Ruth E. Neilan

CASA Central and South America GPS geodesy

In January 1988, scientists from over 25 organizations in 13 countries and territories cooperated in the largest Global Positioning System (GPS) campaign in the world to date (Table 1) [Neilan et al., 1988]. From January 18 to February 5, 1988, 43 GPS receivers collected about 590 station-days of data in American Samoa, Australia, Canada, Colombia, Costa Rica, Ecuador, New Zealand, Norway, Panama, Sweden, United States, West Germany, and Venezuela. The experiment was entitled CASA UNO, an acronym for Central and South America—and “uno” is Spanish for “one,” designating first-epoch measurements. The CASA UNO experiment was the first civilian effort at implementing an extended GPS satellite-tracking network and established the first major GPS network in the northern Andean margin and the western Caribbean.

Operational aspects of CASA UNO '88-the first large scale international GPS geodetic network

For three weeks, from January 18 to February 5, 1988, scientists and engineers from 13 countries and 30 international agencies and institutions cooperated in the most extensive GPS (Global Positioning System) field campaign, and the largest geodynamics experiment, in the world to date. This collaborative experiment concentrated GPS receivers in Central and South America. the predicted rates of motions are on the order of 5-10 cm/yr. Global coverage of GPS observations spanned 220 degrees of longitude and 125 degrees of latitude using a total of 43 GPS receivers. The experiment was the first civilian effort at implementing an extended international GPS satellite tracking network. Covariance analyses incorporating the extended tracking network predicted significant improvement in precise orbit determination, allowing accurate long-baseline geodesy in the science areas. >

GGOS working group on ground networks and communications

Properly designed and structured ground-based geodetic networks materialize the reference systems to support sub-mm global change measurements over space, time and evolving technologies. Over this past year, the Ground Networks and Communications Working Group (GN&C WG) has been organized under the Global Geodetic Observing System (GGOS) to work with the IAG measurement services (the IGS, ILRS, IVS, IDS and IGFS) to develop a strategy for building, integrating, and maintaining the fundamental network of instruments and supporting infrastructure in a sustainable way to satisfy the long-term (10–20 year) requirements identified by the GGOS Science Council. Activities of this Working Group include the investigation of the status quo and the development of a plan for full network integration to support improvements in terrestrial reference frame establishment and maintenance, Earth orientation and gravity field monitoring, precision orbit determination, and other geodetic and gravimetric applications required for the long-term observation of global change. This integration process includes the development of a network of fundamental stations with as many co-located techniques as possible, with precisely determined intersystem vectors. This network would exploit the strengths of each technique and minimize the weaknesses where possible.

The Global Geodetic Observing System (GGOS): Detecting the Fingerprints of Global Change in Geodetic Quantities

Modern geodetic observations from a wide range of space and terrestrial technologies contribute to our knowledge of the solid Earth, atmosphere, ocean, cryosphere, and land water storage. These geodetic observations record the “fingerprints” of global change processes and thus are a crucial independent source of high accuracy information for many global change studies. Many of the geodetic techniques require a globally distributed ground infrastructure, and associated space segment elements. In the past decade and half a variety of technique-specific services have been established under the auspices of the International Association of Geodesy (IAG) to facilitate global coordination of geodetic activities and to ensure the generation of high accuracy and reliable geodetic products to support geoscientific research. The Global Geodetic Observing System (GGOS) is an important component of the IAG, and is intended to be an “umbrella” for the IAG Services, with a primary coordinating function to ensure the development of an adequate global geodetic infrastructure, and a suite of integrated multi-technique products, that will meet the needs of scientific users. Coordination means bringing together the different geodetic observing techniques, services and analysis methods so as to ensure that the same standards, conventions, models and parameters are used in the data analysis and modelling of “Earth system” processes. Integration implies the combination of geometric, gravimetric, and rotational observations in data analysis and data assimilation, and the joint estimation and/or modelling of all the necessary parameters representing the difference components of the Earth system. The geodetic observations collected during the last decades have facilitated major scientific discoveries related to geohazards, climate and the global water cycle. Geodesy has the potential to contribute even more to global change studies, particularly if coordination and integration of the geodetic activities are continued.

GPS Network Operations for the International GPS Geodynamics Service

As GPS technology comes of age in the 1990’s, it is evident that an internationally sponsored GPS tracking system is called for to provide consistent, timely ground tracking data and data products to the geophysical community. The planning group for the International GPS Geodynamics Service (IGS), sponsored by the International Association of Geodesy (IAG), is addressing all elements of the end-to-end tracking system, ranging from data collection to data analysis and distribution of products (Mueller, 1992). Part of the planning process is to formulate how these various elements work together to create the common infrastructure needed to support a wide variety of GPS investigations. A key element for any permanent satellite tracking system is certainly the acquisition segment; the reliability and robustness of the ground network operations directly determine the fates and limitations of final products. The IGS planning group therefore included a committee tasked to develop and establish standards governing data acquisition and site-specific characteristics deemed necessary to ensure the collection of a high quality, continuous data set. An abbreviated version of these standards is included as art appendix to this paper.