A covariance analysis shows that Global Positioning System (GPS) baseline precision in northern South America is substantially improved when fiducial stations (used to improve satellite orbit solutions and establish a reference frame) in North America are supplemented by stations in Hawaii, Australia and New Zealand. The formal errors for a variety of fiducial networks, some including stations in South America, are almost identical when their locations are held fixed. However, the consider error, which is the computed sensitivity to uncertainties in the fiducial station locations and is an estimate of the systematic error, is strongly dependent on the fiducial network geometry. If fiducial station locations are estimated with small a priori uncertainties, the various networks are practically identical, although the total uncertainty is slightly larger for the fiducials-fixed networks. The sensitivity analysis indicates that the baselines of interest are very sensitive to uncertainties in the locations of the closet fiducial stations. If the uncertainty in the fiducial station locations is reduced to approximately 1 cm from the 4 cm assumed, any of the networks used in this analysis would be acceptable for GPS geodesy in northern South American. ¿ America Geophysical Union 1988