INTERNATIONAL GEODETIC SERVICES

For each one of the four primary geodetic techniques (VLBI, SLR, GNSS/GPS, and DORIS), an international service coordinated within the International Association of Geodesy (IAG) organizes and coordinates the data that are acquired, manages the analysis of that data, and generates products for users. These services develop the necessary standards and conventions and encourage international adherence to those conventions in the processing of the data provided by each technique. These services are critical components of the geodetic infrastructure.

As an example, the International VLBI Service (IVS) is essential to acquiring the basic data since VLBI measurements must be coordinated between the various stations to observe the same radio sources at the same time. The stations cannot work independently, and each observing run must be planned in detail. The IVS interacts with the users of VLBI products to integrate VLBI into the global Earth observing system, sets performance standards for the observing stations, establishes conventions for data formats and products, and issues recommendations for analysis software. Finally, the IVS sets the standards for analysis documentation and institutes appropriate product delivery methods in order to ensure product quality and timeliness. The IVS consists of about 30 Network Stations acquiring high-performance VLBI data; three Operation Centers coordinating the activities of a network of network stations; six Correlators processing the acquired data, providing feedback to the stations, and providing processed data to the analysts; six Data Centers distributing the products to users and providing storage and archiving functions; 21 Analysis Centers analyzing the data and producing the results and products; eight Technology Development Centers developing new VLBI technology; and one Coordinating Center coordinating daily and long-term activities. This adds up to more than 75 components representing more than 30 institutions in 16 countries.

The International GNSS Service (IGS) is a collaboration of approximately 200 organizations comprising diversely funded government agencies, universities, and other groups around the world that contribute data and analysis capability for GNSS/GPS. The products generated by the IGS analysis centers are freely available and include core products such as precise orbits and clock corrections for GNSS/GPS satellites (currently GPS and GLONASS), and tropospheric and ionospheric parameter estimates. The IGS orbit products are available on time scales ranging from near-real-time and predicted orbits (ultra-rapid product) to the most accurate orbits (final orbit product), which are available with 2-week latency. Comparisons among the IGS analysis centers show that the IGS ultra-rapid orbits have an internal consistency of better than five centimeters, while the IGS final orbits currently have an internal consistency at the one to two centimeter level. Comparisons with SLR tracking of the two GPS satellites that have reflector arrays indicate an absolute orbit accuracy better than five centimeters. The IGS products, particularly the precise orbits and GPS clock solutions, have become integral in a wide variety of civil and scientific activities. Even U.S. government agencies commonly rely upon the routine availability of the IGS products although some of these agencies such as the National Geospatial-Intelligence Agency have the capacity to generate such products independently. It is now common for geodetic missions to operate a GNSS/GPS receiver on board their satellites for precision orbit determination, relying upon either the IGS orbits and clock solutions or the ground data collected by the IGS network for computing their own solutions. The IGS analysis centers also now provide the dominant contribution to the determination of the Earth’s polar motion. Like the other services, the IGS sets antenna monumentation standards for the observing stations, establishes conventions for data formats and products, and issues recommendations for analysis models, constants, and procedures.

The International Laser Ranging Service (ILRS) provides satellite and lunar laser ranging data and their related products to support geodetic and geophysical research activities. Satellite tracking priorities are agreed upon by the ILRS to best reflect the tracking data needs of each mission and its overall contribution to the geodetic community with special tracking campaigns sometimes



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