Chapter 4 of this report describes voluntary international collaborations, such as the International GNSS Service (IGS), which set standards for participation, including those for site documentation and data provenance; oversee data analysis and quality control through analysis centers; and make data and data products freely available to users. From a global viewpoint, data from the IGS supports the GNSS/GPS component of the ITRF by enabling high-accuracy satellite orbit determination and clock calibration. As such, IGS products are a natural starting point for applications requiring the highest accuracies and are routinely used by researchers and by U.S. federal agencies, even by agencies with their own “in house” capabilities (such as the U.S. Naval Observatory and the National Geospatial-Intelligence Agency). For this reason, among others, IGS is a critical asset to the United States’ geodetic infrastructure for science and commerce. In addition, its importance as an adjunct to the national high-precision GPS network recommended above cannot be overstated.
A well-distributed geodetic network around the globe leads to higher accuracy and reduces dependence on data from particular stations. Poor geographic coverage leads to lower accuracy and greater dependence on particular sites, which are given undue weight in the solutions. If such stations experience temporary equipment failures, the determination of orbit parameters and the realization of the ITRF can be affected excessively, a problem that is exacerbated by the existence of systematic model errors that become more detrimental as we push to achieve greater accuracies. Stations currently in the IGS network are sparsely distributed in the southern hemisphere relative to the northern hemisphere (see Figure 5.1). To balance the data from stations in the IGS network for the purpose of maintaining the ITRF, it is therefore necessary to select an optimal subset of northern stations while depending heavily on data from all available sites located south of the equator. Consequently, maintaining and upgrading the IGS stations of the southern hemisphere and expanding the southern portion of the network to the extent possible should receive the highest priority by the IGS.
NASA currently supports the United States’ contribution to the IGS network, which includes approximately 70 GPS stations both within and outside of U.S. borders. Outside the United States, some of these sites are on U.S. military bases or scientific installations, and some are operated cooperatively with local agencies or universities. Those sites that are co-located with other geodetic systems play a particularly important role in determination of the ITRF. A significant number of these co-located stations are in the southern hemisphere or other areas where the IGS network has poor geographic coverage or is otherwise weak. As a result, eliminating these stations would have unfortunate consequences for the IGS and its contribution to the ITRF.
Given the geographic and temporal gaps in coverage, degrading infrastructure, and potential loss of data continuity for key geodetic observing systems, and given the increased leverage of collaboratively funded efforts, it is in the interest of the United States to make a long-term commitment to a strong IGS network. This commitment includes support for operation and maintenance of a global network of homogeneous, high-quality sites supporting IGS standards within and outside the United States. These sites should be capable of real-time data transmission to support the recommended national GPS network.
In addition to strengthening the global GPS network to enhance the United States’ own geodetic infrastructure, playing a leading role in the IGS enables the United States to exert a strong and lasting influence on IGS standards and practices for the global network and IGS products. To sustain this influence, participation by United States investigators in the bureaus, analysis centers, working groups, and projects of the IGS must be supported.