FIGURE 5.3 Estimating the size of the needed global fundamental station network from simulations. As the size of the network is increased from 8 to 32 stations, the accuracy of the determination of ITRF, in terms of origin and scale, is improved, but the improvement is relatively modest with more than 24 sites. This indicates that the ITRF accuracy goals can be largely achieved with approximately 24 stations with co-located SLR and VLBI stations. SOURCE: Courtesy of E. Pavlis, NASA.

reflectors could help specifically to improve the GPS satellite transmitter phase center modeling and to refine orbit modeling.

Regional and national geodetic reference systems are essential for a variety of civil, legal, and public safety applications. These systems, however, have been traditionally realized through extensive ground-based surveys, are expected to have significant errors at the national scale (due to the accumulation of error inherent in leveling surveys), and are not always mutually consistent. Since the start of the ITRF development and the advent of improved positioning, however, national geodetic agencies have undertaken significant efforts to redefine and modernize continental and national geodetic systems so that they are compatible with the ITRF. For example, the European Terrestrial Reference System 1989 (ETRS89), the North American Datum of 1983 (NAD83), the Geocentric Datum for Australia (GDA), and other national geodetic systems are linked to the ITRF through conventionally adopted transformation parameters and formulas, and are often defined by fixed coordinates at a given epoch.³ For example, NAD83 is now defined in terms of a 14-parameter transformation from ITRF96. Regional organizations,