To address the behavior of the solar-terrestrial system on a global scale, the National Aeronautics and Space Administration (NASA), in coordination with the European Space Agency (ESA), the Japanese Institute of Space and Astronautical Science (ISAS) and the Russian Space Agency (IKI), established the International Solar Terrestrial Physics (ISTP) Initiative. In the spirit of the "Great Observatories," this initiative, combining spaceborne observations, ground-based measurements and theoretical investigations, represents the most comprehensive program ever implemented to study the Sun-Earth system. The Global Geospace Science (GGS) Program represents the primary NASA-contribution to ISTP, and includes the WIND and POLAR satellites, two instruments on the Japanese GEOTAIL satellite, four ground-based investigations, and four theory and modeling groups. The scientific objectives of the GGS Program are to: Measure the mass, momentum and energy flow and their time variability throughout the solar wind-magnetosphere-ionosphere system that comprises the geospace environment; Improve the understanding of plasma processes that control the collective behavior of various components of geospace and trace their cause and effect relationships through the system; Assess the importance to the terrestrial environment of variations in energy input to the atmosphere caused by geospace plasma processes. The GGS program was designed from its inception as a highly efficient, distributed system to study geospace from a global perspective and with the fundamental objective of acquiring detailed, quantitative information about the flow of energy, mass and momentum throughout geospace. To this effect spacecraft were launched and placed in strategic orbits from which fundamental physical processes taking place in four key regions of geospace (solar wind, geomagnetic tail, auroral regions, equatorial magnetosphere) could be observed simultaneously, with the purpose of establishing more definitive cause-effect relationships and quantitative data against which theoretical models could be developed and tested. Due to resource limitations only three spacecraft were developed and launched under GGS, (WIND, POLAR, GEOTAIL). Complementary data from equatorial regions were obtained from existing geosynchronous spacecraft, such as GOES and those operated by the Los Alamos National Laboratory, as well as the venerable IMP-8 spacecraft, and low altitude polar data from NASA’s FAST satellite. GGS benefits from simultaneous measurements by other spacecraft, coordinated through ISTP. These include: SOHO through the European Space Agency, INTERBALL through the Russian Space Agency, AKEBONO and YOHKOH (in addition to GEOTAIL) through Japan's Institute for Space and Astronautical Science, and many other space and ground based coordinated measurements contributed by the international community. Parallel efforts were required to implement a ground system capable of acquiring and processing the large volumes of data generated by the experimental and theoretical systems, spanning international boundaries and a multiplicity of computing and data processing systems, networks and advanced data visualization tools and standards.

Figure 1.1 ISTP-GGS Constellation