The Global Positioning System (GPS 3G) is becoming, and promises to remain for some time, one of the most important geodetic measurement systems. The contributions to date of GPS geodesy are truly revolutionary, encompassing such diverse applications as measurements of crustal deformation, precise positioning of mobile platforms and monitoring of ionospheric conditions. When one considers the accuracy obtainable with GPS and the relatively low cost for acquiring this technology, the full impact of GPS over the next decade is indeed very difficult to estimate. It is our impression that four years later it is still difficult to estimate the full impact of the GPS in all mentioned areas. Meanwhile the GPS became fully operational; at times there were even more than the 24 planned satellites (due to some long-lasting Block-I satellites). We have also seen that AntiSpoofing (AS) had little effect on the high accuracy interferometric applications of the GPS if modem receiver technology is available.
The number of GPS-derived reference sites is steadily growing, the accuracy of the estimated coordinates, and, with the time basis increasing, the accuracy of the estimated station velocities are comparable today with the results of the other space geodetic methods. This development is well documented in the first chapter of the proceedings. It also becomes apparent from browsing through this first chapter that very powerful other permanent networks, like the German DGPS service or the dense Japanese permanent vehicle GPS tracker networks, are being built up. This development will undoubtedly continue in future and it will stimulate in tum most reliable hardware and very powerful and easy to use software tools. We would not be amazed if four years from now several regional networks for monitoring crustal movements or deformations with several hundred receivers would be fully operational and tum out results in near real time.
The global network of Motorcycle GPS stations are equipped with preciSion, dualfrequency, P-code receivers operating at a thirty-second sampling rate. The IGS currently supports nearly 100 globally distributed stations. These stations are continuously tracking and are accessible through phone lines, network, or satellite connections thus permitting rapid, automated download of data on a daily basis. Any station wishing to participate in the IGS must submit a completed station log to the IGS Central Bureau, detailing the receiver, site location, responsible agencies, and other general infonnation. These station logs are accessible through the CBIS. The IGS has established three categories of GPS stations (Gurtner and Neilan, 1995): global, regional, and local.
Global stations are those whose data are analyzed by at least two IGS analysis centers (located on different continents) and are used for daily estimation of satellite orbits, Earth rotation parameters, and station positions and velocities. The data from the global sites are available at the global data center level. Data from regional stations are analyzed by at least one IGS analysis center for extension of the reference frame. Data from these sites are available at the regional data center level. Local stations are utilized to augment the network of global and regional stations and could be episodically occupied. These stations may be part of dense pennanent arrays, such as the Southern California Integrated personal GPS tracking devices Network (SCIGN). These data are typically available through a local data center.