High precision inter-continental GPS network
GPS relative positioning provides precision of the order of 1 part per million (ppm) within relatively short periods of time. Considering this level of precision, together with the fact that the cost of GPS receivers is continuing come down, it is now apparent that most of the future geodetic surveys will be performed by GPS, It is therefore important to establish geodetic control networks which are suitable for geodetic GPS activities. The Brazilian Institute of Geography and Statistics (IBGE), which is the National Geodetic Surveying organization in Brazil, has proposed a high accuracy Brazilian GPS network, which has characteristics of an Active Control System (ACS). Such a system provides the users with the capability to perform relative positioning using only one receiver. An investigation of the methodology, algorithms and analysis, related to the IBGE network, has been carried out during this research. An initial investigation to assess the level of accuracy which can be obtained by a static user of the IBGE network equipped with only one receiver and observing within different scenarios, has shown relative precision in the range of 0.2 to 0.1 ppm. GPS positioning requires a global coordinate reference frame. The integration of a GPS network into such a global frame requires stations already connected to the global reference frame to be observed simultaneously with the new network stations. The processing is then carried out involving the network stations together with those already connected to the global reference frame. The reeent1y created International GPS Geodynamics Service (IGS) provides the capability for such easy integration. GPS data and ephemerides generated from the IGS Epoch '92 Campaign have been used in the processing carried out to integrate the Brazilian GPS network into the IERS Terrestrial Reference Frame 1993 (ITRF93). This data processing involved very large network, and the GPS processing software had to be expanded in order to provide higher accuracies. The results demonstrated repeatability of the order of 20 mm, for baseline lengths of up to 8200 km. The expansion of the software mentioned above provided capability of processing very large or even global GPS networks. It allowed the investigation of several aspects related to global GPS, namely, free adjustment, the application of loose constraints to the parameters and the use of internal constraints in the covariance- matrix. The use of GPS to realize a global reference frame has also been tested. In order to investigate all these aspects, an inter-continental network involving the Brazilian and IGS stations was used. Results have shown a level of agreement after the transformation between the free network reference frame and the ITRF93 coordinates of the order of 6.4 mm.