IGS Analysis Center Workshop - IGS - NASA

and Earth orientation parameters (EOP), The question of exact] y how global parameterdetermination improves with the number of stations is subject to further research, andcannot be answered fully without using real data. Nevertheless, there appears to be ageneral consensus that the computation of GPS satellite ephemerides and earthorientation parameters can be accomplished adequately with the current distribution ofglobal stations with some densification required in equatorial regions and in thesouthern hemisphere, for a total of about 30 to 40 well distributed stations. However,the IGS is a service for geodynamics which requires that the major tectonic plates (i.e.,the global polyhedron) and plate boundaries be sampled adequately, Withimprovements in technology and lower equipment costs, it is very likely that permanentGPS arrays will become attractive alternatives to campaign-type (epoch) GPS surveysor even replace them altogether. In our opinion, we can anticipate within a decade a“core” global network (polyhedron) of about 200 stations with 10-20 regional clustersof 20-100 stations each, all tracking continuously. In fact, it is already difficult todistinguish between global and regional stations.It is clear that there needs to be a mechanism to integrate densely sampled networksacross plate boundaries into the global scheme. Regional arrays in plate boundaryzones require (perhaps periodic) ties to the global reference frame through thecoordinates and velocities of the global tracking stations in order to determine“absolute” displacements of these internally deforming regions (e.g., southernCalifornia). One could argue that the primary purpose of the global IGS network is tosupport the regional arrays. What is the IGS role in this scenario? Does it concentrateits efforts exclusively on the core network? Even in this scenario the computationalburden on the analysis and data centers will be heavy. Can any one center manage 200core stations? In our view the IGS has two main objectives. The first one is to supportepoch and campaign type GPS surveys for geophysical/geodetic applications withglobal information such as orbits, earth orientation, terrestrial reference frame, etc.This is the most obvious and primary current focus of the IGS. The second, and asimportant, objective is to integrate (continuous and campaign-type) regional clusterswith the global core network. With this motivation we outline below the purpose ofthis paper.1.2 Purpose(a) Assess the role of the IGS with regard to the data analysis of clusters of regional GPSnetworks, including recommended procedures, standards, feedback, qualityassessment, customer services, the determination of network kinematics, and unifieddatum control for GPS surveys. (Network operations and data management arebeyond the scope of this paper).(b) Formulate a top level system design tocarry out the above roles.(c) Investigate technical options for combining regional and global solutions, both from theIGS point of view (maintenance of the polyhedron) and the user point of view (precisesolutions in a consistent reference frame for an arbitrary epoch),(d) Present several examples of the integration of a continuously operating regional arraywith thp global IGS network and distributed processing.(e) Converge on a consensus on the issues and come up with a series of recommendations.(~ List those questions that require further discussion and/or research.64