.
• Analysis centre software Software that computes orbits, clocks and station positions from the network's data, with products combined by the Central Bureau.
• Hydrogen maser clocks Some stations have a hydrogen maser as a frequency standard, allowing satellite clocks to be compared accurately.

The people

• Gerhard Beutler Astronomer and geodesist; first chair of the IGS Governing Board.
• Ruth E. Neilan Director of the IGS Central Bureau.

Timeline

• 1992 IGS test campaign and pilot service begin.
• 1994 Official service starts on 1 January as the International GPS Service for Geodynamics.
• 1999 Renamed the International GPS Service.
• 2005 Renamed the International GNSS Service to include GLONASS products.

Key lessons for observation today

• Federation without absorption Hundreds of agencies contribute and keep their stations. A federation works when contributors keep control and the standard is shared.
• Compare independent analyses Several analysis centres compute the same products and the combination checks them. Independent redundancy is a quality system.
• Check the licence before you republish The terms of use are open to participants but not clearly commercial, so the data stay out of our products. Licence review belongs at the start of a data pipeline.

Sources

• IGS, International GPS Service (McGraw-Hill Yearbook article)
• IGS Reference Chapter
• IGS 1999 Technical Report, executive summary